Selina Concise Chemistry Class 6 ICSE Solutions

Selina Concise Chemistry Class 6 ICSE Solutions – Pure Substances and Mixtures ; Separation of Mixtures

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Selina Concise Chemistry Class 6 ICSE Solutions – Pure Substances and Mixtures ; Separation of Mixtures

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

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.

Selina Class 6 Chemistry ICSE SolutionsPhysicsBiologyMathsGeographyHistory & Civics

Selina Concise ICSE Solutions for Class 6 Chemistry Chapter 5 Pure Substances and Mixtures ; Separation of Mixtures

POINTS TO REMEMBER

  1. Element is made up of atoms of the same kind i.e. atoms of gold are similar in every respect i.e. have same melting point and same boiling pts. same colour, mass and odour.
  2. Molecules of a compound are similar in every respect i.e. same composition taste, colour and odour i.e. compound water is made up of similar molecules.
  3. Pure substances are either elements or compounds have definite set of properties.
  4. Pure substances are required to maintain good health of human beings. Chemists, technologists and scientists need pure substances for manufacture of medicines, chemicals in industry and for scientific purposes.
  5. Milk, air, bronze, sugar and water solution, salt and water solution, salt, iron filings and sand, petrol are all mixtures.
  6. Mixtures, two or more components mixed in any ratio, undergo ho chemical change and retain their individual properties, can be separated by simple physical methods.
  7. Mixture has no formula, no change in volume, no change in mass, energy is neither needed nor produced.
  8. Mixture is impure substance.
  9. Homogeneous mixture : in which constituents are uniformaly distributed throughout its volume, e.g. salt and water solution.
  10. Heterogeneous mixture: The components are not uniformly distributed through its volume and components can be easily seen separately, e.g. water, oil solution.
  11. Milk is emulsion but this mixture cannot be separated. Oh shaking a mixture of mustard oil and water vigorously mixture becomes milky and is called an emulsion after some time water and oil get separated.
  12. Alloys are homogeneous mixtures of metal with metals or metal with non-metals.
  13. Salt and sand can be seperated by solvent extraction method where one of the solid component (salt) is soluble in liquid.
  14. Coagulation or loading : When alum (a chemical) is added to mixture dissolves in water and form clusters with clay and fine dust particles making them heavier, increases the rate of sedimentation.
  15. Centrifugation method is used to separate solids from liquids where mixture is homogeneous. Cream is separated being lighter on churning floats on a liquid (milk).
  16. Loading : The process of adding a chemical substance to help the suspended solid particles in liquid to form a sediment is called loading.

EXERCISE – I

Question 1.
Select homogeneous and heterogeneous mixtures from the following:
Salt solution, petrol and water, sand and charcoal, alcohol and water, air dissolved in water, air, sea water, fruit juices, mist, brass.
Answer:
Homogeneous mixture : Salt solution, alcohol and water, air dissolved in water, sea water, brass.
Heterogeneous mixture: Sand and charcoal, air, fruit juice, mist, petrol and water.

Question 2.
Define the following :

(a) Pure substance
(b) Impure substance
(c) Alloy
(d) Solution
(e) Heterogeneous mixture
(f) Homogeneous mixture

Answer:

(a) Pure substance : “Pure Substance is either element or compound. It contains the same kind of atom or molecules and has a definite set of physical and chemical properties.”
(b) Impure substance : “A substance in which some other substances are also present in smaller or larger amounts is called an impure substance. Mixtures are impure substance.
Example of impure substance is air.
(c) Alloy : “A homogeneous solid mixture of two or more metals or a metal and a non-metal is called an alloy.”
(d) Solution : “The homogeneous mixture of water (or any other solvent) and a substance soluble in it is called a solution.”
(e) Heterogeneous mixture : “A mixture in which the components are not uniformally distributed through its volume and can be easily seen separately is called heterogeneous mixture.”
(f) Homogeneous mixture : “A mixture in which its constituents are uniformly distributed throughout its volume and cannot be seen separately is called a homogenous mixture.”

Question 3.
List four characteristics of a mixture.
Answer:
Four characteristics of a mixture :

  1. Mixture has no fixed composition.
  2. To form a mixture energy is neither produced nor evolved.
  3. Mixture has no fixed melting’point and boiling points.
  4. Mixture retain the properties of its components.
  5. Components of mixtures can be seperated by simple physical methods.

Question 4.
Give reasons :

(a) Why do sugar and water retain their individual properties in a sugar solution ?
(b) Why do petrol and water form a heterogeneous mixture ?
(c) Why sulphur does dissolve when carbon disulphide is added to a mixture of iron and sulphur but not when it is added to iron sulphide ?

Answer:

(a) As sugar solution is a mixture and mixtures has not any specific set of properties. They show the properties of the individual components from which they are formed.
(b) Petrol and water forms a heterogeneous mixture as its constituents can be seen separately and are not uniformly distributed throughout its volume.
(c) Sulphur has the property to dissolve in carbon disulphide whereas iron does not dissolve and retains its individual property. However, On heating Iron and Sulphur, they chemically combined forming Iron Sulphide. In this, Iron and Sulphur particles do not exists separately as such they loose their individual property.

Question 5.
Give two examples for each of the following types of mixture.

(a) solid-solid
(b) solid-liquid
(c) liquid-gas
(d) gas-gas

Answer:
Two examples of :
(a) solid – solid –

  1. sand and sugar
  2. sand and iron filling.

(b) solid – liquid –

  1. salt and water
  2. charcoal and water.

(c) liquid – gas –

  1. coca cola
  2. mist.

(d) gas – gas –

  1. air
  2. helium and hydrogen in air balloon,
  3. perfumes and air.

Question 6.
Name the components present in the following mixtures:

(a) Brass
(b) Duralumin
(c) Tap water
(d) Bronze
(e) Crude petroleum oil .

Answer:

(a) Brass → Copper and Zinc.
(b) Duralumin → Aluminium + Copper with little manganese and magnesium.
(c) Tap water → air, dissolved salts.
(d) Bronze → Copper, Tin and zinc.
(e) Crude petroleum oil → petrol, kerosene, diesel, LPG, mixed with salt, water and earth particles.

Question 7.
State:

(a) Three differences between water and air.
(b) Four differences between compounds and mixtures.

Answer:
(a)
Water :

  1. The components of water are hydrogen and oxygen which are chemically combined in a fixed ratio of 1 : 8 by mass.
  2. The chemical composition of water remains same from whatever source it is obtained.
  3. The properties of water are completely different from the properties of elements from which it is formed i.e. hydrogen and oxygen.
  4. Energy change occurs in the formation of water.
  5. A molecule of water is represented by a definite formula H2O.

Air :

  1. The main components of air are nitrogen, oxygen, carbon -dioxide, water vapour which are not chemically combined.
  2. The composition of air varies from place to place. During rainy season the air becomes humid due to presence of more water vapour. Some impurities like sulphur dioxide, hydrogen sulphide etc. also changes its composition at some places.
  3. The components of air retain their individual properties but not air.
  4. No energy change occurs when components of air are mixed together.
  5. Air cannot be represented by any chemical formula.

(b)
Compound :

  1. A compound is a pure substance.
  2. Compounds are always homogeneous.
  3. A compound has a fixed composition, i.e., it is formed when two or more pure substances chemically combine in a definite ratio by mass.
  4. Formation of a compound involves change in energy.
  5. Compounds have specific set of properties.
  6. Components of compounds can be separated only by complex chemical processes.

Mixture :

  1. A mixture is an impure substance.
  2. Mixtures may be homogeneous or heterogeneous.
  3. A mixture has no fixed composition, i.e., it is formed by mixing two or more substances in any ratio without any chemical reaction.
  4. Formation of a mixture does not involve any change in energy.
  5. Mixtures do not have any specific set of properties.
  6. Components of mixtures can be separated by simple physical methods.

EXERCISE – II

Question 1.
Define:

(a) Filtration
(b) Sublimation
(c) Evaporation
(d) Crystallisation
(e) Miscible liquids
(f) Immiscible liquids

Answer:

(a) Filtration : The process of separating solid particles from liquid by allowing it to pass through a filter paper is called filtration.
(b) Sublimation : The process in which a solid changes directly into its vapours on heating is called sublimation.
(c) Evaporation : Is the process of converting a liquid into its vapours state either by exposing it to air or by heating.
(d) Crystallisation : Evaporation of liquid from a homogeneous liquit-solid mixture and collecting solid in the form of crystals is called
crystallisation.
(e) Miscible liquids: Homogeneous liquid-liquid mixtures are called miscible liquids.
(f) Immiscible liquids : Heterogeneous liquid-liquid mixtures are called immiscible liquids.

Question 2.
Why do we need pure substances?
Answer:
We need pure substances because of the following reasons:

  1. A pure substance has a fixed melting and fixed boiling point.
  2. A pure substance has its characteristic taste, colour and odour.
  3. Pure substances can not be broken further into more simple substances by any physical means.

Question 3.
Give one example for each of the following types of mixtures.

(a) Solid-solid heterogeneous mixture
(b) Solid-liquid heterogenous mixture
(c) Solid-liquid homogeneous mixture

Answer:

(a) Iron and sulphur.
(b) Sand and water, rice and water.
(c) Sugar from its solution in water.

Question 4.
Name the process by which the components of following mixtures can be separated.

(a) Powdered glass and sugar
(b) Chalk powder and iron filings
(c) Chaff and grain
(d) Salt and water
(e) Wheat and sugar
(f) Sand and camphor
(g) Sugar and water

Answer:

(a) FILTRATION : Glass and sugar on dissolving in water and filtering, glass separates out as residue on the filter paper. Filtrate of sugar solution is heated to remove water by evaporation, sugar is collected as crystals.
(b) MAGNETIC SEPERATION : With the help of a magnet, iron filings can be separated leaving behind chalk powder.
(c) WINNOWING : It separates chaff (lighter) from heavier grains in two different heaps.
(d) EVAPORATION : This method is used to separate the components of a homogeneous solid-liquid mixture, like salt from sea water. Sea water is collected in shallow beds and allowed to evaporate in the sun. When all the water is evaporated, salt is left behind.
(e) EVAPORATION : Wheat and sugar are put in water in a beaker. Sugar dissolves and mixture is passed through strainer and separated and dried. Sugar is obtained by evaporating sugar solution.
(f) SUBLIMATION : Camphor sublimes on heating leaving behind sand.
(g) CRYSTALLISATION : Pure sugar is obtained from its solution in water by the process of crystallisation. At first the sugar solution is heated to evaporate Water at a faster speed. When very less of water is left the solution is cooled. On cooling sugar dissolved in it starts separating out in the form of crystals.

Question 5.
Name:

(a) two substances which can sublime
(b) two substances soluble in water
(c) two substances insoluble in water
(d) four substances that can be used as filters.

Answer:

(a) Camphor and Naphthalene
(b) Sugar and salt (NaCl)
(c) Sand and chalk powder.
(d) (i) Filter paper, (ii) A bead of sand, (iii) Charcoal, (iv) A piece of muslin cloth.

Question 6.
Give reasons :

(a) Sand and saw dust cannot be separated by hand picking.
(b) Magnet is used to separate a mixture of iron and sulphur.
(c) Alum is used in purification of river water.

Answer:

(a) Because in hand picking method substances should be large enough in size to be recognized and picked out by hand but sand and saw dust particles are very small in size so they can’t be picked by hand. It can be separated by filtration.
(b) Mixtures of iron and sulphur can be separated by moving a magnet over them. Iron gets attached to the magnet is separated.
(c) Water from a river, pond or lake contains very fine clay particles. To make them settle at a faster rate, a chemical substance called alum in powdered form is added to such mixtures. It dissolves in water and forms clusters with clay and dust particles making them heavier and increasing the rate of sedimentation.

OBJECTIVE TYPE QUESTIONS

1. Fill in the blanks

(a) The substances that make a mixture are called its constituents or components.
(b) Evaporation or crystallisation is a process to separate solids dissolved in liquids.
(c) Mist is a heterogeneous (liquid in gas) mixture of droplets of water and air.
(d) Clay is separated from water by the method of loading and decantation.
(e) When cereals are washed before cooking, water is separated from the cereals by decantation.
(f) Crystallisation is a process to obtain a very pure form of a solid dissolved in a liquid.
(g) Ammonium chloride can be separated from common salt by the method of sublimation.
(h) The solid particles which remain on the filter paper are called residue and the liquid which passes through it is called filtrate.
(i) The process of transferring the clear liquid layer above the solid particles which settle at the bottom of the container is known as decantation.
(j) Filtration is a method used for the separation of an insoluble solid from a solid-liquid mixture.

2. Write “true” or “false” for the following statements 

(a) A pure substance consists of only one kind of atom or molecule.
Answer. True

(b) Common salt is separated from its solution in water by decantation.
Answer. False
Correct : Common salt is separated from its solution in water by evaporation.

(c) Winnowing is a process to remove small stones from grains.
Answer. False
Correct : Winnowing is a process to remove husk from grains.

(d) Jewellery gold is a homogeneous mixture of metals.
Answer. False
Correct : Jewellery gold is a heterogeneous mixture of metals.

(e) Air can be separated from water by filtration.
Answer. False
Correct : Air can be separated from water by heating.

(f) Salt and air dissolved in water add taste to water.
Answer. True

(g) Steel is an alloy of iron and aluminium.
Answer. False
Correct: Steel is an alloy of iron and carbon.

MULTIPLE CHOICE QUESTIONS

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

1. The process of adding a chemical substance to help the suspended solid particles to deposit as sediment fastly is called

  1. loading
  2. sedimentation
  3. decantation
  4. filtration

2. Salt is separated from sea water by

  1. evaporation
  2. sublimation
  3. crystallisation
  4. filtration

3. A mixture of mustard oil and water forms

  1. a compound
  2. a homogeneous mixture
  3. an alloy
  4. a heterogeneous mixture

4. A heterogeneous mixture is

  1. made up of only one kind of atom
  2. made up of only one kind of molecule
  3. made up of different kinds of atoms and molecules.
  4. that looks uniform

5. Example of a homogeneous mixture is

  1. distilled water
  2. tap water
  3. sand and water
  4. sawdust and water

6. A set of mixture is

  1. gold, common salt, water, alloy
  2. alloy, ink, honey, icecream
  3. alloy, mercury, air, sea water
  4. milk, duralumin, brass, silver

7. A gas dissolved in a liquid can be separated by :

  1. filtration
  2. boiling
  3. using magnet
  4. by crystallisation

8. Copper is not a part of the alloy :

  1. brass
  2. bronze
  3. steel
  4. duralumin

9. Which is not a mixture?

  1. sugar solution
  2. tap water
  3. milk
  4. distilled water

10. Give one word name for the following

(a) The solid which is left on the filter paper after filtration residue.
(b) The solid particles which separate out from the solution on slow evaporation crystals.
(c) The solid particles that settles at the bottom of the beaker in a heterogeneous mixture of a solid and a liquid decantation.
(d) The clean liquid which is poured out after sedimentation supernatant liquid.
(e) The technique used to separate the light particles from heavy particles using the flow of wind winnowing.

Selina Concise Chemistry Class 6 ICSE Solutions – Air and Atmosphere

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Selina Concise Chemistry Class 6 ICSE Solutions – Air and Atmosphere

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

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.

Selina Class 6 Chemistry ICSE SolutionsPhysicsBiologyMathsGeographyHistory & Civics

Selina Concise ICSE Solutions for Class 6 Chemistry Chapter 6 Air and Atmosphere

POINTS TO REMEMBER

  1. Air cannot be seen as it is transparent we only feel its presence when it blows.
  2. The thick layer of air around earth upto 320 km above the surface is called atmosphere.
  3. Air is a mixture of various gases, water vapours, which protects us from harmful rays of Sun. It consists of nearly 80% nitrogen, 20% oxygen.
  4. The components of air can be separated by simple physical means. Has no formula.
  5. N2 is inert gas, reduces the effect of O2 present in air, N2 is helpful in plant growth as it provides nitrates used by plant for formation of protein which is important nutrient for animals.
  6. Oxygen is supporter of combustion no burning is possible without O2 and no life is possible without it.
  7. CO2 is must for plant life, plants prepare their food taking CO2 from air. CO2 maintains temp, of earth and without CO2 our planet would be too cold.
  8. Ozone in the upper layers of atmosphere protects the earth from harmful ultraviolet rays of Sun.
  9. Air is useful in many ways : Acts as medium for propagation of sound, birds, insects to fly in air, air ships, aeroplanes to fly only must due to presence of air. Air helps in movement of yachts, parachutes, aircrafts etc. Air is must for animals and plants on earth and air dissolved in water for aquatic life.
  10. Respiration : Is a slow chemical process when O2 present in the inhaled air reacts with the digested food material in the body to release energy, CO2 and water. Sugar + Oxygen -> Carbon dioxide + Water + Energy.
    Breathing : The complete process in which air is inhaled and exhaled is called breathing.
  11. Combustion : Burning of substance in air completely producing CO2, heat and light.
  12. Photosynthesis : Preparation of food by green plants taking CO2 from air, water from soil in presence of sun light.
  13. Oxygen in air remains constant it is not depleted as plants during day time produce lot of O2 in air.
  14. Soil contains air trapped in between its particle and organisms living there breath this oxygen. Due to this earth worms come out of soil during heavy rains as water gets into the pores in the soil.
  15. Common pollutant of air are smoke, dust, coal, emitted by industries and automobiles.
  16. Gases like SO2, NO2 also pollute air as they form corresponding acids when they come in contact with water vapours in air and fall down in the form of acid rain with rain water.
  17. These pollutants reduce the visibility which lead to accidents, cause global warming.

EXERCISE-I

Question 1.
What is atmosphere ?
Answer:
The earth is surrounded by a thick layer of air called the atmosphere that extends upto a height of about 320 kilometres above the surface of the earth.

Question 2.
Why can’t we see air ?
Answer:
We can’t see air because it is colourless, odourless and transparent gaseous matter.

Question 3.
What is wind ?
Answer:
Fast moving air is called wind.

Question 4.
What would have happened if there would have been no atmosphere around the earth ?
Answer:
Without atmosphere life would not be possible as atmosphere protects us from harmful gases. We could not live without
air present in atmosphere. In absence of the atmopshere, the earth would get so cold at night that we would not be able to survive. No CO2 and N2 for plants without atmosphere.
P.Q. When water is heated, we see bubbles rising up. Why ?
Answer:
Water has a lot of air dissolved on it. This is what allows breathing to fishes and other aquatic beings. The solubility of gases decreases when the temperature is raised, and that is why the dissolved air bubbles out from the water.

Question 5.
Why is air called a mixture ? Give five facts in support of your answer.
Answer:
Air is a mixture because :

  1. Air has no formula, a mixture has no formula whereas compound has a formula.
  2. No energy changes are involved to form air from various gases.
  3. When air is formed out of its constituents no change in mass and no change in volume takes place.
  4. Properties of air vary from place to place and time to time. i.e. there is more CO2 in towns as compared to villages where more oxygen prevails as compared to towns.
  5. Components of air can be separated by simple physical methods.

Question 6.
What are the main components of air ? Write down the composition of three main gases present in air by volume.
Answer:
Main components of air are :
Selina Concise Chemistry Class 6 ICSE Solutions Chapter 6 Air and Atmosphere 1
Selina Concise Chemistry Class 6 ICSE Solutions Chapter 6 Air and Atmosphere 2

Question 7.
What do you observe when

(a) Ice cold water is filled in a glass tumbler.
(b) A burning candle is covered with an inverted jar.
(c) Carbon dioxide gas is passed through lime water.
(d) A beam of light is allowed to enter in a closed dark room through a small hole.

Answer:

(a) We will observe that fine water droplets get deposited on the outer wall of the glass tumbler.
(b) The candle burn more brightly because candle gets oxygen support in burning.
(c) When CO2 gas is passed through lime water it forms calcium carbonate which is white precipitate (turbidity). This gives the milky white appearance to the solution.
(d) We will observe randomly moving dust particles in the beam of light. This confirms the presence of dust particles in the air.

Question 8.
Write the chemical name of –

(a) Lime water
(b) The white insoluble solid formed on reaction of carbon dioxide with lime water.

Answer:

(a) Calcium hydroxide Ca(OH)2
(b) Calcium carbonate – CaCO3

These droplets were present in air as moisture which got condensed on the cooler walls of glass tumbler. This shows the presence of water vapours in air.

EXERCISE-II

Question 1.
Name two important processes supported by oxygen present in air.
Answer:
Two processes supported by oxygen present in air are :

  1. Combustion
  2. Respiration

Question 2.
Give two uses of the following components present in air:

(a) oxygen
(b) nitrogen
(c) carbon dioxide
(d) water vapour

Answer:

(a) Oxygen: For respiration and breathing no life is possible without oxygen.
Oxygen is the most vital component which is responsible for two most important processes,
(i) Respiration and
(ii) Combustion.
(b) Nitrogen : Present in air gets fixed up in the soil as nitrates used by plants for their growth and for the formation of protein an important nutrient for animals. It is used to make fertilizers and various nitrogenous products.
(c) Carbon dioxide : Carbon dioxide is used by plants to prepare their food by photosynthesis. Dry ice (solid carbon-dioxide) is used as a refrigerant. Carbon dioxide is used to prepare fizzy drinks like soda water.
(d) Water vapour : Water vapors present in air provide moisture for both plants and animals. It also helps in predicting climatic conditions of a particular area as its amount varies from place to place and time to time.

Question 3.
Define the following:

(a) Respiration
(b) Photosynthesis
(c) Combustion.

Answer:

(a) Respiration : Respiration is a chemical process that takes place in all living beings (slow in plants). In this process, oxygen present in inhaled air reacts with the digested food material in the body. This results in release of energy, carbon-dioxide and water.
Sugar + Oxygen → CO2 + Water + Energy.
(b) Photosynthesis : Process of producing food by green plants taking CO2 from air, water from soil by roots and in presence of sun light and chlorophyll is called photosynthesis.
(c) Combustion : Combustion also called burning “is burning of substance in oxygen of air completely producing heat and light is called combustion.”

Question 4.
What are fuels ? Give two examples of modern fuels.
Answer:
Fuels : The substances which burn in air to produce large amount of energy in the form of heat and light are called fuels.
Examples :

1. CNG (Compressed natural gas)
2. LPG (Liquefied natural gas)

Question 5.
Give reasons :

(a) Aquatic animals and plants are able to survive in matter.
(b) A burning candle stops burning if covered with a glass tumbler.
(c) Mountaineers and divers carry oxygen cylinders with them.
(d) When water is heated, we see bubbles rising up.

Answer:

(a) Aquatic animals and plants use oxygen dissolved in water and survive.
(b) Oxygen is necessary for combustion when covered with a glass tumbler supply of oxygen stops.
(c) For artificial respiration mountaineers carry oxygen cylinders as at high altitudes, the air is thin and breathing becomes difficult. Also divers carry oxygen cylinder for artificial respiration as there is less oxygen dissolved in water (less dense) and breathing becomes difficult.
(d) These bubbles come from the air dissolved in water. The marine life uses air dissolved in water.

Question 6.
Name the processes which maintain the balance between oxygen and carbon dioxide in the air. How is it done?
Answer:
Respiration and combustion are the processes which maintain the balance between oxygen and CO2 in the air.

  1. Respiration : Respiration is a chemical process that takes place in all living beings. In this process, oxygen present in the inhaled air reacts with the digested food material in the body. This results in the release of energy, carbon dioxide and water.
  2. Combustion : Burning or combustion is a process in which a substance reacts chemically with oxygen and gets oxidised, with the release of energy in the form of heat and light. It is a fast process. During the process of burning, along with energy, carbon dioxide and water vapour are also produced.

Question 7.
State two similarities and two differences between respiration and burning.
Answer:
Similarities:
Burning

  1. Oxygen is needed to combine with carbon and hydrogen in compound.
  2. CO2 and H2O are formed with release of energy.

Respiration

  1. Oxygen is needed tocombine with C and H2 of food.
  2. CO2 and H2O are formed with release of energy.

Differences :
Burning

  1. It occurs at higher temperature.
  2. Is fast process.
  3. A natural and continous process.

Respiration

  1. It occurs at body temperature.
  2. Is slow process.
  3. An artificial & discont inuous process.

Question 8.
Define rusting ? What are the two necessary conditions for rusting of iron. Give the chemical name of rust.
Answer:
Rusting : Slow conversion of iron into its hydrated oxide in the presence of moisture and air is called rusting.
Conditions for rusting:

  1. Presence of moisture (water).
  2. Presence of oxygen (air).

Chemical name of rust is hydrated iron oxide [Fe20rxH20]

Question 9.
How is air useful to :

(a) water boats
(b) agriculture
(c) windmills
(d) scooters and cars.

Answer:

(a) Air helps movement of water boats.
(b) Air speeds up drying up of agricultural products like grains, pulses fruits etc. Air helps in pollination of flowers and dispersel of seeds.
(c) Windmills work where there is sufficient movement of air.
(d) Air filled tyres of cars move smoothly on road as there is less friction.

Question 10.
State the full form of LPG and CNG ? How are the two different in their composition ?
Answer:
LPG (Liquefied Petroleum Gas): It is obtained from crude petroleum oil. It mainly contains gaseous compounds known as isobutane and butane. Popularly it is known as cooking gas. It is the best fuel for domestic purposes and in laboratories. It is available in cylinders. It is also supplied through pipes in big cities.
CNG (Compressed Natural Gas): It is produced along with crude oil. It mainly contains methane gas. It has become a popular fuel for vehicles like three wheeler scooters, cars and buses. It is a cheap fuel as well as pollution free. It is used as a substitute of petrol.
Difference in composition
LPG is obtained from crude petroleum oil. It mainly contains gaseous compounds known as isobutane and butane. While
CNG is produced along with crude oil. It mainly contains methane gas.

Question 11.

(a) Why is nitrogen important to all living beings ?
(b) What is nitrogen fixation ?

Answer:

(a) Nitrogen constitutes 78% of air by volume. It is of vital importance to the plants, animals and human beings as it is needed to prepare vital nutrient ‘protein’ to every living being which is necessary for their growth.
(b) Nitrogen cannot be absorbed directly by plants. It is first fixed up in the soil as nitrites and nitrates and then absorbed by the plants in soluble forms. This phenomenon is called nitrogen fixation.

EXERCISE-III

Question 1.
What is air pollution ?
Answer:
Air Pollution : “Mixing of UNWANTED and HARMFUL SUBSTANCES in air is called AIR POLLUTION.”

Question 2.
Mention five causes of air pollution.
Answer:
CAUSES OF AIR POLLUTION ARE:

  1. Burning of FOSSIL FUELS and FIBRES.
  2. Cutting of forests.
  3. Erruption of volcanoes.
  4. Increase in POPULATION.
  5. Agricultural activities like use of chemical fertilisers, insecticides, pesticides and burning of husks etc.

Question 3.
Name two air pollutants which

(a) affect our health
(b) cause acid rain
(c) cause global warming.

Answer:

(a) Two air pollutants that affect our health are :

  1. DUST
  2. SMOKE

(b) Two air pollutants that cause acid rain are :

  1. SULPHUR DIOXIDE (SO2) and
  2. NITROGEN DIOXIDE (NO2)

(c) Two air pollutants that cause global warming are :

  1. CARBON DIOXIDE (CO2)
  2. METHANE (CH4)

Question 4.
What is meant by ozone depletion?
Answer:
Ozone is present in the upper layer of atomosphere called stratosphere. It protects the earth from harmful ultraviolet rays of the sun.
Some air pollutants like chlorofluorocarbon (CFC) react with ozone present in the atmosphere. These pollutants reduce the density of the air. The ultra-violet rays from the sun reach directly on the earth which affect human health, causing skin diseases and cancer and also increase the earth’s temperature.

Question 5.
State four steps to be taken to control air pollution.
Answer:
Four steps taken to control air pollution :

  1. Tall chimneys be installed in factories and power houses so that smoke and gases rise high and get diluted.
  2. GROW more tall trees to absorb CO2 Also plants help in bringing rains.
  3. Use unleaded petrol and CNG in all public transport vehicles
  4. Industries should be located far away from residential areas.

Question 6.
Name three greenhouse gases.
Answer:
Carbon dioxide, methane, nitrous oxide, etc.

OBJECTIVE TYPE QUESTIONS

1. Fill in the blanks

(a) The layer of air around the earth is called the atmosphere.
(b) Although we cannot see air, we can feel it.
(c) Air is a mixture of gases.
(d) Plants and animals maintain the balance of carbon- dioxide and oxygen in air.
(e) Polluted air is harmful for health.
(f) The supporter of combustion in air is oxygen.
(g) Green plants need CO2 water and light to prepare food.
(h) Oxygen is used in burning the food to get energy.
(i) Aquatic plants and animals use dissolved oxygen in water.
(j) The reddish brown powder on the surface of iron nails exposed to air and moisture is called rust.

2. Write ‘true’ or ‘false for the following

(a) Air is a compound.
Answer. False
Correct: Air is a mixture.

(b) Carbon dioxide is given out during photosynthesis.
Answer. False
Correct : Oxygen is given out during photosynthesis.

(c) Respiration needs nitrogen.
Answer. False
Correct: Respiration needs oxygen.

(d) The composition of air was discovered by Lavoisier.
Answer. True

(e) The major component of air is oxygen.
Answer. False
Correct: The major component of air is nitrogen.

MULTIPLE CHOICE QUESTIONS

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

1. Air consists of

  1. only oxygen
  2. only nitrogen
  3. only carbon dioxide
  4. all of these

2. Air pollution is due to the

  1. cutting of green plants
  2. gases like carbon monoxide, sulphur dioxide etc.
  3. smoke given out by factories
  4. all of the above

3. The gases which cause acid rain are

  1. sulphur dioxide and oxygen
  2. nitrogen and oxygen
  3. carbon dioxide and water vapour
  4. nitrogen dioxide and sulphur dioxide

4. Rust is

  1. hydrated iron oxide
  2. hydrated copper sulphate
  3. anhydrous iron oxide
  4. none of the above

5. Photosynthesis is a process in which plants

  1. take in oxygen and give out carbon dioxide
  2. take in carbondioxide and give out oxygen
  3. take in nitrogen and give out oxygen .
  4. none of the above.

6. Fuels which do not leave any residue on burning are

  1. coal and wood
  2. coal and LPG
  3. wood and CNG
  4. LPG and CNG

7. Respiration

  1. is a slow process
  2. is a natural and continuous process
  3. takes place at body temperature
  4. all of the above

8. Which of the following is common in combustion and respiration

  1. oxygen
  2. release of heat and light
  3. natural process
  4. nitrogen

9. Which of the following is not a green house gas ?

  1. carbondioxide
  2. sulphur dioxide
  3. methane
  4. nitrogen

10. The substance which accelerates the speed of a reaction without itself undergoing any change is called

  1. catalyst
  2. pollutant
  3. fuel
  4. none of the above.