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Selina Concise Chemistry Class 6 ICSE Solutions – Elements, Compounds, Symbols and Formulae

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Selina Concise Chemistry Class 6 ICSE Solutions – Elements, Compounds, Symbols and Formulae

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APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 6 Chemistry. You can download the Selina Concise Chemistry ICSE Solutions for Class 6 with Free PDF download option. Selina Publishers Concise Chemistry for Class 6 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

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Selina Concise ICSE Solutions for Class 6 Chemistry Chapter 4 Elements, Compounds, Symbols and Formulae

POINTS TO REMEMBER

  1. Pure substances : “A single substance of definite composition.” Pure substances are homogeneous. They are made up of only one kind of atoms and compounds or made up of only one kind of molecules.
  2. Elements : An element is defined as a pure substance made up of only one kind of atoms that cannot be converted into anything simpler than itself by any physical or chemical process.
  3. Metals : Most of the elements known to us are metals. Example: Sodium potassium, iron, gold, silver are elements. Metals are lustrous, hard, ductile and malleable, good conductor of electricity. Mercury (liquid) have High M.P. and High B.P. but exceptions are there.
  4. Non-metals : Carbon, phosphorous, sulphur all solids, Bromine (liquid) iodine(s) and rest are gases, dull, do not shine. Cannot be drawn into wires, non malleable, have low M.P. and B.P. do not produce sound when struck, do not conduct electricity. Exceptions are there.
  5. Boron, sillicon, arsenic, antimony resemble both metals and non-metals and are called metalloids.
  6. Unreactive gases, also called noble gases are gases which are very unreactive i.e. He, Ne Ar, Kr, xenon and radon.
  7. Symbols are abbreviations that are used to denote a chemical element which is usually first letter of its name in English or Latin.
  8. O is the symbol of element oxygen.
    C is symbol of element carbon.
    Cu is symbol of element copper (taken from Latin name Cuprum)
  9. Compound is formed by the chemical combination of two or more elements in definite ratio (by mass).
  10. Molecule is the smallest unit of a compound.
  11. Elements are the basic substances from which all other substances are made.
  12. Compound : Consist of two or more elements combined in definite proportion, pure and homogeneous, physical and chemical properties are entirely new and different from its consititutent elements, energy is either needed or produced when a compound is formed.
  13. Atoms : Atom is the smallest unit of an element which cannot be further broken into simpler parts, may or may not have independent existence.
  14. Molecule of element : When two or more atoms of the same element combine it forms a molecule of an atom. e.g. N2,O2
  15. Molecule of compounds : When atoms of two or more element combine, they form a molecule of a compound. H20, HC1, CuSO4.

EXERCISE-I

Question 1.
Classify the following substances into elements and compounds.
Answer:
Mercury, sulphur, sugar, water, sand, gold, coal, oxygen, alcohol.
Ans. Element: Mercury, sulphur, gold, coal, oxygen.
Compound : Sugar, water, sand, alcohol.

Question 2.
Give the symbols of: Carbon, calcium, copper, chlorine, cobalt, argon.
Answer:
Carbon is C         Chlorine is Cl
Calcium is Ca     Cobalt is Co
Copper is Cu       Argon is Ar

Question 3.
Define a pure substance. How many types of pure substances do you know ?
Answer:
Pure substances : “A substance of a definite composition which has consistent properties throughout, is called a pure substance”
Types of pure substances: Pure substances are of two types (i) Elements, (ii) Compounds.

Question 4.
Define : (a) Elements (b) Compounds.
Name the particles from which elements and compounds are made of.
Answer:
(a) Elements : An element is defined as a pure substance made up of only one kind of atoms that cannot be converted into anything simpler than itself by any physical or chemical process.
(b) Compounds : Compounds are pure substances composed of two or more elements in definite proportion by mass and has a definite set of properties. Compound is made up of only one kind of molecules.

Question 5.
Give two examples for each of the following :
(a) Metals
(b) Non-metals
(c) Metalloids
(d) Noble gases
Answer:
(a) Metals : Iron, silver, gold.
(b) Non-metals : Carbon, sulphur, oxygen.
(c) Metalloids : Antimony, silicon, boron.
(d) Noble gases : Helium, argon, neon.

Question 6.
Name the elements which form water. How will you justify that water is a compound ?
Answer:
The elements which form water are (i) Hydrogen and oxygen.
Justification : Water has entirely different properties (i.e. is a liquid, extinguishes fire) from the elements it is made up of i.e. Hydrogen a gas catches fire oxygen a gas supporter of combustion.

  1. Energy is needed to form water on combining O2 with H2.
  2. We can not seperate the constituents of water by simple physical means.

Question 7.
Give three differences between metals and non-metals.
Answer:
Metals

  1. Metals are ductile i.e. can be drawn into wires.
  2. Metals are malleable i.e. can be beaten to form sheets.
  3. They are sonorous.

Non-metals

  1. Non-metals are mostly soft solids cannot be drawn into wires.
  2. They are mostly gases and are not malleable.
  3. They donot produce sound when struck.

Question 8.
How is sodium chloride different from its constituent elements, sodium and chlorine ?
Answer:
Sodium is a metal that is stored in kerosene oil as it reacts very fast with air and water. Chlorine is a reactive greenish yellow gas which is poisonous. When these two elements combine chemically they form common salt sodium chloride which is non poisonous colourless solid substance that we use in our food to add taste and to obtain some nutrition.

Question 9.
State four important characteristics of compounds.
Answer:

  1. When compound is formed energy like heat, light or electricity is either needed or produced.
  2. A compound has properties entirely different from the properties of its constituents.
  3. Change in weight takes place.
  4. It cannot be separated into its constituents by simple physical means.

Question 10.
Give two examples for each of the following :

(a) Non-metals which are solids
(b) Metals which are soft
(c) Non-metals which are lustrous
(d) Elements which are liquids.
(e) Inert gases
(f) Metalloids

Answer:

(a) Phosphorus, Sulphur
(b) Lead and Sodium
(c) Radium, Graphite
(d) Mercury, Bromine
(e) Helium, Neon
(f) Antimony, Arsenic

Question 11.
Name the elements present

(a) Sugar
(b) Ammonia
(c) Marble
(d) Washing soda

Answer:

Compounds
(a) Sugar
(b) Ammonia
(c) Marble
(d) Washing soda
Elements present
(a) Carbon, hydrogen & oxygen
(b) Nitrogen and hydrogen
(c) Calcium, carbon & oxygen
(d) Sodium, carbon & oxygen

Question 12.
What is the proportion of elements present in the following compounds?
(a) H2O
(b) CO2
(c) CaO
(d) NO2
Answer:
Selina Concise Chemistry Class 6 ICSE Solutions Chapter 4 Elements, Compounds, Symbols and Formulae 1

Question 13.
Name two compounds which dissolve in water.
Answer:
Two compounds which dissolve in water are sugar, table salt.

EXERCISE-II

(ATOMS & MOLECULES)

Question 1.
Define:

(a) Atom
(b) Molecule
(c) Atomicity
(d) Formula

Answer:
(a) Atom : An atom is the smallest indivisible unit of an element which exhibits all the properties of that element and may or may not have independent existence.
(b) Molecule : A molecule can be defined as the smallest unit of an element or a compound which exhibits all the properties of that element or compound and has independent existence. They are divisible into atoms.
(c) Atomicity : The number of atoms in a molecule of an element is called its atomicity.
(d) Formula : Formula is a short way of representing the molecule of an element or a compound.

Question 2.
Why are symbols and formulae of substances important?
Answer:
Importance of symbols and Formulae :
Symbols and formulae of substance gives a lot of information like.

  1. Types of elements present in the compound. E.g. (H20 is made of two elements hydrogen and oxygen).
  2. Number of each kind of atoms in one molecule. E.g. (water has 2 atoms of hydrogen combined with 1 atom of oxygen.)
  3. Mass of one molecule of the compound. E.g. [H2O has mass (1 × 2) + 16 = 18 g].

Question 3.
Mention three gaseous elements and write their molecular formulae.
Answer:
Selina Concise Chemistry Class 6 ICSE Solutions Chapter 4 Elements, Compounds, Symbols and Formulae 2

Question 4.
State the informations obtained from the formula of a compound.
Answer:
A formula gives us the following information about a compound.

  1. Types of elements present in the compound.
  2. Number of each kind of atoms in one molecule of the compound.
  3. Mass of one molecule of the compound.

Example:
A molecule of carbon dioxide gas is represented by CO2 It indicates that a carbon dioxide molecule is formed by the combination of two elements i.e. carbon and oxygen. The number of carbon atom is one and that of oxygen atom is two. The mass of one molecule of carbon dioxide can be calculated by adding the mass of one atom of carbon and two atoms of oxygen.

Question 5.
What is meant by
(a) 2H and H2
(b) H20 and 3H2O ?
Answer:
(a) 2H is two atoms of hydrogen. H2 is one molecule of hydrogen gas.
(b) H20 represents one molecule of water. 3H20 represents 3 molecules of water.

Question 6.
State the number of atoms of each kind, present in
(a) C6 H12O6
(b) H2SO4
(c) HNO3
(d) CaCO3
Also name these compounds.
Answer:
Selina Concise Chemistry Class 6 ICSE Solutions Chapter 4 Elements, Compounds, Symbols and Formulae 3

Question 7.
Write the molecular formulae of compounds calcium oxide, hydrogen sulphide, carbon monoxide and lead sulphide.
Answer:
Compound Calcium oxide is formed of elements calcium (Ca) and oxygen (O)
Selina Concise Chemistry Class 6 ICSE Solutions Chapter 4 Elements, Compounds, Symbols and Formulae 4
Symbols combining power Here subscript number is same Ca2 Formula of calcium oxide is CaO Compounds Hydrogen sulphide is formed of elements
Selina Concise Chemistry Class 6 ICSE Solutions Chapter 4 Elements, Compounds, Symbols and Formulae 5

EXERCISE-III

Question 1.
Name:
(a) Three different forms of carbon.
(b) A form of carbon used as a gem.
(c) Two substances used to make electric wires.
(d) Two substances used to make jewellery.
(e) A substances used as an insulator.
Answer:
(a)

  1. Diamond
  2. Graphite
  3. Coal

(b) Diamond is used as gem.
(c)

  1. Copper.
  2. Aluminium as these are good conductors of electricity.

(d)

  1. Gold.
  2. Silver as these are shining, lustrous, and ductile.

(e) Plastic is used as insulator as it is bad conductor of electricity.

Question 2.
Give one use of each of the following substances :

(a) Iron
(b) Brass
(c) Coal

Answer:
(a) Iron : To make machines tools and building material.
(b) Brass : To make water taps and utensils.
(c) Coal: Coal is used as fuel also used in thermal power plant to produce electricity.

Question 3.
Give reason:

(a) A frying pan is made up of steel but its handle is made up of wood.
(b) Graphite is used to make lead of the pencils.
(c) Argon is filled in electric bulbs.

Answer:

(a) Steel is good conductor of heat to cook food, pan is made of steel where as wood is insulator of heat and to hold, handle is made up of wood.
(b) Graphite leaves mark on the paper and makes it black.
(c) Argon is inert gas and protects the element of bulb from oxidation and burning. Hence increases bulb’s life.

Question 4.
Answer the following questions:

(a) Why are copper and aluminium used to make electric wires?
(b) What do you understand by the statement: ‘metals are ductile and malleable’?
(e) Give the advantages of using symbols instead of names of elements or compounds.
(d) When iron is mixed with sulphur at room temperature, it does not form a compound. Why?
(e) Find the atomicity of the Tollowing molecules:

  1. calcium chloride
  2. aluminium suiphide
  3. acetic acid
  4. dinitrogen oxide
  5. carbon monoxide

Answer:
(a) Copper and aluminium are good conductors of heat and electricity. They can be drawn into wires and beaten into sheets. Therefore, they are used to make electric wires.
(b) Metals are ductile, i.e., they can be drawn or stretched into thin wires. They are malleable, i.e., they can be beaten into thin sheets.
(c)

  1. Symbols increases scientific communication across the world.
  2. Symbols helps to make equations and data shorter and concise.
  3. Symbols are helpful for Scientists as it would take time and paper to do their job. They had to write out the full name of every element instead of its symbol.

(d) When iron is mixed with sulphur at room temperature, it does not form a compound because the mixture of iron and sulphur requires heat to form a compound i.e. iron sulphide.
Selina Concise Chemistry Class 6 ICSE Solutions Chapter 4 Elements, Compounds, Symbols and Formulae 6

OBJECTIVE TYPE QUESTIONS

1. Fill in the blanks

(a) Atomicity refers to the number of atoms in the molecule of an element.
(b) The most abundant element in the earth’s crust is oxygen.
(c) A metal which is a liquid at room temperature is mercury.
(d) The most abundant element in the atmosphere is nitrogen.
(e) A metal which is a poor conductor of electricity is tungsten.
(f) A diatomic gaseous element is oxygen.
(g) A liquid non-metal is bromine.

2. Match the columns
Selina Concise Chemistry Class 6 ICSE Solutions Chapter 4 Elements, Compounds, Symbols and Formulae 7

3. Indicate whether the following statements are true or false.

(a) A compound is made up of just one kind of atom.
Answer. True False
Correct : A compound is made up of two or more elements is a fixed proportion by mass.

(b) Metals reflect light and are good conductors of electricity.
Answer. True

(c) Metals can be polished.
Answer. True

(d) Elements are made up of compounds.
Answer. False
Correct : Elements are made up of atoms.

(e) All elements are artificially prepared.
Answer. False
Correct: All elements are made up of a limited number of basic substances.

(f) Molecules can exist independently.
Answer. True

(g) Molecules combine to form atoms.
Answer. False
Correct : Atoms combine to form molecule.

(h) Noble gases are high reactive.
Answer. False
Correct: Noble gases are non-reactive.

(i) Ozone is a triatomic molecule.
Answer. True

MULTIPLE CHOICE QUESTIONS

Tick (√) the correct alternative from the choice given for the following statements :

1. All pure substances have

  1. the same physical state .
  2. the same colour
  3. the same composition
  4. a definite set of properties

2. Sugar is a compound which consists of the elements

  1. carbon and hydrogen
  2. hydrogen and oxygen
  3. carbon, hydrogen and oxygen
  4. hydrogen, carbon and sulphur

3. Atoms of different kinds combine to form molecules of

  1. an element
  2. a compound
  3. a mixture
  4. all of the above

4. Sulphur and carbon are

  1. metals
  2. non-metals
  3. metalloids
  4. noble gases

5. Gold is used to make jewellery because

  1. it is dull
  2. lustrous and attractive
  3. highly reactive
  4. very cheap

6. The most abundant elements in the universe are

  1. neon and argon
  2. hydrogen and helium
  3. aluminium and copper
  4. oxygen and nitrogen

7. The compound used as common salt is

  1. sodium chloride
  2. calcium chloride
  3. sodium oxide
  4. hydrogen chloride

8. Brass and bronze are

  1. elements
  2. mixtures
  3. compounds
  4. all of the above

9. Sand is a compound of

  1. silicon and nitrogen
  2. silicon and oxygen
  3. oxygen and sulphur
  4. none of the above

10. From the list given below select the correct substance which is most suitable to the statements given : (oxygen, diamond, zinc, graphite, gold)

  1. A metal which is brittle.
  2. A non-metal which is a good conductor of electricity.
  3. The hardest naturally occurring substance.
  4. The most ductile metal.
  5. A gaseous non-metal.
    Answer.
    (a) Zinc
    (b) Graphite
    (c) Diamond
    (e) Oxygen

Selina Concise Biology Class 8 ICSE Solutions – Endocrine System and Adolescence

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Selina Concise Biology Class 8 ICSE Solutions – Endocrine System and Adolescence

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 8 Biology Chapter 5 Endocrine System and Adolescence. You can download the Selina Concise Biology ICSE Solutions for Class 8 with Free PDF download option. Selina Publishers Concise Biology for Class 8 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

Selina Class 8 Biology ICSE SolutionsChemistryPhysicsMathsGeographyHistory & Civics

Selina Concise ICSE Solutions for Class 8 Biology Chapter 5 Endocrine System and Adolescence

REVIEW QUESTIONS

Multiple Choice Questions:

1. Put a tick mark (✓) against the correct alternative in the following statements:

(a) Cortisone hormone is secreted by:

  1. Medulla of adrenal
  2. Cortex of adrenal
  3. Pancreas
  4. Thyroid

(b) Which one of the following hormones stimulates the breakdown of glycogen in the liver into glucose:

  1. Insulin
  2. Adrenaline
  3. Glucagon
  4. Thyroxine

(c) Which one of the following hormones converts excess of glucose into glycogen:

  1. Glucagon
  2. Thyroxine
  3. Insulin
  4. Adrenaline

(d) Which one of the following glands is also called master gland:

  1. Pituitary gland
  2. Adrenal gland
  3. Thyroid gland
  4. Ovary

(e) The emergency hormone to face the danger or to fight is secreted by:

  1. Islets of Langerhans
  2. Adrenal cortex
  3. Pituitary
  4. Adrenal medulla

(f) Which one of the following endocrine glands produces its hormone in large quantities as a result of emotional stimulation?

  1. Thyroid
  2. Islets of Langerhans
  3. Adrenal medulla
  4. Adrenal cortex

Adrenal medulla produces its hormone in large quantities as a result of emotional stimulation.

(g) In humans, increased thyroxine production results in (tick the correct answer):

  1. Increased metabolism
  2. Decreased metabolism
  3. Dwarfism
  4. Cretinism

Short Answer Questions:

Question 1.
What is a hormone?
Answer:
The secretions of the endocrine glands are called Hormones, which are poured directly into the blood and are carried to the target organs.

Question 2.
In table given below, fill in the blanks by naming endocrine glands, the hormones they secrete, and the function they perform, in a normal person.
Answer:

S.No.Name of the glandproducedFunction
1.ThyroidThyroxineControl of metabolic rate
2.PancreasInsulinRegulation of sugar in blood.
3.AdrenalAdrenaline and cortisonePreparing the body for action
4.Pituitary(i) Growth hormone
(ii) Thyroid stimulating hormone		(i) For growth
(ii) Stimulates thyroid gland to secrete thyroxine

Question 3.
Match the items in Column A with those in Column B. Column A Column B
Selina Concise Biology Class 8 ICSE Solutions – Endocrine System and Adolescence 1

Answer:

Selina Concise Biology Class 8 ICSE Solutions – Endocrine System and Adolescence 2

Question 4.
Name the source and the function of each of the following hormones:

HormoneSourceFunction
(a) Glucagon
(b) Thyroxine
(c) Adrenaline
(d) Insulin
(e) Cortisone

Answer:

HormoneSourceFunction
(a) GlucagonPancreasBreakdown of glycogen to glucose. Raises sugar in the blood.
(b) ThyroxineThyroid glandControl of metabolic rate
(c) AdrenalineAdrenal glandPrepare the body to face emergency, stress
(d) InsulinPancreasRegulation of sugar in blood.
(e) CortisoneAdrenal cortexRegulates carbohydrate metabolism. Its deficiency causes Addison’s disease.

Question 5.
What is the difference between an exocrine gland and an endocrine gland?
Answer:
The salivary glands, pancreas, etc., are exocrine glands, they send their secretions through ducts directly to the target orgOn the other hand, the endocrine glands are ductless glands. Their secretions are called hormones, which are poured directly into the blood and are thus carried to the target organs.

Question 6.
Why is pitnitary gland is called “master gland”?
Answer:
The pituitary gland is called “master gland” because it produces hormones that control other glands and many body functions including growth (growth hormone, Thyroid and Gonad stimulating hormone).

Question 7.
Briefly write about the importance of physical hygiene during adolescence.
Answer:
Physical hygiene also named as Personal hygiene plays a crucial role in maintaining healthy during adolescence. The teenager should follow the below mentioned activities to promote their health:

  1. Proper and Safe Food: Adolescence is a stage of rapid growth and development. Hence, a teenager should take proper care of their diet. They should take proper balanced diet that provides protiens, carbohydrates, fats, minerals and vitamins. They should take freshly prepared food and avoid stale food. They should take milk, fruits and fresh vegetables.
  2. Proper life Style: Regular Exercise and sleep are necessary for maintaining good health. Teenager should avoid long hours of continous table work, television watching. Teenager should not consume alcohol, drugs or smoking.
  3. Cleanliness: Teenager should take bath regularly. They should always wash their hands before and after having meals. Brushing up teeths after every meal. They should always change and wear washed clothes especially undergarments. Regular toilet habits should be adopted for maintaining good health. Teenager must keep their feet cleaned and protected. Injuries due to bacteria like tetanus, hookworms and insects may be issued if barefoot walk is undertaken. They must wash and comb their hairs regularly. All body parts must be washed and cleaned everyday. If cleanliness is not maintained there may occur chances of catching bacterial infection. Girls should take special care of cleanliness during the time of menstrual period.
  4. Physical Exercise: Inorder to keep the body fit and healthy, teenager’s should walk, exercise and play outdoor games regularly in fresh air. Playing Outdoor games reduces the stress and strain of adolescence.

Question 8.
Briefly discuss any four activities which can be practiced to overcome stress.
Answer:
Stress is a state of mental or emotional strain and in simple terms it is called as tension.
The stress can be controlled or reduced by following the below mentioned steps:

  1. Yoga: It is a mind-body practice that combines physical poses, controlled breathing, and meditation or relaxation. Yoga helps in reducing steps by:
    (a) Increasing Flexibility
    (b) Increasing muiscle strength and tone.
    (c) Improving respiration, energy and vitality.
  2. Exercise: Exercise or Running or Jogging for 30 to 45 minutes at least three times a week reduce stress and keep’s the body much healthier.
  3. Proper sleep schedule: Improving your sleep schedule also helps in reducing the stress.
  4. Reading is a great way to calm your mind and to gain more knowledge.
  5. Practice Hobbies of your interest: One should must keep practicing their hobbies as it helps in reducing the stress. Enjoy playing guitar, paino or listening music or doing riddles.

Selina Concise Chemistry Class 8 ICSE Solutions – Carbon and Its Compounds

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Selina Concise Chemistry Class 8 ICSE Solutions – Carbon and Its Compounds

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 8 Chemistry Chapter 9 Carbon and Its Compounds. You can download the Selina Concise Chemistry ICSE Solutions for Class 8 with Free PDF download option. Selina Publishers Concise Chemistry for Class 8 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

Selina Class 8 Chemistry ICSE SolutionsPhysicsBiologyMathsGeographyHistory & Civics

Selina Concise ICSE Solutions for Class 8 Chemistry Chapter 9 Carbon and Its Compounds

Points to Remember:

  1. Carbon occurs in the earth’s crust in the free as well as in the combined state.
  2. In the free state, it occurs as coal, diamond and graphite.
  3. In the combined state, carbon occurs in atmosphere (CO2) natural gas, food nutrients and carbonates.
  4. Diamond is the hardest naturally occuring substance known.
  5. Fullerenes are discovered only recently.

Exercise – I

Question 1.
Fill in the blanks.

(a) Carbon is present in both living and non-living things.
(b) The tendency of an element to exist in two or more forms but in the same physical state is called Allotropy.
(c) Crystalline and non- crystalline are the two major crystalline allotropes of carbon.
(d) Diamond is the hardest substance that occurs naturally.
(e) The name ‘carbon’ is derived from the Latin word carbo.

Question 2.
Choose the correct alternative.

(a) In combined state, carbon occurs as
(i) coal
(ii) diamond
(iii) graphite
(iv) petroleum

(b) A crystalline form of carbon is
(i) lampblack
(ii) gas carbon
(iii) sugar
(iv) fullerene

(c) Graphite is not found in
(i) Bihar
(ii) Maharashtra
(iii) Orissa
(iv) Rajasthan

(d) Diamond is used for
(i) making the electrodes of electric furnaces.
(ii) making crucible for melting metals.
(iii) cutting and drilling rocks and glass.
(iv) making carbon brushes for electric motors.

(e) Carbon forms innumerable compounds because
(i) it has four electrons in its outermost shell.
(ii) it behaves as a metal as well as a non- metal.
(iii) carbon atoms can form long chains.
(iv) it combines with other elements to form covalent compounds.

Question 3.
Write ‘true’ or ‘false’ against the following statements.

(a) Carbon constitues 0.03% of the earth’s crust. – True
(b) Graphite is the purest form of carbon. – False
(c) Coloured diamonds are costlier than colourless and transparent diamonds. – False
(d) Graphite has layers of hexagonal carbon bondings. – True
(e) Diamond is insoluble in all solvents. – True.

Question 4.
Define the following terms:
(a) Allotropy (b) Carat
(c) Crystal (d) Catenation
Answer:
(a) Allotropy: Allotropy is defined as the phenomenon due to which an element exists in two or more forms in the same physical state with identical chemical properties but with different physical properties.

(b) Carat – The weight of diamond is expressed in carats [ 1 carat = 0.2 g]

(c) Crystal – A crystal is a homogeneous solid which particles (atoms, molecules or ions) are arranged in difinite pattern due to which they have definite geometrical shape with plane surfaces e.g. sugar and sodium chloride.

(d) Catenation – The large number of organic compounds is due to the ability of carbon atom to form long chains with other carbon atoms through the sharing of electrons. This unique property of carbon is known as catenation.
selina-concise-chemistry-class-8-icse-solutions-carbon-and-its-compounds-1

Question 5.
State the terms:

(a) Substances whose atoms or molecules are arranged in a definite pattern. – Crystals.
(b) Different forms of an element found in the same physical state. – Allotropy.
(c) The property by which atoms of an element link together to form long chain or ring compounds. – Catenation

Question 6.
Name the following:

(a) The hardest naturally occurring substance. – Diamond.
(b) A greyish black non- metal that is a good conductor of electricity. – Graphite.
(c) The third crystalline form of carbon. – Fullerenes.

Question 7.
Answer the following questions:
(a) Why is graphite a good conductor of electricity but not diamond?
(b) Why is diamond very hard?
(c) What are fullerenes? Name the most common fullerenes.
(d) What impurity is present in black diamond?
(e) Explain the softness of graphite with reference to its structure.
Answer:
(a) In a graphite molecule, one valence electron of each carbon atom remains free, Thus making graphite a good conductor of electricity. Whereas in diamond, they have no free mobile electron. Thats why diamond are bad conductor electricity.

(b) A diamond is a giant molecule. The number of valence electrons in carbon atom is four. As such each carbon atom is linked with four neighboring carbon atoms. Thus forming a rigid tetrahedral structure. It is the strong bonding’that makes diamond the hardest substance.

(c) Fullerenes: Fullerenes are the third crystalline form of carbon.
Though they were discovered only recently. They have.been found to exist in interstellar dust as well as in the geological formations of the earth.
Common fullerenes are C – 32, C – 50, C – 70 and C – 76

(d) Black diamonds have copper oxide present in them as impurity.

(e) In a graphite molecule of each carbon atoms is linked with three neighboring carbon atoms. Thus forming a hexagonal arrangement of atoms. These hexagonal grouping of carbon atoms are arranged as layers or sheets piled one the top of other. The layers are held together by weak forces such that they can slide over one another. That is why graphite is soft.

Question 8.
Give two uses of (a) graphite (b) diamond.
Answer:

(a) Uses of graphite:

  1. For making the electrodes of electric furnaces.
  2. For making crucibles for melting metals due to its high melting points.

(b) Uses of Diamond:

  1. Diamond is used in jewellery as a gem
  2. It is used for cutting and drilling rocks, glass,

Question 9.
Write three differences between graphite and diamond.
Answer:
Difference between diamond and graphite.

Diamond

  1. Pure diamond is colourless and transparent.
  2. It is the hardest naturally occurring substance.
  3. It has high density i.e. 3.5 g/cm3
  4. It is bad conduct of electricity.
  5. It bums in air at 900°C to form carbon dioxide.

Graphite

  1. Graphite is greyish black opaque and shiny.
  2. It is soft and greasy to touch.
  3. It has low density i.e. 2.39 g / cm3
  4. It is good conductor of electricity.
  5. It bums in air at 700° C to form carbon dioxide.

Exercise – II

Question 1.
Fill in the blanks:

(a) Charcoal is formed when charcoal is burnt in a limited supply of air.
(b) Coal is a amorphous form of carbon.
(c) Peat is the most inferior form of coal.
(d) Wood charcoal is a bad conductor of heat and electricity.
(e) lampblack is used in making black shoe polish.

Question 2.
Choose the correct alternative

(a) Anthracite is
(i) an inferior type of coal
(ii) a superior type of coal
(iii) a cheapest form of coal
(iv) none of above

(b) Destructive distillation of coal yields
(i) coal tar
(ii) coal gas
(iii) coke
(iv) all of the above

(c) Lamp black is
(i) an amorphous form of carbon
(ii) a crystalline form of carbon
(iii) a pure form of carbon
(iv) a cluster of carbon atoms

(d) The process by which decayed plants slowly convert into coal is called.
(i) petrification
(ii) carbonisation
(ii) carbonification
(iv) fermentation

(e) The purest form of the amorphous carbon is
(i) wood charcoal
(ii) sugar charcoal
(iii) bone charcoal
(iv) lampblack

Question 3.
Write ‘true’ or ‘false’ against the following statements:

(a) Charcoal is a good adsorbent. True
(b) Coke is obtained by destructive distillation of sugar. False
(c) Activated charcoal is a good conductor of electricity. False
(d) Wood charcoal is an important constituent of gun powder. True
(e) Coal gas is used in the preparation of artificial ferilizers. False.

Question 4.
Define the following:
(a) Carbonization
(b) Adsorption
(c) Bone black
Answer: 
(a) Carbonization: The process of the slow conversion of vegetable matter into carbon-rich substances is called carbonization.
(b) Adsorption: Adsorption is the property due to which a substance absorbs gases, liquids and solids on its surface.
(c) Bone black: The Carbon content of bone charcoal is separated by treating the latter with hydrchloride acid, which dissolves the calcium phosphate. Carbon is then filtered out of the solution and in this form it is called bone black.

Question 5.
Name the following:

(a) Substances whose atoms or molecules are not arranged in a geometrical pattern. – Amorphous
(b) The best variety of coal. – Bituminous
(c) The purest form of amorphous carbon. – Anthracite
(d) An amorphous form of carbon that contains about 98% carbon. – Anthracite
(e) Mixture of carbon monoxide and hydrogen. – Water gas.

Question 7.
Answer the following questions:
(a) What is destructive distillation? What are the products formed due to the destructive distillation of coal?
(b) Why is wood charcoal used in water filters and gas masks?
(c) How is wood charcoal made locally? What other substances are formed in the process.
(d) How many carbon atoms are there in Buckminster fullerenes?
Answer: 
(a) Destructive Distillation: When a substances is heated in the absence of air. The process is called destructive distillation.
Products formed are: Coke, Coal tar, Coal gas and ammonia solution

(b) Due to its high adsorbing capacity, wood charcoal is used as gas masks to adsorb harmful gases. Wood charcoal is porous, that is why it is used to filter water.

(c) Wood charcoal is prepared when wood is heated in a limited supply of air. Locally wood charcoal is prepared by piling logs of wood one above the other with a gap in the centre of the pile. The pile is covered with wet clay to prevent the entry of air. A few holes are left at the bottom of the pile. The wood is set on fire. After some time when fire dies out, wood charcoal is left behind. The other substances are -wood tar, pyroligneous acid and wood gas.
selina-concise-chemistry-class-8-icse-solutions-carbon-and-its-compounds-2

(d) 60 carbon atoms are arranged in spherical structure in Buck minster fullerences.

Question 7.
(a) Descirbe the formation of coal,
(b) Name four types of coal with percentage of carbon present in each, with uses.
Answer:
(a) Formation of coal:- The formation of coal took millions of years. Coal was formed by the bacterial decomposition of ancient vegetable matter hurried under successive layers of the earth. Under in action of high temperature and pressure, and in the abcence of air, the decayed vegetable matter converted into coal.
(b) Types of Coal:

  1. Peat: It is light brown in colour and contains only 50 – 60% carbon. It is the most inferior form of coal.
  2. Lignite: it contains more than 60% carbon. It is brown in colour and harder than peat.
  3. Bituminous: It has 90%, 80%, 70 – 75% carbon contents. Bituminous coal is the most common variety of coal and used as house hold coal.
  4. Anthracite: It is the purest variety of coal. Its carbon contents vary between 92 – 98%. It is hard, dense and black, difficultto ignite.

Uses of coal:

  1. Coal is used as both domestic and industrial fuel.
  2. It is used to prepare coke, coal gas and coal tar.

Question 9.
Name the products formed when:
(a) wood is burnt in the absence of air.
(b) bone is heated in the absence of air.
(c) diamond is burnt in air at 900°C.
(d) graphite is subjected to high pressure and 3000°C temperature.
Answer:
(a) Wood charcoal is formed when wood is burnt in limited supply of air.
(b) Bone charcaol, bone oil and organic compound pyridine.
(c) Carbon dioxide.
(d) Artificial diamond.

Question 9.
Give two uses for each of the following:
(a) coal
(b) coke
(c) wood charcoal
(d) sugar charcoal
(e) bone charcoal
(f) lampblack
Answer: 
(a) Uses of coal

  • It is used as both a domestic and an industrial fuel.
  • It is used to prepare coke, coal gas and coal tar.

(b) Uses of coke

  • Coke is used as a smokeless fuel, in smelting furnaces.
  • It is used in the manufacturing of water and producer gas.

(c) Uses of wood charcoal:

  • Wood charcoal is used as a fuel.
  • It is an important constituent of gun powder.

(d) Sugar charcoal:

  • Sugar charcoal is mostly used as a reducing agent.
  • It is used to decolourise coloured solutions.

(e) Bone charcoal:

  • It is extensively used to decolourise cane-sugar in the process of manufacturing sugar.
  • It is also used in the manufacture of large number of phosphorous compounds.

(f) Uses of lamp black:

  • It is used in making black shoe polish.
  • It is used in the manufacture of tyres and gun powder.

Question 10.
Give balanced equations for the following chemical reactions:
(a) wood charcoal and cone, nitric acid
(b) coke and steam
(c) wood charcoal and lead monoxide.
Answer:
selina-concise-chemistry-class-8-icse-solutions-carbon-and-its-compounds-3

Exercise – III

Question 1.
(a) Name the chemicals required for the preparation of carbon dioxide in the laboratory.
(b) How will you collect the gas ?
(c) Write the balanced chemical equation for the above reaction.
(d) Draw a labelled diagram for the preparation of CO2 in the laboratory.
(e) Why is sulphuric acid not used for the preparation of carbon dioxide in the laboratory ?
Answer:
(a) Calcium carbonate and dilute hydrochloric acid.
(b) By upward displacement of air.
(c) CaCO3 + 2HCl → CaCl2 + H2O + CO2.
(d) Laboratory preparation of carbon dioxide
selina-concise-chemistry-class-8-icse-solutions-carbon-and-its-compounds-4
(e) Dilute sulphuric acid reacts with calcium carbonate. But it is not used because the calcium sulphate which is formed during the reaction is insoluble in water. It covers the marble chips and stops the reaction.
selina-concise-chemistry-class-8-icse-solutions-carbon-and-its-compounds-5

Question 2.
Write the balanced chemical equations for the preparation of carbon dioxide by:
(a) heating calcium carbonate.
(b) the action of acetic acid on sodium bicarbonate.
(c) the action of dilute sulphuric acid on sodium bicarbonate.
Answer:
selina-concise-chemistry-class-8-icse-solutions-carbon-and-its-compounds-6

Question 3.
What happens when:
(a) a lit splinter is introduced into a jar containing carbon dioxide ?
(b) moist blue litmus paper is placed in a jar containing carbon dioxide ?
(c) carbon dioxide is passed through lime water first in small amounts and then in excess ?
(d) a baking mixture containing baking powder is heated?
(e) a soda water bottle is opened ?
Answer:
(a) Lit splinter extinguishes.
(b) Blue litmus paper turns red.
(c) When CO2 is passed through lime water in small amount, it turns milky, when passed in excess milkiness disappears.
(d) Carbon dioxide is formed.
(e) When the pressure is released the bottled gas escapes with a bristling effervescence that ads fizz to the drink.

Question 4.
Give reasons for the following:
(a) An excess of carbon dioxide increases the temperature of the earth.
(b) Soda acid and foam types of fire extinguisher are not used for extinguishing electrical fires.
(c) Solid carbon dioxide is used for refrigeration of food.
Answer:
(a) Excess of carbon dioxide increases the temperature of the earth. Due to rise in temperature ice in the polar regions may melt causing floods in coastal regions island.
(b) In both of these fire extinguishers, the solutions are prepared in water, which conducts electricity. As a result an electric shock might result, which might lead to short- circuiting and another fire.
(c) Solid carbon dioxide serves as a coolant and refrigeration for preserving food articles.

Question 5.
What is a fire extinguisher ? What is the substance used in the modern type of fire extinguishers ? How is it an improvement over the soda acid-type and the foam-type fire extinguishers ?
Answer:
Fire Extinguisher— Fire extinguishers are a device in which carbon dioxide is produced in different forms for use as the extinguishing agent. It is a modem type of fire extinguisher in which liquid carbon dioxide is stored in a steel cylinder under pressure. Soda-acid and foam types of extinguisher cannot be used for extinguishing fire as they prepared in water, which conducts electricity and there can be short circuiting, causing another fire.

Question 6.
Explain the term ‘green house effect’. How can it be both beneficial and harmful for life on earth ?
Answer:
Green house effect— The trapping of the earth’s radiated energy by carbon dioxide present in air, so as to keep the earth warm, is called ‘green house effect’.
Green house is beneficial because this principle is applied to grow plants in colder regions.
Carbon dioxide increases the temperature of atmosphere. Due to rise in temperature; ice in the polar regions may melt, causing floods. So it is harmful for life on earth.

Question 7.
What steps should be taken to balance carbon dioxide in the atmosphere ?
Answer:
As global warming will cause an unbalanced ecological system, serious efforts should be made to balance the percentage of carbon dioxide in the atmosphere. Some of these steps are:

  • Growing more trees and plants.
  • Using smokeless sources of energy like solar energy, biogas, etc.
  • Using filters in the chimneys of factories and power houses.

Question 8.
State three ways by which carbon dioxide gas is added into the atmosphere.
Answer:

  1. By planting more trees.
  2. By combustion of fuels
  3. By decay of dead animals, plants and plants products.

Exercise – IV

Question 1.
Fill in the blanks:

(a) Carbon monoxide is formed when carbon is burnt in a limited supply of air or oxygen.
(b) Carbon monoxide bums in air with a pale blue flame to form carbon dioxide.
(c) Carbon monoxide is a products of incomplete combustion.
(d) A mixture of 95% oxygen and 5% carbon dioxide is called carbogen
(e) Carbon dioxide is used as a reducing agent in the extraction of pure metals from their corresponding ores.

Question 2.
Match the following.
selina-concise-chemistry-class-8-icse-solutions-carbon-and-its-compounds-7

Question 3.
How is carbon monoxide gas formed?
Answer:
Mostly carbon monoxide is formed when a large amount of carbon or its compounds is burnt in a limited supply of air or oxygen.
selina-concise-chemistry-class-8-icse-solutions-carbon-and-its-compounds-8

Question 4.
State the poisonous nature of carbon monoxide?
Answer:
Carbon monoxide is highly poisonous gas. If air containing 0.5% carbon monoxide by volume is inhaled, death can result This is because carbon monoxide combines with the haemogloblin present in the blood cells of our body to form a stable compound called carboxyl-haemoglobin. This does not allow to absorb oxygen. Thus depriving our body cells of oxygen. This cause obsruction in respiration and causes death.

Question 5.
Give two uses of carbon monoxide.
Answer:
Uses of carbon monoxide:

  • Carbon monoxide is a strong reducing agent.
  • Carbon monoxide is used in the extraction of pure metals from their ores.

Question 6.
Why is carbon monoxide called silent killer ?
Answer:
Carbon monoxide is produced by burning coal or wood in a limited supply of air. Since the gas is colourless and a barely detectable smell, people do not feel it and it can be proved as a silent killer.

Question 7.
Explain the reducing action of carbon monoxide.
Answer:
Reducing action of carbon- monoxide: Carbon monoxide is a strong reducing agent. It reduces the oxides of the less active metals to their respective metals and itself gets oxidised to carbon dioxide.
selina-concise-chemistry-class-8-icse-solutions-carbon-and-its-compounds-9

Question 8.
Write two remedies for carbon monoxide poisoning.
Answer:

  1. The victim should immediately brought out into the open.
  2. The victim should be given artificial respiration with caibogen.

Question 9.
Complete the reactions and balance them.
(a) CuO + CO →
(b) Fe2O2 + CO →
Answer:
selina-concise-chemistry-class-8-icse-solutions-carbon-and-its-compounds-10

Selina Concise Physics Class 6 ICSE Solutions – Simple Machines

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Selina Concise Physics Class 6 ICSE Solutions – Simple Machines

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APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 6 Physics. You can download the Selina Concise Physics ICSE Solutions for Class 6 with Free PDF download option. Selina Publishers Concise Physics for Class 6 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

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Selina Concise ICSE Solutions for Class 6 Physics Chapter 4 Simple Machines

  • Synposis
  • Work is said to be done when a force applied on a body moves it. If the body does not move on applying a force on it, no work is done by the force.
  • The capacity of doing work is called energy.
  • A machine is a device which helps us to do work more easily.
  • A machine enables us to apply a less effort for a load greater than the effort or to apply the effort at convenient point and in a desired direction.
  • Some machines are simple and some are complex.
  • The mechanical advantage of a machine is the ratio of the load to the effort, i.e., Mechanical advantage = Load / Effort
  • Smaller the effort required for a certain load, greater is the mechanical advantage of the machine.
  • The efficiency of a machine is the ratio of the useful work done on the load by the machine to the work put into the machine by the effort, i.e.
    Efficiency = Work output / Work input
  • The efficiency of an ideal machine is 1 (or 100 per cent).
  • The efficiency of an actual machine is less than 1 because some part of the work put into the machine is lost in overcoming the friction between the moving parts of the machine.
  • A lever is a simple machine which we most commonly use in our daily life. It is a rod which can turn about a fixed point called the fulcrum.
  • The mechanical advantage of a lever is equal to the ratio of the effort arm to the load arm, i.e. Mechanical advantage of a lever = Effort arm / Load arm
    The levers are of three kinds :
    Class I levers which have fulcrum in between the load and the effort.
    Class II levers which have load in between fulcrum and the effort.
    Class III levers which have effort in between the fulcrum and the load.
  • The mechanical advantage of class I lever can be 1, more than 1 or less than 1.
  • The mechanical advantage of class II levers is always more than 1.
  • The mechanical advantage of class III levers is always less than 1.
  • A pulley is a simple machine which is used for raising a load up by applying the effort downwards.
  • The mechanical advantage of an ideal pulley is 1. In an actual pulley, due to friction, the mechanical advantage is less than 1 (i.e., the effort is more than the load).
  • The pulley allow us to apply the effort downwards which is a convenient direction.
  • The wheel and axle is a simple machine having a wheel and an axle. The linear motion of axle is obtained by rotating the wheel so as to reduce friction. Example: Steering wheel, screw drivers, water tap etc.
  • An inclined plane is a simple machine which is used to move a load up with a less effort. It is a sloping (or slanting) surface.
  • Less the slope of the inclined plane, less is the effort needed to push a load up.
  • The mechanical advantage of an inclined plane is greater than 1 (i. e. a less effort is required to push a heavy load up an inclined plane).
  • A wedge is a sharp edge formed by joining the two inclined planes together. Example: nail, knife, axe, plough etc.
  • A screw is a modified form of an inclined plane.
  • A screwjack is a simple machine having a combination of a screw and a lever. It is used to lift the heavy vehicles such as cars, trucks, buses etc.
  • Machines are used for our convenience. Therefore, we should take proper care of a machine by painting the machine parts to avoid rusting, lubricating its parts to reduce friction etc. This increases the life span of the machine.

Test yourself

A. Objective Questions

1. State whether the following statements are True or False.

(a) A boy does work while pushing a wall.
Answer. False

(b) A machine performs work by itself.
Answer. False

(c) In an ideal machine, work done on load is equal to the work done by effort.
Answer. True

(d) All levers are force multipliers.
Answer. False

(e) A pulley changes the direction of force.
Answer. True

(f) An inclined plane always has the mechanical advantage more than 1.
Answer. True

2. Fill in the blanks

(a) The useful work done by an actual machine is always less than the work done on the machine.
(b) In class II levers, the load is in between fulcrum and effort.
(c) The mechanical advantage of class III lever is always less than 1.
(d) A pulley is used to change the direction of effort.
(e) Mechanical advantage of an inclined plane is always greater than 1.

3. Match the following

Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 1

Answer.

Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 2
Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 3

4. Select the correct alternatives

(a) For an ideal machine, the efficiency is

  1. greater than unity
  2. less than unity
  3. equal to unity
  4. depends on the value of load

(b) Mechanical advantage of a machine is defined as:

  1. Load X Effort
  2. Load / Effort
  3. Load + Effort
  4. Effort / Load

(c) The mechanical advantage of a lever is equal to:

  1. Load arm / Effort arm
  2. Effort arm / Load arm
  3. Load arm + Effort arm
  4. Load arn — Effort arm

(d) A pulley is used because it

  1. has the mechanical advantage greater than one
  2. has 100% efficiency
  3. helps to apply the force in a convenient direction
  4. requires more effort to raise a less load.

(e) Wheel is used with axle because

  1. sliding friction is less than the rolling friction
  2. rolling friction is less than the sliding friction
  3. they work as the inclined plane
  4. They help us to change the direction of force.

B. Short/Long Answer Questions

Question 1.
When is work said to be done by a force ?
Answer:
Work is said to be done when a force moves an obj ect through a distance in its own direction.

Question 2.
What is energy ?
Answer:
Energy: The ability or capacity to do work is called energy.

Question 3.
What do you understand by a machine ?
Answer:
Machine: A machine is a device that allows us to do work with less effort. Machines make our work easier to do. Machines have made our li ves comfortable and faster.

Question 4.
What is the principle on which a machine works ?
Answer:
Principle of a Machine: The work output of a machine is equal to the work input.

Question 5.
State two functions of a machine.
Answer:
Various functions that a machine can perform are:

  1. Changing the direction of applied force — Example: When a flag is hoisted with the help of a pulley.
  2. Changing the magnitude of applied force — Example: Bottle opener multiplies the applied force and much less effort is required to open the cap.
  3. Applying force at a convenient point — Example: In a pair of scissors, the input force is applied at the handle of the scissors which cuts the paper at the other end of the blade.
  4. Changing the speed of an object — Example: While riding a bicycle, force is applied on pedals which multiplies the speed.

Question 6.
Name six simple machines. Give an example of each machine.
Answer:
The Simple Machines and there examples are as follows:

  1. The lever: Examples are a crow bar, claw hammer, a pair of pilers etc.
  2. The Inclined plane: Examples are ramp, staircase, hilly roads etc.
  3. The wedge: Examples are knife, axe, plough, nail etc.
  4. Screw: Examples are A screw.
  5. The wheel and axle: Examples are steering wheel of a car, bicycle pedal etc.
  6. The pulley: Examples are a pulley used in raising a load.

Question 7.
Define the term ‘work input’ and ‘work output’ in relation to a machine.
Answer:
Work input is work done on a machine equal to the effort force times the distance through which the force is applied.
Work output is work that is done by a machine equals resistance force times the distance through which the force applied.
For an ideal machine, the work output is equal to the work input i. e. the efficiency.

Question 8.
Explain the term mechanical advantage of a machine.
Answer:
The mechanical advantage of a machine is the ratio of the load to the effort. In other words
Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 4

Question 9.
Define the term efficiency of a machine.
Answer:
The ratio of the work done by the machine to the work done on the machine is called efficiency of a machine
Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 5
(Work done by a machine is called the output energy and the work done on a machine is called the input energy.)

Question 10.
What is an ideal machine ?
Answer:
A machine is which no part of the work done on the machine is wasted, is called an ideal or perfect machine. Thus, for an ideal machine, the work output is equal to the work input, i.e., the efficiency of an ideal machine is 1 (or 100 per cent).

Question 11.
Can a machine have an efficiency of 100% ? Give a reason to support your answer.
Answer:
Efficiency of a machine is always less than 100% as output energy is always less than the input energy, because some energy is lost to overcome friction.

Question 12.
A machine is 75% efficient’. What do you understand by this statement ?
Answer:
If a machine is 75% efficient, it means that 75% of the work input to the machine is obtained as the useful work output. The remaining 25% of the work input has been lost in overcoming the friction.

Question 13.
What is a lever ?
Answer:
Lever: A lever is a simple rigid bar which is free to move around a point called fulcrum.

Question 14.
Describe three orders of levers giving an example of each. Draw neat diagrams showing the positions of fulcrum, load and effort in each kind of lever.
Answer:
The levers are of three kinds :
Class I levers which have fulcrum in between the load and the effort.
Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 6
Class II levers which have load in between the fulcrum and the effort.
Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 7
Class III levers which has effort in between the fulcrum and the Load
Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 8

Question 15.
What do you mean by the mechanical advantage of a lever ?
Answer:
The mechanical advantage of a lever is equal to the ratio of the effort arm to the load arm. This is also called the principle of a lever.

Question 16.
Which class of lever has the mechanical advantage always more than 1 ? Give an example.
Answer:
The mechanical advantage of class II levers is always more- than 1.
Example – Nut cracker, wheel barrow, bottle opener etc.

Question 17.
Which class of lever has the mechanical advantage always less than 1 ? Give an example.
Answer:
The mechanical advantage of class III levers is always less than 1.
Example: a pair of tongs, sugar tongs, knife, forceps etc.

Question 18.
Give one example of class I lever in each case where the mechanical advantage is

  1. more than 1
  2. equal to 1
  3. less than 1.

Answer:

  1. more than 1: Load arm of pliers
  2. equal to 1: See – saw
  3. less than 1: The load arm of a pair of scissors.

Question 19.
Name the class to which the following levers belong:
Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 9
Answer:
(a) A pair of scissors — Class I lever
(b) a lemon squeezer — Class II lever
(c) a nut cracker — Class II lever
(d) a pair of sugar tongs — Class III lever
(e) a beam balance — Class I lever
(f) an oar rowing a boat — Class I lever
(g) a wheel barrow — Class II lever
(h) a see saw — Class I lever
(i) a pair of pilers — Class I lever
(j) a crow bar — Class I lever

Question 20.
The diagram given below shows the three kinds of levers. Name the class of each lever and give one example of each class.
Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 10
Answer:
Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 11
Examples : The examples of class I levers are : a see saw, a pair of scissors, a pair of pilers, crow bar, common balance, spoon opening the lid of a tin can, handle of a hand pump.
Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 12
Examples : The examples of lever of class II are : nut cracker, wheel barrow, paper cutter, mango, lemon squeezer, bottle opener.
Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 13
Examples: The examples of levers of class III are : a pair of tongs, sugar tongs, knife, forceps,-forearm of a person holding a load, spade for lifting soil or coal.

Question 21.
Draw diagrams to illustrate the positions of fulcrum, load and effort, in each of the following:
(a) a see saw
(b) a beam balance
(c) a nut cracker
(d) a pair of forceps
Answer:
Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 14

Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 15

Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 16

Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 17

Question 22.
How can you increase the mechanical advantage of a lever ?
Answer:
The mechanical advantage of a lever can be increased by increasing the effort arm or reducing the load arm.

Question 23.
How does the friction at the fulcrum affect the mechanical advantage of the lever ?
Answer:
Friction at the fulcrum reduces the mechanical advantage.

Question 24.
State three differences between the three classes of levers.
Answer:
Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 18

Question 25.
What is a pulley ?
Answer:
Pulley: It is a flat circular disc with a groove in its edge and a rope passing through the groove. It is capable of rotating around a fixed point passing through its central axis called axle.

Question 26.
What is the mechanical advantage of an ideal pulley ?
Answer:
In an ideal pulley, the effort applied is equal to the load to be lifted.
i.e. Effort = Load
Mechanical advantage = Load / Effort = 1

Question 27.
The mechanical advantage of an actual pulley is less than 1. Give a reason. What is the justification for using the pulley then ?
Answer:
In an actual pulley due to friction, the mechanical advantage is less than 1 (i.e. the effort is more than the load).
The reason for using the pulley when its mechanical advantage is equal to 1 or less than 1 is that the pulley allows us to apply the effort downwards i.e. in a convenient direction. To raise a load directly upwards is difficult. But with the help of a pulley, the effort can be applied in the downward direction to move the load upwards. One can hang on it to make use of his own weight also in order to apply the effort.

Question 28.
Draw a neat labelled diagram showing a pulley being used to lift a load. How are load and effort related in an ideal situation?
Answer:
To raise a load, the load is attached to one end of the string and the effort is applied at the other end by pulling it is downward direction . as shown in fig.
Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 19

Question 29.
What is an inclined plane? What is its use ? Give two examples where ¡t is used.
Answer:
An inclined plane is a rigid sloping surface over which heavy loads can be raised or lowered to a certain height or depth.
The mechanical advantage of an inclined plane is the ratio of the length of the plank to the vertical height of the load raised. Its value is greater than one. Therefore, an inclined plane acts as a force multiplier. Thus, it can be used to lift heavy loads.
Example : If a heavy box needs to be loaded on a lorry, it is far easier to push it over an inclined plane than to lift it up. Steeper the inclined plane, greater will be the effort required to push up the load.
Sloping ramps, flyovers, roads on hills and staircases are all examples of inclined planes.

Question 30.
What is a screw ? Give two examples.
Answer:
A screw is a simple machine which appears like an inclined plane wound around a rod with a pointed tip.
Examples : ajar lid, a drill.

Question 31.
What is wheel and axle ? Give two examples.
Answer:
The wheel and axle is a simple machine having a wheel and an axle. The linear motion of axle is obtained by rotating the wheel so as to reduce friction. Example: Steering wheel, screw drivers, water tap etc.

Question 32.
How does a wheel help in moving the axle ?
Answer:
Wheel-and-axle arrangement consists of two cylinders of different diameters joined together such that if one is made to rotate, the other also rotates. The axle is a cylindrical rod fixed to the centre of a circular disc-like object called the wheel.
Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 20
This machine acts as a speed multiplier device.
In riding a bicycle, when we apply force on the wheel (by pedal), the fixed axle rotates with it easily. This force that turns the axle produces a much larger movement of the wheel.

Question 33.
What is a wedge ? Give two examples.
Answer:
A wedge is a double inclined plane such that the two sloping surfaces taper to form either a sharp edge or a pointed edge. Examples : A knife, an axe, a chisel.
In some special cases, the number of inclined planes used can be more than two as well. In such cases, the sloping surfaces generally taper to form either a very sharp or a pointed edge to split or pierce materials. Pins, nails and needles are examples of pointed wedges. The front end of a boat is shaped like a wedge so that it can easily cut across the flowing water.
The wedge works on a principle of an inclined plane.

Question 34.
Name the machine to which the following belong :

  1. Beam balance
  2. Lemon crusher
  3. Sugar tongs
  4. Ramp
  5. Door knob
  6. Needle

Answer:

  1. Beam balance — A lever (lever of class I)
  2. Lemon crusher — A lever (lever of class II)
  3. Sugar tongs — A lever (lever of class III)
  4. Ramp — An inclined plane
  5. Door knob — Wheel and axle
  6. Needle — Wedge

Question 35.
What care would you take to increase the life span of a machine which you use ?
Answer:
Taking care of machines: Some of the ways in which machines should be cared for are given below :

  1. Machines should be kept in a clean environment, which is free from dust and moisture.
  2. When not in use, machines should be kept covered to prevent collection of dust on them.
  3. Machines made of iron should be protected from rust by coating them with paint.
  4. The moving parts of a machine should be regularly oiled with a good-quality machine oil to reduce friction and wear and tear. The above care of machines increases their life.

Question 36.
Select the correct statement :
(a) A wheel barrow is a lever of class I.
(b) The efficiency of a machine is always 100%
(c) Friction in moving parts of a machine reduces its efficiency.
(d) No lever has the mechanical advantage greater than 1.
(e) It is easier to lift a load vertically up than to push it along an inclined plane.
(f) A screw is made by two inclined planes placed together.
Answer:
(c) Friction in moving parts of a machine reduces its efficiency.

C. Numericals

Question 1.
In a machine an effort of 10 kgf is applied to lift a load of 100 kgf. What is its mechanical advantage ?
Answer:
Given,
Load = loo kgf
Effort = 10 kgf
Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 21

Question 2.
The mechanical advantage of a machine is 5. How much load it can exert for the effort of 2 kgf ?
Answer:
Given,
Mechanical advantage = 5
Effort 2 kgf
Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 22

Question 3.
The mechanical advantage of a machine is 2. It is used to raise a load of 15 kgf. What effort is needed ?
Answer:
Given,
Mechanical advantage = 2
Load = 15 Kgf
Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 23

Question 4.
A lever of length 100 cm has effort of 15 kgf at a distance of 40 cm from the fulcrum at one end. What load can be applied at its other end ?
Answer:
Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 24

Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 25

Question 5.
In a lever, fulcrum is at one end at a distance of 30 cm from the load and effort is at the other end at a distance of 90 cm from the load. Find :
(a) the length of load arm,
(b) the length of effort arm, and
(c) the mechanical advantage of the lever.
Answer:
Selina Concise Physics Class 6 ICSE Solutions Chapter 4 Simple Machines 26

Selina Concise Biology Class 7 ICSE Solutions – Photosynthesis and Respiration

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Selina Concise Biology Class 7 ICSE Solutions – Photosynthesis and Respiration

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APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 7 Biology. You can download the Selina Concise Biology ICSE Solutions for Class 7 with Free PDF download option. Selina Publishers Concise Biology for Class 7 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

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Selina Concise ICSE Solutions for Class 7 Biology Chapter 4 Photosynthesis and Respiration

Synopsis —

  • The process of preparing or synthesising food using water from the soil and carbon dioxide from the air, together chlorophyll and sunlight is called photosynthesis.
  • Photosynthesis occurs in the mesophyll cells of the leaf. These cells contain numerous plastids called chloroplasts.
  • With in the plastids is present the green pigment called chlorophyll.
  • On the lower surface of the leaf are numerous pores called stomata, which open into small air cavities inside the leaf.
  • Stomata are tiny openings found mainly on the lower surface of leaves. These openings are surrounded by a pair of bean¬shaped cells called guard cells.
  • All plants have a transport system called vascular system. It is composed of two types of tissues called xylem and phloem.
  • There are two end-products of photosynthesis :
    • Glucose (C6H12O6)
    • Oxygen
  • The energy released during respiration is stored as chemical energy in the form of ATP — adenosine tri-phosphate.
  • Aerobic respiration —
    C6 H12O6+6O2  →  6CO2+6H2O+38ATP
    Anaerobic respiration —
    C6H12O6 →  2C2H5OH+2CO2+2ATP
  • During vigorous exercise, the cells respire anaerobically and form lactic acid which accumulate in the muscle cells causing fatigue and pain.
  • Cellular respiration is called internal respiration which takes place in the cells of living organisms.
    Selina Concise Biology Class 7 ICSE Solutions - Photosynthesis and Respiration 1
  • The frog respires through
    1. skinmouth
    2. lining
    3. lungs.
  • The respiratory organs of the fish are gills.
  • Transpiration is the loss of water in the form of water vapour from the aerial parts of the plant.
  • Transpiration occurs through stomata.
  • The factors which affect the rate of transpiration are 
    1. Sunlight
    2. Temperature
    3. Wind
    4. Humidity
  • The significance of transpiration
    •  Maintain the concentration of the sap inside the plant body.
    • Causes cooling effect

Review Questions

Multiple Choice Questions
1. Put a tick (✓ ) against the most appropriate alternative in the following statements.
(i) Carbohydrates are stored by plants in the form of:
(a) Vitamins
(b) Proteins
(c) Fats
(d) Glucose 

(ii) Stomata are present on the surface of:
(a) Leaves
(b) Roots
(c) Stem
(d) Flower petals

(iii) Which one of the following is an end-product of photosynthesis ?
(a) Fructose
(b) Glucose
(c) Cellulose
(d) Lactose

Short Answer Questions

1. Why do leaves generally look green ?
Answer:
The green colour of a leaf is due to the presence of chlorophyll.

2. Which four of the following are needed for photosyn-thesis in a leaf:
(i) Carbon dioxide:
(ii) Oxygen:
(iii) Nitrates :
(iv) Water:
(v) Chlorophyll:
(vi) Soil:
(vii) Light:
Answer:
(i) Carbon dioxide: from air
(iv) Water: from soil
(v) Chlorophyll: contained in leaf
(vii) Light: from sunlight

3. What is the source of energy for photosynthesis ?
Answer:
Light is the ultimate source of energy in photosynthesis because plants take in the sunlight CO2 and H2O and converts it into glucose.

4. Which gas is taken in and which one is given out by the leaf in bright sunlight ?
(i) Taken in :
(ii) Given out:
Answer:
(i) Carbon dioxide
(ii) Oxygen

5. Suppose we compare the leaf with a factory, match the items in Column A with those in Column B.
ColumnA Column B
Selina Concise Biology Class 7 ICSE Solutions - Photosynthesis and Respiration 2

Answer:
Selina Concise Biology Class 7 ICSE Solutions - Photosynthesis and Respiration 3

6. State whether the following statements are True or False:

(i) Green plants prepare their food by using two raw materials, oxygen and water.
False. Green plants prepare their food by using raw materials, CO, chlorophyll and water.
(ii) The chlorophyll enables the plants to use light energy.
 True
(iii) The free oxygen in the atmospheric air is the result of photosynthesis.
 True
(iv) Photosynthesis occurs only in chlorophyll-containing parts of the plant.
True

7. Differentiate between aerobic and anaerobic respiration. Write the overall chemical equations of the two kinds of respiration in plants.
(i) Aerobic:
(ii) Anaerobic:
Answer:
Aerobic respiration

  1. It occurs in the presence of oxygen.
  2. Here there is complete breakdown of glucose releasing carbon-di-oxide, energy. water and energy.
  3. More energy is released (38 ATP).

Anaerobic respiration

  1. It occurs in the absence of oxygen.
  2. There is partial breakdown of glucose into ethyl alohol, carbon-di-oxide and
  3. Lesser energy is released (2 ATP).

(i) Aerobic respiration:
Selina Concise Biology Class 7 ICSE Solutions - Photosynthesis and Respiration 4
(ii) Anaerobic respiration:
Selina Concise Biology Class 7 ICSE Solutions - Photosynthesis and Respiration 5
Question 8.
 Explain how photosynthesis is different from respiration.
Answer:
Photosynthesis

  1. Anabolic process
  2. Food is produced here.
  3. Oxygen is by-product.
  4. Chlorophyll and sunlight are required.
  5. Occur only during daytime.
  6. Occurs in green plants only.

Respiration

  1. Catabolic process
  2. Food is broken down form here to release energy
  3. Carbon-di-oxide is the by-product.
  4. Chlorophyll and sunlight are notrequired.
  5. Occurs all the time
  6. Occurs in all living beings including plants.

Photosynthesis :
Selina Concise Biology Class 7 ICSE Solutions - Photosynthesis and Respiration 6

Respiration:

C6H12O6+6O→6CO2+6H20+energy

Question 9.
 Do the plants respire all day and night or only during the night ?
Answer:
The plants respire only during the night. Plants take in oxygen and give out carbon dioxide. Hence, there is some truth in the belief that one should not sleep under the trees at night.

Question 10.
 What happens to the energy liberated during respiration?
Answer:
The energy liberated during respiration is utilised for carrying out various life processes.
Some of the energy liberated during the breakdown of 03 the glucose molecule, is in the form of heat, but a large part of it is converted into chemical energy called Adenosine Triphosphate (ATP). Any activity inside the cell is carried out by the energy released by these ATP molecules.

Long Answer Questions

Question 1.
In order to carry out photosynthesis, what are the substances that a plant must take in ? Also mention their sources.
Answer:
For preparing food, the plants require the following :

  1. Water (from soil)
  2. Carbon dioxide (from air)
  3. Chlorophyll (contained in the leaf)
  4. Energy (from sunlight)

The process of preparing or synthesising food using water from the soil and carbon dioxide from the air, together chlorophyll and sunlight is called photosynthesis. This entire process is a series of complex chemical reactions Photosynthesis is represented as follows:
Selina Concise Biology Class 7 ICSE Solutions - Photosynthesis and Respiration 7

Question 2. 
What is the role of chlorophyll in photosynthesis ?
Answer:
This pigment is of utmost importance as it absorbs the sunlight which provides energy for the process of photosynthesis occurs inside the chloroplast of the leaf.

Question 3.
Do plants need oxygen ? If so, what is its source ?
Answer:
Yes plants need oxygen, all the free oxygen in the atmospheric air is the result of photosynthesis. No animal can survive without oxygen as it is needed for respiration. Even the plants use the same oxygen in dark for their own respiration.