New Simplified Chemistry Class 8 ICSE Solutions – Matter

New Simplified Chemistry Class 8 ICSE Solutions – Matter

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

Simplified ChemistryChemistryPhysicsBiologyMathsGeographyHistory & Civics

EXERCISE

Question 1.
Explain the meaning of the term ‘matter’.
Answer:
Matter is defined as anything that has mass, occupies space and can be felt by the senses.

Question 2.
Matter in any state is composed of small particles – molecules, atoms or ions. Differentiate the terms above in Italics.
Answer:
Molecules : A molecule is the smallest particle of a substance that can normally exist separately and retain the characteristics of the substance, e.g. O2, H2, Cl2 are molecules.
Atoms : An atom is the smallest particle of an element which can take part in a reaction and is considered the basic unit of matter, e.g. O, H, Cl are atoms.
Ions : An ion is any atom or a group of atoms which has a resultant charge due to loss or gain of electrons, e.g. O2-, H1+, Cl1-– are ions.

Question 3.
Differentiate between the-two characteristics of matter – ‘mass’ & ‘weight’.
Answer:
Mass :

  1. The amount of matter in a body.
  2. It is measured in Kilograms (kg).

Weight :

  1. Due to the pull of gravity on a body.
  2. It is measured in Newtons (N)

Question 4.
State which of the three states of matter Le. solids, liquids or gases – have

(a) No definite volume
(b) A definite shape
(c) High density
(d) No free surfaces
(e) Particles – which diffuse very easily.

Answer:

(a) Gases
(b) Solids
(c) Solids
(d) Gases
(e) Gases

Question 5.
State the main postulates of the kinetic theory with special reference to –

(a) Inter-particle space
(b) Inter-particle attraction
(c) Energy possessed by particles of matter.

Answer:

(a) Inter-particle space : The particles are arranged in a way such that they have spaces existing between them. i.e. inter molecular space.
(b) Inter-particle attraction : All particles attract each other with a force which is maximum if the particles are close to each other and minimum if the particles are at a distance from each other i.e. decreases with increase distance and vice-versa.
(c) Energy possessed by particles of matter : Since particles are in continuous motion, they, possess energy called kinetic energy. Application of heat increases kinetic energy of particles – which then move more randomly.

Question 6.
State in which of the following examples ie. a piece of wood, water, a light gas is the

(a) Inter-particle space maximum
(b) Inter-particle attraction maximum
(c) Energy possessed by particles of matter, very large.

Answer:

(a) A light gas
(b) a piece of wood
(c) a light gas

Question 7.
In which of the three states of matter – solids, liquids or gases is the movement of atoms about their own position. Give a reason for the same.
Answer:
Movement of atoms[molecules] in solids are about their own position.

New Simplified Chemistry Class 8 ICSE Solutions Chapter 1 Matter 1

This is because of Minimum Inter Particle space and very strong inter – particle attraction among atoms.

Question 8.
‘Inter-particle attraction between atoms of gases is very weak’. State five properties of gases which correlate as a consequence of the weak inter-particle attraction between particles of gases.
Answer:
Gases are

  1. least rigid
  2. highly compressible

Inter-Particles attraction between atoms of gases is very weak. The five properties of gases which correlate as a consequence of the weak inter-particle attraction between particles of gases are :

  • They have no definite volume.
  • They have no definite shape.
  • They have no free surface.

Gases Rapidly diffuse with other gas particles,

New Simplified Chemistry Class 8 ICSE Solutions Chapter 1 Matter 2

Question 9.
What is inter-conversion of matter. Give the meaning of the terms involved in inter-conversion of matter

(a) Melting
(b) Vaporisation
(c) Liquefaction or condensation,
(d) Solidification or freezing
(e) Sublimation.

Answer:
INTERCONVERSION OF MATTER :
“Change of state of matter from one state to another state and back to its original state is called inter-conversion of matter.”

(a) MELTING : Conversion of a solid into a liquid on heating e.g. ice to water.
(b) VAPORISATION : Conversion of a – liquid into vapours (or gas) on heating e.g. water to water vapour.
(c) LIQUEFACTION or CONDENSATION : Conversion of VAPOUR (or gas) into a LIQUID on cooling e.g. water vapour to water.
(d) SOLIDIFICATION OR FREEZING : Conversion of a LIQUID into a SOLID e.g. water to ice.
(e) SUBLIMATION : Conversion of a — SOLID directly in GASEOUS (vapour) state and on cooling directly back to SOLID e.g. Naphthalene sublimes at room temperature.

Question 10.
Draw a labelled schematic diagram representing the terms (a) to (e) involved in the inter-conversion of matter.
Answer:
(a) Melting

New Simplified Chemistry Class 8 ICSE Solutions Chapter 1 Matter 3
(b) Vaporisation

New Simplified Chemistry Class 8 ICSE Solutions Chapter 1 Matter 4
(c) Liquefaction or condensation

New Simplified Chemistry Class 8 ICSE Solutions Chapter 1 Matter 5
(d) Solidification or freezing

New Simplified Chemistry Class 8 ICSE Solutions Chapter 1 Matter 6
(e) Sublimation

New Simplified Chemistry Class 8 ICSE Solutions Chapter 1 Matter 7

Question 11.
With reference to inter-conversion of matter – on the basis of kinetic theory – explain in brief the conversion of :

(a) A solid into a liquid
(b) A liquid into vapour [or gas]
(c) Vapour [or gas] into a liquid
(d) A liquid into a solid

With special reference to inter-particle space & inter particle attraction at the different stages of conversion.
Answer:
(a) A solid into a liquid : Kinetic energy increases

  1. Heat energy is absorbed by the molecules and stored in the form of potential energy.
  2. Inter-particle space-increases.
  3. Inter-particle attraction-decreases.
  4. Collision-increases and push apart.
    Solid changes into liquid.

(b) A liquid into vapour [or gas] :

  1. Heat energy absorbed -stored as potential energy.
  2. Inter-particle space-increases.
  3. Inter-particle attraction-decreases (almost negligible).
  4. Collision-increase and particles push apart become free and escapes gas.
    Liquid changes into vapours.

(c) Vapour [or gas] into a liquid :

  1. Potential energy released in the form of heat energy.
  2. Inter-particle space-decreases considerably.
  3. Inter-particle attraction-increases.
  4. Collision-decrease to negligible particles are not in a position to be free.
    Vapours changes into liquid state.

(d) A liquid into a solid :

  1. Potential energy (stored) is-released.
  2. Inter-particle space-decreases.
  3. Inter-particle attraction-increases. Particles are not in position to be free.
  4. Collision-decreases.
    Liquid changes into solid.

Question 12.
On the basis of kinetic theory explain why, ammonium chloride sublims and goes from solid state directly into vapour state.
Answer:
Ammonium Chloride sublimes and goes from solid state directly into vapour state because if low inter-particle attraction.
Sublimable solids have very low force of attraction between their particles.

Question 13.
State the ‘law of conservation of mass’. State the main points of Landolt’s experiment for experimental evidence of the law.
Answer:
Law of Conservation of Mass : “In any chemical reaction, the total mass of the reacting substances is equal to the total mass of the products of the reaction provided masses are measured under similar conditions.”
Landolt’s experiment : To illustrate the law of conservation of mass. Two solutions – NaCl in limb A and AgNO3 sol. in limb B were taken in U-shaped tube and weighed.

New Simplified Chemistry Class 8 ICSE Solutions Chapter 1 Matter 8
Now tube was tilted so that two solutions get mixed and react with each other to form new products. Tube is weighed again
Weight after reaction was found to be same as before reaction.

New Simplified Chemistry Class 8 ICSE Solutions Chapter 1 Matter 9
Total mass of reactants = Total mass of products This verifies the law.
Two other examples are :

  1. We can take two solutions of lead acetate Pb (CH300)2 in limb A and sodium sulphate Na2So4 in limb B in the U-shaped tube.
    Pb(CH3COO)2 + Na3So4 → PbSo4 + 2CH3COONa
  2. We can also take two solutions of iron [II] sulphate FeSo4 in limb A and silver sulphate Ag2So4 in limb B in U- shaped tube still the result is found to be same.
    2FeSo4 + Ag2So4 → 2Ag ↓ + Fe2(So4)3

OBJECTIVE TYPE QUESTIONS

Q.1. Select the correct answer from A, B, C, D & E for each statement given below:
A: Solid
B: Vaporization
C: Ion
D: Gases
E: Heat

Question 1.
An atom or group of atoms – having a resultant charge.
Answer:
C : Ion

Question 2.
The state of matter which has – least density & no free surfaces.
Answer:
D : Gases

Question 3.
In Landolt’s experiment – the form in which the chemical energy stored up in the reactants – is released.
Answer:
E : Heat

Question 4.
The process of change of a liquid into vapour, [gas] on heating.
Answer:
B : Vaporization

Question 5.
The state of matter, where the inter-particle attraction between particles is maximum.
Answer:
A : Solid

Q.2. With reference to the liquid state of matter – answer the following

Question 1.
On heating the liquid, do the particles gain or lose energy.
Answer:
Gain energy.

Question 2.
The heat energy supplied to the liquid is absorbed by its molecule & stored as which form of energy.
Answer:
Potential energy.

Question 3.
How does the stored energy, have effect on inter-particles space.
Answer:
Stored potential energy increases the space.

Question 4.
State why the inter-particles attraction decreases to negligible.
Answer:
This is so because when distance between particles increases, and force of attraction decreases leading to Inter-particle attraction to almost negligible.

Question 5.
At what point will the particles become free and escape as gas.
Answer:
At boiling point.

Q.3. Complete the statement by filling the blanks with the correct word from the brackets.

Question 1.
In naphthalene, the inter-particle attraction is __ [high/low].
Answer:
In naphthalene, the inter-particle attraction is low.

Question 2.
The law of conservation of mass, is strictly valid if mass and __ [energy/volume] are considered together.
Answer:
The law of conservation of mass, is strictly valid if mass and energy are considered together.

Question 3.
When inter-particle space increases, the inter-particle attractive force __ [decreases/increases].
Answer:
When inter-particle space increases, the inter-particle attractive force decreases.

Question 4.
Kinetic energy of molecules in helium is __ [large / very large] compared to the kinetic energy of molecules in water.
Answer:
Kinetic energy of molecules in helium is large compared to the kinetic energy of molecules in water.

Question 5.
Conversion of vapour [or gas] into a liquid is termed __ [liquefaction/vaporization].
Answer:
Conversion of vapour [or gas] into a liquid is termed liquefaction.

Q.4. Give reasons for the following 

Question 1.
Particles of matter possess energy called kinetic energy.
Answer:
Particles of matter are contiunously moving, that is, they possesses kinetic energy.

Question 2.
Solids cannot be compressed, but gases are highly compressible.
Answer:

  • In solids : Particles are closely packed. Inter-particle space is minimum and hence cannot be compressed.
  • In gases : Particles are least compact and inter-particle spaces are maximum. Hence are highly compressible.

Question 3.
Kinetic energy of molecules of gases is very large & of solids, the least.
Answer:
The Kinetic Energy of molecules of gases is very large because :

  1. The inter-particle space between the particles is maximum.
  2. The Inter-particle attraction between the particles is negligible. Whereas The kinetic energy of molecules of solids is least

because :

  1. The Inter-particle space between the particles is maximum.
  2. The Inter-particle attraction between the particles is very strong.

Question 4.
On heating a sublimable solid, the molecules break free & escape from surface of the solid directly into vapour.
Answer:
On heating a sublimable solid the inter-particle attraction is overcome and the molecules break-free and escapes from the surface of the solid directly into the vapour.

Question 5.
Particles of matter move more rapidly on application of heat.
Answer:
Application of force increases the K.E. of particles and space increases and inter-particle attraction decreases. Thus particles
move rapidly.

Q.5. Complete the blanks with reference to interconversion of matter on basis of kinetic theory-with the word ‘increase’, ‘decreases’, ‘gain’, ‘lose’ or ‘overcome’ in each case.

  1. During melting of solids, the inter-particles space increases.
  2. During vaporization, the liquid particles gain energy.
  3. During liquefaction, the particles lose energy.
  4. During soldification, the inter-particle space decreases gently.
  5. During sublimation the inter-particle attraction is overcome.

New Simplified Chemistry Class 8 ICSE Solutions – Physical & Chemical Changes

New Simplified Chemistry Class 8 ICSE Solutions – Physical & Chemical Changes

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

Simplified ChemistryChemistryPhysicsBiologyMathsGeographyHistory & Civics

EXERCISE

Question 1.
Change – is the law of nature which occurs in everyday life.
State when a substance [Le. matter] undergoes a kind of change.
Answer:
Change is the law of nature which occurs in everyday life. A substance [i.e. matter] undergoes a kind of change when- subjected to energy changes.
Matter is said to undergo certain changes when energy is added to the matter or removed from the matter.
Examples of Changes are :

  1. Ice melts on supplying heat to it.
  2. Milk changes into curd when a little curd is added to hot milk.

Question 2.
Give a reason why

(a) Freezing of water is a reversible change while burning of a candle – an irreversible change,
(b) Change of seasons is a periodic change while change of weather is a non-periodic change.

Answer:

(a) Freezing of water is reversible change as on heating frozen ice changes to water by heating.
Burning of a candle is irreversible change as on reversing the conditions we can not get candle back out of CO2, water vapours and wax vapours.
(b) Change of season is periodic change as it occurs at regular intervals. Change of weather is non-periodic change because, it does not occur periodically at regular interval. Weather can change suddenly and it may rain in the evening.

Question 3.
State which of the following pertain to – chemical changes.

(a) Substance retains its identity
(b) The change is permanent
(c) No new substance is formed
(d) Is easily reversible
(e) Produces one or more new substances by change in its composition
(f) The change is temporary
(g) The composition and properties of the original substance are not altered.

Answer:
Chemical change :

(b) The change is permanent.
(e) Produces one or more new substances by change in its composition.
(g) The composition and properties of the original substance care not altered.

Question 4.
Give four reasons why burning of a magnesium ribbon in air is considered a chemical change.
Answer:
BURNING OF MAGNESIUM RIBBON is chemical change as : It is

  1. Irreversible change
  2. New product with new properties is formed (MgO)
  3. It is permanent change
  4. Composition is changed
  5. Energy is required to bring the change.

Question 5.
Compare the energy changes involved during a physical and a chemical change.
Answer:

  • Physical change : No gain or loss of energy during the physical change.
    Energy required for completion of a physical change is released when the change is reversed.
  • Chemical change : Energy is either absorbed or evolved during the chemical change.
    Energy in the form of light, percussion (pressure) or generally heat energy is required for a chemical change.

Question 6.
Is change of state of matter – a physical or a chemical change. Give reasons.
Answer:
Change of state of matter is a PHYSICAL change since on reversing the conditions, original state can be attained.

Question 7.
State which of the following terms connected with change of state of matter ie.

(a) Melting
(b) Vaporisation
(c) Condensation
(d) Freezing

pertain to –

  1. Conversion of a liquid into a vapour or gas
  2. Conversion of a liquid into a solid.
  3. Conversion of a solid into a liquid on heating.

Answer:

  1. (b) Vaporisation
  2. (d) Freezing
  3. (a) Melting

Question 8.
Potassium chloride is added to water and stirred. A salt solution is obtained which is then boiled leaving behind a residue.

(a) Is the above change physical or chemical
(b) Name the residue which remains behind after the salt solution is boiled.
(c) Is the change reversible or not reversible.
(d) Are the composition and properties of the original substance altered.
(e) Give a reason why the above experimentation would not be possible, if calcium carbonate is taken, in place of potassium chloride.

Answer:

(a) Physical change.
(b) White residue of potassium chloride.
(c) Change is reversible.
(d) No, the composition and properties are not altered.
(e) When in place of KC1, we take calcium carbonate the above experiment cannot be possible as no new substance with new properties is produced, since CaCo3 is insoluble in water. Boiling will also not work as CaCo3 is in solid form in water.

Question 9.
Give a reason why –

(a) Heating a platinum wire is a reversible change but, heating a magnesium wire is an irreversible change.
(b) Addition of zinc nitrate to water is a physical change but, addition of zinc to dilute nitric acid is a chemical change.
(c) Rusting of iron is a chemical change but, magnetization of iron is a physical change.

Answer:

(a) Heating a platinum wire, final product is again platinum wire (as platinum is noble metal and does not react with air), i.e. no change in composition and is physical change. Heating magnesium wire forms MgO i.e. composition changes a new substance white powder of magnesium oxide is formed is chemical change.
(b) Zinc nitrate (soluble in water) disolves in water and is reversible. No change in composition takes place is a physical change where as zinc reacts with dil. nitfic acid to produce H2 gas which gets oxidized to water by nitric acid (oxidizing agent) and hence is a chemical change.
(c) Rusting of iron is a chemical change as composition changes, reddish brown flaky powder is formed with new properties and is irreversible, permanent change but magnetization of iron is temporary and reversible change with no change in composition is a physical change.

Question 10.
Select the chemical changes from the following list of changes

(a) Vaporisation of water into water vapour.
(b) Boiling of milk.
(c) Respiration in mammals.
(d) Rotting of eggs.
(e) Drying of a fruit
(f) The carbon cycle.
(g) Occurrence of lightening.
(h) Breaking of glass.
(i) Butter turning rancid.
(j) Glowing of an electric bulb.
(k) Crystallisation of a salt from its solution.
(l) Change of seasons.
(m) Preparation of carbon dioxide from calcium carbonate & dilute hydrochloric acid.

Answer:

(a) Change of water into water vapours is a PHYSICAL change as no change in composition, only state changes, is reversible change on cooling vapour water is formed.
(b) Boiling milk is PHYSICAL change as no change in composition, temporary and reversible, no new substance with new properties is formed.
(c) Respiration in mammals is a CHEMICAL change, we breathe in oxygen and breathe out carbon dioxide. It is irreversible and composition changes.
(d) Rotting of eggs is CHEMICAL change, change in composition takes place, is irreversible process.
(e) Drying of a fruit is PHYSICAL change as composition remains same and only state changes, no new substance with new properties is produced.
(f) Carbon cycle is a PHYSICAL change as carbon remains as carbon, mass of carbon remains same in the cycle, is temporary and only change in state takes place.
(g) Occurance of lightening is a CHEMICAL change, energy is produced, new substance with new properties is produced.
(h) Breaking of glass is a PHYSICAL change as no change in composition takes place, no new substance is formed.
(i) Butter turning rancid is a CHEMICAL change as composition changes, new substance with new properties is formed, irreversible and permanent change.
(j) Glowing of an electric bulb is a PHYSICAL change as no change in composition of filament takes place is reversible, only filament glows when electricity is passed and returns to its original form on switching off the electric current.
(k) Crystallisation of a salt from its solution is a PHYSICAL change as only state changes (crystals are formed), composition does not change, is reversible, no change in mass takes place.
(l) Change of season is a PHYSICAL change, only change in temperature is seen and is reversible change.
(m) Preparation of carbondioxide from calcium carbonate and dilute hydrochloric acid is a CHEMICAL change as new K substance with new properties is formed, the change is permanent and irreversible.

OBJECTIVE TYPE QUESTIONS

Q.1. Select the correct answer from the choice in bracket to complete each sentence

Question 1.
A change in which a substance retains its identity is a __ [physical/chemical] change.
Answer:
A change in which a substance retains its identity is a physical change.

Question 2.
Matter is added or removed during a __ [chemical/ physical] change.
Answer:
Matter is added or removed during a chemical change.

Question 3.
Energy required for completion of a physical change is __ [released/absorbed] when the change is reversed.
Answer:
Energy required for completion of a physical change is released when the change is reversed.

Question 4.
Dissolution of lead nitrate in water is deemed as a __ [physical/chemical] change.
Answer:
Dissolution of lead nitrate in water is deemed as a physical change.

Question 5.
A change in which matter undergoes a change but the total mass of substance is unaltered is a __ [physical/ chemical] change.
Answer:
A change in which matter undergoes a change but the total mass of substance is unaltered is a chemical change.

Q.2. State which of the following 1 to 5 pertain to – A: Physical Change B: Chemical change :

  1. Sublimation.
  2. Fermentation.
  3. Liquefaction or condensation.
  4. Magnetization.
  5. Respiration.

Answer:

  1. Sublimation – A : Physical Change
  2. Fermentation – B : Chemical change
  3. Liquefaction or condensation – A : Physical Change
  4. Magnetization – A : Physical Change

Q.3. Give reasons why the following are considered as chemical changes.

Question 1.
Copper carbonate on heat gives copper oxide and carbon dioxide.
Answer:
Copper carbonate [CuCO3] green colour on heating gives copper oxide [Black colour CuO] and CO2.
Hence composition is changed new substances CuO [Black] and CO2 with new properties are produced and it is irreversible reaction hence it is a chemical change.

New Simplified Chemistry Class 8 ICSE Solutions Chapter 2 Physical & Chemical Changes 1

Question 2.
A bright light is seen evolved when a strip of magnesium is heated.
Answer:
Magnesium is heated producing a bright light to form MgO in air is a chemical change as the composition changes (MgO white powder) is formed, is irreversible. We cannot get magnesium back from MgO.

Question 3.
Sulphur when burnt in air evolves a gaseous acidic product.
Answer:
Is a chemical change as sulphur (yellowish green) on burning produces SO2 gas which is acidic in nature and a new substance with new properties is formed.

Question 4.
An iron nail kept open in the atmosphere rusts.
Answer:
Rust of iron is a chemical change as iron rust [FeO3 χ H20] is brown flaky powder having different composition from original iron metal, is irreversible change.

New Simplified Chemistry Class 8 ICSE Solutions Chapter 2 Physical & Chemical Changes 2

Question 5.
A piece of magnesium strip is dropped into a beaker containing dilute hydrochloric acid.
Answer:
When a piece of magnesium strip is dropped in dilute hydrochloric acid, hydrogen gas is evolved and hence it is a chemical change.

New Simplified Chemistry Class 8 ICSE Solutions Chapter 2 Physical & Chemical Changes 3
as new substances MgCl2 and H2 with new properties are produced and the reaction is irreversible.

Q.4. Match the examples of changes in List I with the correct type of change in List II.

New Simplified Chemistry Class 8 ICSE Solutions Chapter 2 Physical & Chemical Changes 4
Answer:

New Simplified Chemistry Class 8 ICSE Solutions Chapter 2 Physical & Chemical Changes 5

Q.5. Name the following.

Question 1.
The salt obtained when a chemical change takes place on addition of iron to dilute sulphuric acid.
Answer:
FeSO4 (Iron Sulphate)

Question 2.
The product obtained during a physical change when water converts from a liquid into a solid.
Answer:
Ice.

Question 3.
The product of the chemical change on keeping a polished iron nail exposed to the atmosphere.
Answer:
Iron III oxide [Fe2O3x H20]

Question 4.
A form of energy required for a chemical change.
Answer:
Generally HEAT ENERGY is required but light, pressure or electricity can also be needed.

Question 5.
The gaseous product of the chemcial change which takes place during respiration in living organisms.
Answer:
CO2 carbon dioxide.

New Simplified Chemistry Class 8 ICSE Solutions – Elements, Compounds & Mixtures

New Simplified Chemistry Class 8 ICSE Solutions – Elements, Compounds & Mixtures

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

Simplified ChemistryChemistryPhysicsBiologyMathsGeographyHistory & Civics

 

EXERCISE

Question 1.
Represent with the help of a simple chart how matter is classified into pure or impure substances & further into elements, compounds & mixtures, with elements further segmented.
Answer:

New Simplified Chemistry Class 8 ICSE Solutions Chapter 3 Elements, Compounds & Mixtures 1

Question 2.
Define the terms elements, compounds & mixtures with a view to show their basic difference.
Answer:
Element is a pure substance.

  1. It is the basic unit of matter and cannot be broken down into two or more simpler substances by any means.
  2. It is mainly classified into metals, non-metals, metalloids and noble gases.

Compound is a pure substance.

  1. It is formed by combination of two or more elements.
  2. The elements are combined together in a fixed ratio.
  3. It can be broken down into its elements by chemical means.

Mixture is an impure substance.

  1. It is formed by combination of two or more elements, compounds or both.
  2. The substances are mechanically mixed together in any ratio.

Question 3.
‘An atom is the basic unit of an element’. Draw a diagram of an atom – divisible as seen today.
Answer:
‘An atom is the basic unit of an element’.

New Simplified Chemistry Class 8 ICSE Solutions Chapter 3 Elements, Compounds & Mixtures 2

Question 4.
‘The modern periodic table consists of elements arranged according to their increasing atomic numbers’. With reference to elements with atomic numbers 1 to 20 only in the periodic table – differentiate them into – metallic elements, metalloids, non-metals & noble gases.
Answer:
Names and symbols of metal, non-metals, metalloids and noble – gases out of 1st 20 elements.

New Simplified Chemistry Class 8 ICSE Solutions Chapter 3 Elements, Compounds & Mixtures 3

Question 5.
Elements are broadly classified into metals & non-metals. State six general differences in physical properties of metals & non-metals. State two metals & two non-metals which contradict with the general physical properties – giving reasons. State one difference in property between metalloids & noble gases.
Answer:
Metallic elements :

(a) Have lustre – i.e. shine.
(b) Are malleable – i.e. can be beaten into sheets.
(c) Are ductile – i.e. can be drawn into wires.
(d) Are good conductors – of heat and electricity.
(e) have high – melting and boiling points.
(f) Have high – density.
(g) Contain – one type of atoms – monoatomic

Non-metallic Elements :

(a) Do no have lustre.
(b) Are non-malleable – i.e. cannot be beaten into sheets.
(c) Are non-ductile – i.e. cannot be drawn into wires.
(d) Are poor conductors – of heat and electricity.
(e) Have low – melting and boiling points.
(f) How low – density.
(g) Contain – monoatomic or diatomic atoms.

Two metals which contradict properties :

  1. Mercury — is liquid at room temperature whereas metals are mostly solid.
  2. Zinc — is NON-MALLEABLE and NON-DUCTILE contradicts metal which are MALLEABLE and DUCTILE.

Two Non-metals are :

  1. Iodine — is lustrous contradicts non-metals have no lustre.
  2. Graphite — is good conductor of electricity where as non – metals are poor conductor of electricity.

Question 6.
With reference to elements – define the term ‘molecule’. Give two examples each of a monoatomic, diatomic & polyatomic molecule.
Answer:
Molecule : Atoms of the same element or different elements combine to form a molecule. It is the smallest particle of a substance – which can normally exist independently and can retain, the physical and chemical properties of the substance.
Examples
Monoatomic molecule : Metals – Na, Mg etc.
Diatomic molecule : H2, O2, N2.
Polyatomic molecule : O3, P4.

Question 7.
Define the term ‘compound’. In the compound carbon dioxide – the elements carbon & oxygen are combined in a fixed ratio. Explain.
Answer:
A compound is a pure substance made up of two or more different elements [atoms] combined chemically in a fixed proportion.

(a) Contains – two or more kinds of atoms, e.g. compound – carbon dioxide [CO2].
(b) Can be broken down – into two or more simpler substances – by chemical means.
(c) Properties of compounds – differ from those of their elements, e.g. CO2 contains elements – Carbon [C] is combustible and Oxygen [O] is supporter of combustion – but carbon dioxide is non-combustible and non-supporter of combustion.

Question 8.
State five different characteristics of compounds. Give three differences between elements & compounds with relevant examples.
Answer:
Five characteristics of a compound :

(a) Components in a compound are in a definite proportion.
(b) Compound is always homogenous [i.e. identical composition].
(c) Particles in a compound are of one kind.
(d) Compounds have a definite set of properties.
(e) Elements in the compound do not retain their original properties.
(f) Components in a compound can be separated by chemical means only.

Three differences between elements and compounds :
Element :

  1. They contains one kind of atoms only e.g. element (Na) Sodium and [Cl] chlorine.
  2. They elements have ther own set of properties e.g. [H] Hydrogen is combustible.
  3. They cannot be broken down into two or more simpler substances by physical or chemical means.

Compound :

  1. They contains two or more kinds of atoms, e.g. compound — water [H20].
  2. They Properties of compounds are entirly new i.e. water [H20] is a liquid and can extinguish fire.
  3. They Cannot be broken down into two or more simpler substances by chemical means.

Question 9.
Explain the term ‘mixture’. Differentiate between homogenous & heterogenous mixtures. State why brass is considered as a homogenous mixture while a mixture of iron & sulphur – heterogenous. Give an example of two liquids which form (a) homogenous (b) heterogenous – mixtures.
Answer:
Mixture : A mixture is made up of two or more substances elements or compounds or both mechanically mixed together in any proportion.
A mixture retain the properties of its constituent elements or compounds.

Differenance between homogenous & heterogenous mixtures :

Homogenous mixture :

  • There constituents are uniformly mixed.
  • The properties and composition are same throughout the mixture
  • Two solids are Brass [Cu + Zn]

Heterogeneous mixture :

  • There constituents are not uniformly mixed.
  • The properties and composition vary throughout the mixture.
  • Two solids are Iron + sulphur

BRASS is HOMOGENOUS because mixture is just the same through out and its constituents cannot be distinguished from each other i.e. composition is same through out.
Whereas in Heterogenous mixture of iron and sulphur, the constituents. Can be separated easily by a magnet, the particles can be distinguished from each other and composition is not uniform.

(a) Two liquids which form Homogenous mixture are Alcohol and water.
(b) Two liquids which form Heterogenous mixture are oil and water.

Question 10.
Compare the properties of iron [II] sulphide with iron – sulphur mixture, considering iron [II] sulphide as a compound & particles of iron & sulphur mixed together as an example of a mixture.
Answer:
Iron and sulphur mixture :

  1. The constituents i.e. iron and sulphur are seen separately.
  2. Iron attracts and clings to magnet.
  3. On adding dil. HC1 to the mixture H2 gas evolves.

Iron [II] sulphide is a compound

  1. It is black solid.
  2. On bringing a magnet near it we cannot separate iron.
  3. On adding dil. HCl to it H2S gas evolves.

Question 11.
State any one method – to separate the following mixtures –

(a) Two solid mixtures one of which – directly changes into vapour on heating.
(b) Two solid mixtures one of which – dissolves in a – particular solvent and other does not
(c) A solid-liquid mixture containing – an insoluble solid in the liquid component
(d) A solid-liquid mixture containing – a soluble solid in the liquid component
(e) A liquid-liquid mixture containing – two immiscible ”’ liquids having different densities
(f) A liquid-liquid mixture containing – two miscible liquids having different boiling points.
(g) A liquid-gas mixture containing – a gas dissolved in a liquid component.
(h) A gas-gas mixture containing – two gases with different densities.
(i) A mixture of different solid constituents – in a liquid constituent.

Answer:

(a) The method used is sublimation : Sublimable solid sublimes on heating i.e. changes into vapours and condenses on cooling is separated leaving behind non – sublimable solid.
(b) The method is solvent extraction : Soluble solid dissolves in solvent leaving behind the other insoluble solid. The dissolved solid is recovered by evaporation.
(c) The method is filtration : The insoluble solid is obtained on the filter paper as residue.
(d) The method is evaporation or distillation : Liquid evaporates leaving behind solid.
(e) The method is separating funnel : Heavier liquid collects in die flask below an opening tap, while lighter layer remains in the separating funnel.
(f) Method is fractional distillation : The liquid with lower boiling point collects in the receiver while the liquid with higher B.P. remains in distillation flask.
(g) The method used is boiling the mixture : Solubility of gas decreases with increase in temperature, so gas escapes when mixture is boiled and collected separately.
(h) The method is diffusion : The lighter gas diffuses more rapidly on passing the rough porous partition where as heavier gas diffuses less rapidly on passing through porous partition.
(i) The method used is chromatography.

Question 12.
Explain with diagrams the process used to – separate the following substances from the given mixtures.

(a) Ammonium chloride from a mixture of – ammonium chloride & potassium chloride.
(b) Iron from a mixture of – iron & copper
(c) Sulphur from a mixture of – sulphur & copper.
(d) Potassium nitrate from a mixture of – potassium nitrate & potassium chlorate.
(e) Lead carbonate [insoluble] from a mixture of – lead carbonate & water.
(f) Lead nitrate [soluble] from a mixture of – lead nitrate & water Le. lead nitrate solution.
(g) Carbon tetrachloride from a mixture of – carbon tetrachloride [heavier component] & water.
(h) Benzene from a mixture of – benezene [b.p. 80°C] & toluene [b.p. 110°C].
(i) Different dyes – in their liquid constituent ink.

Answer:

(a) By sublimation : On heating the mixture in evaporating dish, ammonium chloride sublimes on the walls of funnel and potassium chloride remains in evaporating dish.

New Simplified Chemistry Class 8 ICSE Solutions Chapter 3 Elements, Compounds & Mixtures 4
Here A is Ammonium Chloride and B is Potassium Chloride.
(b) By magnetic separation : By bringing a magnet near the mixture iron pieces can be separated which will cling to the magnet.

New Simplified Chemistry Class 8 ICSE Solutions Chapter 3 Elements, Compounds & Mixtures 5
Here A is Iron and B is Copper.
(c) By solvent extraction : Mixture of copper and sulphur is added to the beaker containing solvent carbon disulphide and stirred well. Sulphur dissolves. Put this mixture on filter paper in the funnel. Copper remains on filter paper and sulphur passes into the beaker as filtrate. Sulphur separates as carbon disulphide evaporates.

New Simplified Chemistry Class 8 ICSE Solutions Chapter 3 Elements, Compounds & Mixtures 6

New Simplified Chemistry Class 8 ICSE Solutions Chapter 3 Elements, Compounds & Mixtures 7
Here A is for Copper and B is Sulphur.
(d) Potassium nitrate KNO3 is more-soluble than potassium chlorate KClO3.
On heating to get saturated solution and on cooling the saturated solution less soluble (KClO3) crystallise out. .More soluble KNO3 is filtered out from hot saturated solution and is recrystallised from hot water and dried.

New Simplified Chemistry Class 8 ICSE Solutions Chapter 3 Elements, Compounds & Mixtures 8
(e) Evaporation : Lead carbonate can be separated by evaporation. On evaporation, water evaporates leaving behind solid lead carbonate which has higher M.P.

New Simplified Chemistry Class 8 ICSE Solutions Chapter 3 Elements, Compounds & Mixtures 9
(f) Lead nitrate is separated from soluble lead nitrate solution by crystallisation.

New Simplified Chemistry Class 8 ICSE Solutions Chapter 3 Elements, Compounds & Mixtures 10
New Simplified Chemistry Class 8 ICSE Solutions Chapter 3 Elements, Compounds & Mixtures 11
(g) By separing funnel, heavier CCl4 carbon tetrachloride form the lower layer is separated when tap is opened and is collected in the flask. Water the lighter top layer remains in the funnel.

New Simplified Chemistry Class 8 ICSE Solutions Chapter 3 Elements, Compounds & Mixtures 12
(h) By fractional distillation, miscible low boiling point benzene (B.P. 80°C) evaporates on heating the mixture and condenses in and collects in flask ‘Y’ where as higher boiling pt. Toluene (B.P. 110°C) remains in flask ‘X’ after condensation.

New Simplified Chemistry Class 8 ICSE Solutions Chapter 3 Elements, Compounds & Mixtures 13
(i) By chromatography : Different dyes [solid constituents i.e. A, B, C, D] in ink which is the liquid constituent. By placing the ink spot containing different solid constituents [dyes] on the filter paper. Filter paper is hung with it’s lower end completely dipped in the solvent.

New Simplified Chemistry Class 8 ICSE Solutions Chapter 3 Elements, Compounds & Mixtures 14
The solvent flows over the ink spot and the solid constituents [dyes ‘A’, ‘B’, ‘C’, ‘D’] separate out.

OBJECTIVE TYPE QUESTIONS

Q.1. Select the correct answer from A, B, C, D & E for each statement given below :
A: Gunpowder
B: Iodine
C: Boron
D: Helium
E: Bromine

Question 1.
A diatomic molecule.
Answer:
E: Bromine

Question 2.
A metalloid.
Answer:
C: Boron

Question 3.
A non-metal which is lustrous.
Answer:
B: Iodine

Question 4.
A mixture consisting of elements & a compound.
Answer:
A: Gunpowder

Question 5.
A noble gas.
Answer:
D: Helium

Q.2. Match the separation of components in List I with the most appropriate process in List II.

New Simplified Chemistry Class 8 ICSE Solutions Chapter 3 Elements, Compounds & Mixtures 15
Answer:

New Simplified Chemistry Class 8 ICSE Solutions Chapter 3 Elements, Compounds & Mixtures 16

Q.3. The diagram represents fractional distillation for separation of mixtures. Answer the following :

New Simplified Chemistry Class 8 ICSE Solutions Chapter 3 Elements, Compounds & Mixtures 17

Question 1.
Can two immiscible liquids be separated by this process.
Answer:
No, they can be separated by separating funnel.

Question 2.
Separation of liquids by this process is based on which physical property?
Answer:
The physical property on which separation is based “Difference in their boiling points.”

Question 3.
If methyl alcohol & water are to he separated, which liquid would remain in flask ‘X’ after condensation.
Answer:
Water having boiling point 100°C will remain in flask ‘X’.

Question 4.
Give a reason for the above answer.
Answer:
Alcohol having lower B.P. 78°C will evaporate first and condense in the conical flask — receiver ‘Y’.

Question 5.
State the purpose of the fractionating column in the apparatus.
Answer:
The upper part of the ‘Fractionating column is cooler, so as the hot vapours rise up in the column, they get cooled (condense) and trickle back into the distillation flask ‘X’.

Q.4. Select the correct answer from the choice in bracket to complete each sentence :

Question 1.
Dust in air is an example of __ [heterogeneous / homogenous] mixture.
Answer:
Dust in air is an example of Homogenous mixture.

Question 2.
A soluble solid is separated from an insoluble solid by __ [fractional crystallisation / solvent extraction].
Answer:
A soluble solid is separated from an insoluble solid by solvent extraction.

Question 3.
The reactive element from the two monoatomic elements is __ [neon/silicon].
Answer:
The reactive element from the two monoatomic elements is silicon.

Question 4.
Compounds are __ [homogenous or heterogenous / always homogenous] in nature.
Answer:
Compounds are always homogenous in nature.

Question 5.
An example of a monoatomic molecule is __ [hydrogen / helium],
Answer:
An example of a monoatomic molecule is helium.

Q.5. Give reasons for the following statements :

Question 1.
Components in a mixture can be separated by physical methods only.
Answer:
Components of a mixture can be separated by physical methods because particles remain separate without chemical reaction between them.

Question 2.
Centrifugation can be used for separating an insoluble heavier solid, present in an – insoluble solid-liquid mixture.
Answer:
Centrifugation is fast method to separate suspended (heavier) insoluble solid from lighter liquid by rotating the mixture fast. Heavier solid settles down at the bottom.

Question 3.
The filter paper made into a cone & placed in a funnel for filtering out the solid particles in a solid-liquid mixture, should be moistened before placing.
Answer:
Moistening the filter paper cone sticks to the walls of the funnel and also makes filtration convenient.

Question 4.
Brass & bronze are examples of mixtures, while copper sulphate & lead nitrate are examples of compounds.
Answer:
Brass and bronze are mixtures as the composition of elements is not fixed by mass and have no formula.
Lead nitrate and copper sulphate are compounds as they have fixed ratio of elements by mass and have formula like Pb[NO3]2 and CuSO4.

Question 5.
Zinc is considered an element, while zinc sulphide is considered a compound.
Answer:
Zinc [Zn] is an element as zinc is a pure substance made up of one kind of atoms all having same size, atomic number and atomic mass.
Zinc sulphide [ZnS] is pure substance made up of two element [atoms] of zinc and sulphur combined chemically in a fixed proportion.

New Simplified Chemistry Class 8 ICSE Solutions – Atomic Structure

New Simplified Chemistry Class 8 ICSE Solutions – Atomic Structure

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

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EXERCISE

Question 1.
State the main postulates of – Dalton’s atomic theory. Explain how the modern atomic theory contradicted Dalton’s atomic theory.
Answer:
(a) Dalton’s Atomic Theory :
The main postulates of theory are :

  1. Matter consists of small indivisible particles called – atoms i. e. Atom is the basic unit of matter.
  2. Atoms of the same element are – alike in all respects i.e. atoms of hydrogen have same properties like mass, density and atoms of oxygen are alike in all respects.
  3. Atoms of different elements are different from each other. i.e. atoms of hydrogen are different from atoms of oxygen.
  4. Atom can neither be created nor destroyed.
  5. Atoms combine with other atoms in simple whole number ratio forming compounds or molecules.

(b) Modern atomic theory contradicted Dalton Atomic Theory as :

  1. Atom is no longer indivisible as atom has been divided and has sub – atomic particles
    (a) Protons
    (b) Neutrons
    (c) Electrons
  2. Atoms of same element may have different properties
    New Simplified Chemistry Class 8 ICSE Solutions Chapter 4 Atomic Structure 1
  3. Atoms of different elements may have same properties i.e. isobars
  4. Atoms combine with other atoms may not be in simple whole number ratio i.e. C12H22O11 (Sugar).
  5. Atom can be destroyed and converted into energy.

Question 2.
With reference to the discovery of the structure of an atom, explain in brief – William Crookes experiment for the discovery of cathode rays, followed by – J.J. Thomsons experiment pertaining to the constituents of the cathode rays. State which sub-atomic particle was discovered from his experiment.
Answer:
Discovery of the three subatomic particles – electrons, protons and neutrons
Atom are built up of three sub-atomic particles – electrons, protons and neutrons.

New Simplified Chemistry Class 8 ICSE Solutions Chapter 4 Atomic Structure 2
Discovery of cathode rays leading to the discover of ‘electrons’

  • Scientist – William Crookes [1878]
  • Discovery – The cathode rays

Experiment :

New Simplified Chemistry Class 8 ICSE Solutions Chapter 4 Atomic Structure 3

  1. An electric discharge was passed through a tube containing a gas at low pressure.
  2. Blue rays were emitted from the cathode [negative plate] – which were called cathode rays.

Question 3.
Explain in brief – Goldstein’s experiment which led to the discovery of the proton and – Lod Rutherford’s experiment which led to the discovery of the atomic nucleus.
Answer:
Discovery of – Protons

  • Discovery – Constituent of positive rays i.e. particles that contain – protons.

Experiment :

New Simplified Chemistry Class 8 ICSE Solutions Chapter 4 Atomic Structure 4

  • Goldstein used a modified cathode ray tube with a perforated cathode.
  • He observed a new type of rays produced from the anode passing through the holes of the perforated cathode. These rays were called anode rays.

Conclusion :

  • Anode rays or positive rays consist of positively charged particles now called – protons.
  • The positive rays were affected by electric & magnetic fields but – in a direction opposite to that of cathode rays.
  • Thus with the discovery of the positive particles – proton was initiated.

Discovery of – Atomic nucleus :

  • Discovery – Study of the atomic model leading to the discovery of – atomic nucleus.

Experiment :

New Simplified Chemistry Class 8 ICSE Solutions Chapter 4 Atomic Structure 5

  • Rutherford projected alpha particles towards a thin gold foil, – in the path of the rays.
  • He saw that most of the alpha particles went straight through the foil, – but some were deflected slightly & some by large angles.

Conclusion :

  • An atom on the whole is relatively empty but consists of a – concentrated positive mass in the centre, which lead to the deflection of the alpha particles.
  • Thus the discovery of a central positive region – atomic nucleus was initiated.

Question 4.
‘Electrons revolve around the nucleus in fixed orbits or shells called energy levels’. State how these energy levels are represented.
Answer:

(a) Electrons revolve around the nucleus in – fixed ‘orbits’ called energy levels
(b) The energy levels 1, 2, 3… are represented by – integer ‘n’ or as K, L, M, N…
(c) Electrons rotate around the nucleus, in one or .more of the energy levels.

Question 5.
Draw a neat labelled diagram representing an atom. Name the three sub-atomic particles in the atom & represent them symbolically showing the mass & charge of each. State where the sub-atomic particles are present in the atom.
Answer:

New Simplified Chemistry Class 8 ICSE Solutions Chapter 4 Atomic Structure 6

Question 6.
Define the term – ‘atomic number’ of an atom. If an atom ‘A’ has an atomic number of – eleven, state the number of protons & electrons it contains.
Answer:
Atomic number is the number of protons in the atom of an element. Since atom is electrically neutral i.e. is charge less, therefore
number of electrons = number of protons.
∴ It has 11 P and 11 electrons.
Atomic number z = p = e

Question 7.
Define the term – ‘mass number ’ of an atom. If an atom t ‘B’ has mass number 35 & atomic number 17, state the number of protons, electrons & neutrons it contains.
Answer:
Mass number of an element is equal to the sum of protons and neutrons in the nucleus of atom.
Mass number = Number of protons + Number of neutrons
A = p + n A = 35 atomic number p = 17
∴ 35 = 17 + n
∴ n = number of neutrons = 35 – 17 = 18
n = 18
But p = e
∴ e = p = 17
Number of electrons = e = 17

Question 8.
State why the atomic weight of an element is also termed – relative atomic mass.
Answer:

  • Atomic weight : is mass of an atom, the number times it is heavier than an atom of hydrogen.
    Since carbon atom is 12 times heavier than an atom of hydrogen.
  • Relative mass : is equal to the number of times an atom of an element is heavier than 1/12 the mass of an atom of carbon.
    Hence atomic weight of an element is also termed relative atomic mass as it is in comparison with mass of 1/12 th mass of a carbon atom.

Question 9.
State how electrons are distributed in an atom. Explain in brief the rules which govern their distribution.
Answer:
(a) Electrons revolve around the nucleus in imaginary paths called shells or orbits. Shells start from nucleus to outwards.

New Simplified Chemistry Class 8 ICSE Solutions Chapter 4 Atomic Structure 7
Rules : Maximum number of electrons in a shell is given by 2n2. Where n is the number of shell i.e. 1st shell can have maximum of 2 electrons.
2n2 = 2(1)2 = 2 × 1 = 2
2nd shell can have maximum of 8 electrons
2n2 = 2(2)2 = 2 × 4 = 8
3rd shell can have maximum of 18 electrons
2n2 = 2 (3)2 = 2 × 9 = 18 and so on….
(b) Outer most orbit cannot have more than 8 electrons and 18 in penultimate orbit.
(c) A new shell cannot start until previous is filled completely.

Question 10.
If an atom ‘A’ has atomic number 19 & mass number 39, state –

  1. Its electronic configuration.
  2. The number of valence electrons it possesses.

Answer:
Atom ‘A’ has mass number A = 39 and atomic number Z = 19 = p
∴ A = Z + n
A = p + n
39 = 19 + n
n = 39 – 19 = 20
But e = p = 19

  1. A (K, L, M, N)
    19 = 2, 8, 8, 1
    There will be 2 electrons in K-shell or 1st shell
    8 electrons in 2nd shell or L-shell
    8 electrons in 3rd shell or M-shell
    1 electron in 4th shell or nth-shell
  2. The number of valence electrons i.e. in outer most shell = 1 electron.

Question 11.
Draw the atomic diagrams of the following elements showing the distribution of – protons, neutrons & the electrons in the various shells of the atoms.

New Simplified Chemistry Class 8 ICSE Solutions Chapter 4 Atomic Structure 8
[The upper number represents the – mass number & the lower number the – atomic number e.g. calcium – mass number = 40, atomic number = 20]
Answer:

New Simplified Chemistry Class 8 ICSE Solutions Chapter 4 Atomic Structure 9

New Simplified Chemistry Class 8 ICSE Solutions Chapter 4 Atomic Structure 10

New Simplified Chemistry Class 8 ICSE Solutions Chapter 4 Atomic Structure 11

New Simplified Chemistry Class 8 ICSE Solutions Chapter 4 Atomic Structure 12

Question 12.
‘ Valency is the number of hydrogen atoms which can combine with [or displace] one atom of the element [or radical] forming a compound’. With reference to the above definition of valency, state the valency of chlorine in hydrogen chloride, giving reasons.
Answer:
Hydrogen chloride [HCl], one atom of chlorine has combined with one atom of hydrogen and also 1 atom of hydrogen can be replaced by metals like potassium, sodium. Hence valency of chlorine in one.

Question 13.
‘ Valency is also the number of electrons – donated or accepted by an atom so as to achieve stable electronic configuration of the nearest noble gas’. With reference to this definition –

(a) State what is meant by ‘stable electronic configuration’.
(b) State why the valency of –

  1. sodium, magnesium & aluminium is : +1, +2 & +3 respectively.
  2. chlorine, oxygen & nitrogen is : -1, -2 & -3 respectively.

Answer:

(a) Stable electronic configuration means to have 2 electrons in the 1st [or K] outer most shell like He – [Duplet].
OR
8 electrons in outer most orbit like other nearst noble gas – [Octet].

(b)

  1. Valency is the number of electrons donated or lost from the valence shell. Since sodium donates 1 valence electron its valency is +1.
    Magnesium loses 2 electrons and aluminium loses 3 electrons from their valence shell their valency is
    +2 – magnesium
    +3 – Aluminium
  2. Valency of an element is the number of electrons accepted to achieve stable configuration of nearest noble gas.
    Chlorine accepts 1 electron and has valency -1 where as oxygen accepts 2 electrons the valency of oxygen is -2 and nitrogen accepts 3 electrons, valency of nitrogen is -3.

Question 14.
With reference to formation of compounds from atoms by electron transfer – electro valency, state the basic steps in the conversion of sodium & chlorine atoms to sodium & chloride ions leading to the formation of the compound – sodium chloride.
[electronic configuration of : Na = 2, 8, 1 & Cl = 2, 8, 7]
Answer:
Electronic configuration

New Simplified Chemistry Class 8 ICSE Solutions Chapter 4 Atomic Structure 13

OBJECTIVE TYPE QUESTIONS

Q.1. Match the statements in List I with the correct answer from List II.

New Simplified Chemistry Class 8 ICSE Solutions Chapter 4 Atomic Structure 14
Answer:

New Simplified Chemistry Class 8 ICSE Solutions Chapter 4 Atomic Structure 15

Q.2. Select the correct answer from the choice in bracket to complete each sentence :

Question 1.
An element ‘X has six electrons in its outer or valence shell. Its valency is __ [+2/-2/-1].
Answer:
An element ‘X has six electrons in its outer or valence shell. Its valency is -2.

Question 2.
An element ‘Y’ has electronic configuration 2, 8, 6. The element ‘Y’ is a __ [metal/non-metal/noble gas].
Answer:
An element ‘Y’ has electronic configuration 2, 8, 6. The element ‘Y’ is a non-metal.

Question 3.
A __ [proton/neutron] is a sub-atomic particle with no charge and unit mass.
Answer:
A neutron is a sub-atomic particle with no charge and unit mass.

Question 4.
An element Z with zero valency is a __ [metal/noble gas/non-metal].
Answer:
An element Z with zero valency is a noble gas.

Question 5.
Magnesium atom with electronic configuration 2, 8, 2 achieves stable electronic configuration by losing two electrons, thereby achieving stable electronic configuration of the nearest noble gas __ [neon/argon].
Answer:
Magnesium atom with electronic configuration 2, 8, 2 achieves stable electronic configuration by losing two electrons, thereby achieving stable electronic configuration of the nearest noble gas neon.

Q.3. The diagram represents an isotope of hydrogen [H]. Answer the following :

New Simplified Chemistry Class 8 ICSE Solutions Chapter 4 Atomic Structure 22

Question 1.
Are isotopes atoms of the same element or different elements.
Answer:
Isotopes atoms are of the same element.

Question 2.
Do isotopes have the same atomic number or the same mass number.
Answer:
Same atomic number.

Question 3.
If an isotope of ‘H’ has mass no. = 2, how many electrons does it have.
Answer:
One electron.

Question 4.
If an isotope of ‘H’ has mass no. = 3, how many neutrons does it have.
Answer:
Two neutrons. [∵ A = P + n]

Question 5.
Which sub-atomic particles in the 3 isotopes of ‘H’ are the same.
Answer:
Protons and electrons in each isotope are same.

Q.4. State the electronic configuration for each of the following :

New Simplified Chemistry Class 8 ICSE Solutions Chapter 4 Atomic Structure 16
Answer:
Electronic configuration of :

New Simplified Chemistry Class 8 ICSE Solutions Chapter 4 Atomic Structure 17

New Simplified Chemistry Class 8 ICSE Solutions Chapter 4 Atomic Structure 18

Q.5. Draw the structure of the following atoms showing the nucleus containing – protons, neutrons and the orbits with the respective electrons :

  1. Lithium [At. no. = 3, Mass no. = 7]
  2. Carbon [At. no. = 6, Mass no. = 12]
  3. Silicon [At. no. = 14, Mass no. = 28]
  4. Sodium [At. no. = 11, Mass no. = 23]
  5. Isotopes of hydrogen [11H, 21H , 31H]

Answer:
Structure of atoms :
Z is Atomic Number A is mass number

New Simplified Chemistry Class 8 ICSE Solutions Chapter 4 Atomic Structure 19

New Simplified Chemistry Class 8 ICSE Solutions Chapter 4 Atomic Structure 20

New Simplified Chemistry Class 8 ICSE Solutions Chapter 4 Atomic Structure 21

 

New Simplified Chemistry Class 8 ICSE Solutions – Language of Chemistry

New Simplified Chemistry Class 8 ICSE Solutions – Language of Chemistry

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

Simplified ChemistryChemistryPhysicsBiologyMathsGeographyHistory & Civics

EXERCISE

Question 1.
Explain the term ‘symbol’. State a reason why – the symbol of calcium is ‘Ca’ & of copper is ‘Cu’
Answer:
Symbol is a short form or abreviated name – of the element.
OR
“Symbol of an element is the ‘first letter’ or the ‘first letter and another letter’ of the English name or Latin name of the element”.
As the name calcium and copper start with same letter of English alphabet ‘C’ so another letter from the name of the element is added to ‘C’.
Hence symbol of calcium is ‘Ca’ and symbol of copper or cuprum (Latin name of copper) is ‘Cu’.

Question 2.
Define the term ‘valency’. With reference to water & ammonia as compounds respectively, state the valency of oxygen & nitrogen. Magnesium [2, 8, 2] has valency 2+. Give reasons.
Answer:
Valency is the number of hydrogen atoms which combine with (or displace) one atom of the element forming a Compound. Compound water H20, two atoms of hydrogen combine with one atom of oxygen to form water. Hence valency of oxygen atom is 2.
Compound ammonia NH3.
Valency of atom nitrogen is 3, as it combines with 3 atoms of hydrogen.
Valency of magnesium is 2+, Mg [2, 8, 2]
Electronic configuration of Mg is 2, 8, 2 i.e. there are 2 valence electrons which Mg can lose and achieve stable configuration of nearest noble gas Neon.
Hence is cation Mg2+ has valency 2+.

Question 3.
Explain the term ‘variable valency’. Copper having electronic configuration 2,8,18,1 exhibits variable valency. Give a reason for the same & name the compound CuCI & CuCl2.
Answer:
Variable valency : “When an element has more than one valency, its valency is called variable valency.”
Copper exhibit valency ‘1’ and ‘2’ i.e. has variable valency reason for variable valency :
Valency is – the number of electrons lost or gained from the outer shell of an atom of an element – during chemical reaction. Variation in this gain or loss results in ‘variable valency’.
Lower valency ends with – ous
Higher valency ends with – ic
CuCI is named as cuprous chloride (lower valency)
OR
Cu [I] Cl i.e. Copper [I] chloride and CuCl2 – Cupric chloride (higher valency) i.e. Copper [II] chloride Cu[II]Cl2

Question 4.
State the valencies of the following metallic elements –

(a) Potassium
(b) Sodium
(c) Calcium
(d) Magnesium
(e) Zinc
(f) Aluminium
(g) Chromium [write each symbol with the valency]

Answer:
Metals have positive valency 1, 2 or 3 ,
Valency of

(a) Potassium is K+
(b) Sodium 1+
Na+
(c) Calcium 2+
Ca2+
(d) Magnesium 2+
Mg2+
(e) Zinc 2+
Zn2+
(f) Aluminium 3+
Al3+
(g) Chromium 3+
Cr3+

Question 5.
Certain metals exhibit variable valencies which include valencies: 1+, 2+, 3+ & 4+.
State the variable valency of the following metals –

(a) Copper
(b) Silver
(c) Mercury
(d) Iron
(e) tin
(f) Lead

[write each symbol with the variable valency]

Answer:

New Simplified Chemistry Class 8 ICSE Solutions Chapter 5 Language of Chemistry 1

New Simplified Chemistry Class 8 ICSE Solutions Chapter 5 Language of Chemistry 2

Question 6.
State which of the following ions or radicals given below of non-metallic elements exhibit-valency: 1, 2 & 3
(a) Chloride (b) Bromide (c) Iodide (d) Nitrate (e) Hydroxide (f) Bicarbonate (g) Bisulphite (h) Bisulphate (i) Aluminate (j) Permanganate (k) Oxide (l) Sulphide (m) Sulphite (n) Sulphate (o) Carbonate (p) Dichromate (q) Zincate (r) Plumbite (s) Phosphate (t) Nitride
[write each ion or radical with the correct valency]
Answer:

New Simplified Chemistry Class 8 ICSE Solutions Chapter 5 Language of Chemistry 3

New Simplified Chemistry Class 8 ICSE Solutions Chapter 5 Language of Chemistry 4

Question 7.
Differentiate between the terms – ‘Ion’ & ‘radical’ with suitable examples.
Answer:
Ion “is an atom or a group of atoms carrying a positive or negative charge due to loss or gain of electrons.” e.g. cation Na+ and Cl1- is anion.
Radical “is group of atoms of elements that behaves like a single unit & show valency.”
Positive radical [NH H41+ ] Ammonium
Negative radical [HCO31- ], [NO31-] Bicarbonate Nitrate

Question 8.
Write the chemical formula of the following compounds in a step-by-step manner – (a) Potassium chloride (b) Sodium bromide (c) Potassium nitrate (d) Calcium hydroxide (e) Calcium bicarbonate (f) Sodium bisulphate (g) Potassium sulphate (h) Zinc hydroxide (i) Potassium permanganate (j) Potassium dichromate (k) Aluminium hydroxide (l) Magnesium nitride (m) Sodium zincate (n) Copper [II] oxide (o) Copper [I] sulphide (p) Iron [III] chloride (q) Iron [II] hydroxide (r) Iron [III] sulphide (s) Iron [III] oxide.
Answer:

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Question 9.
What’ is a chemical equation. How is it represented. Differentiate between a ‘word equation’ and a ‘molecular equation’ with a suitable example.
Answer:
“Chemical equation is a short form – representing the result of a chemical change.”
OR
“Is the representation of a chemical change.”
Word equation tells which substances react (take part) in chemical reaction and which substances are produced where as molecular equation symbols and molecular formulas are used for both reactants and products.
Example : When zinc reacts with dilute sulphuric acid, both being reactants, products produced are zinc sulphate and hydrogen gas, which are shown as below:
Word equation :
Zinc + Sulphuric acid → Zinc sulphate + Hydrogen
Molecular equation :

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Question 10.
State the information provided by a chemical equation. Chemical equations suffer from a number of limitations. State the main limitations of a chemical equation.
Answer:
Information provided by a chemical equation :

(a) It tells us the formulas and symbols of the reactants and products.
(b) It tells us the ratio in which substances reacts or are produced. If limitations are covered.
(c) It tells the physical state of substances i.e. solid, liquid, gas.
(d) Whether the reaction is endothermic or exothermic.
(e) Conditions for starting the reaction i.e. if catalyst is needed or not.
(f) If reaction is reversible or not.

Limitations :

(a) Physical states of the reactants or products. But now we write along with substances (1) for liquids, (s) for solid and (g) for gas.
(b) Conditions that effect the reaction, i.e. temp, pressure or catalyst.
(c) Concentration of reactants and products we use dil. for dilute and cone, for concentrated.
(d) Nature of the chemical reaction.
(e) Speed – reaction is fast or slow.
(f) Exothermic or endothermic we write + heat or – heat towards products for exothermic and endothermic.
(g) The completion of the reaction.

Question 11.
State what is a balanced equation with a relevant example. Give a reason why an equation is balanced with reference to the law of conservation of matter.
Answer:
A balanced equation : “An equation is said to be balanced if the number of atoms of each element of the reactant is equal to the number of atoms of each element of the product.”
Example :
2Mg + O2 → 2MgO
[Reactants] [Product]
Reactans have 2 atoms of Mg and 2 atoms of oxygen.
Products have 2 atoms of magnesium and 2 atoms of oxygen.
∴ Number of atoms of each element of reactants = Number of atoms of each element of product.

  • Reason for balancing equation : Law of conservation of matter says that “Matter is neither created nor destroyed during a chemical reaction.” This is possible only if number of atoms of each element on both sides of → are equal i.e. in reactants and also in products. Hence a reaction is balanced.

Question 12.
Write balanced molecular equations for the following word equations :

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Answer:

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Question 13.
Balance the following equations :

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Answer:

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OBJECTIVE TYPE QUESTIONS

Q.1. Complete the statements given below by filling in the blank with the correct words.

Question 1.
The formula of silver [I] chloride is ___ [AgCl/AgCl2].
Answer:
The formula of silver [I] chloride is AgCl.

Question 2.
The basic unit of an element is a/an ___ [molecule/atom/ion]
Answer:
The basic unit of an element is a/an atom.

Question 3.
Atoms contains ___ [netron/nucleus/, with positively charged __ [electrons/protons].
Answer:
Atoms contains nucleus with positively charged protons.

Question 4.
Element ___ [calcium/lead/carbon] has the symbol derived from its Laltin name ‘plumbum’.
Answer:
Element lead has the symbol derived from its Laltin name ‘plumbum’.

Question 5.
From the elements – He, Br, Pt & O; the element which forms a polyatomic molecule is ___ & which is liquid at room temperature is ___
Answer:
From the elements – He, Br, Pt & O; the element which forms a polyatomic molecule is O & which is liquid at room temperature is Br.

Question 6.
The valency of iron in FeO is ___ [2+/1+] of chlorine [chloride] in CaCl2 is ___ [1/2] and of dichromate in K2 Cr O7 is [2+/2].
Answer:
The valency of iron in FeO is 2+ of chlorine [chloride] in CaCl2 is 1 and of dichromate in K2 Cr O7 is 2.

Q.2. Match the statements – 1 to 10 below with their correct answers from – A to J.

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Answer:

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Q.3. Match the compounds in List I – 1 to 20 with their correct formulas in List II – A to T.

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Answer:

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Q.4. Underline the incorrectly balanced compound in each equation & rewrite the correct equation.

Question 1.
2Na + 3H20 → 2NaOH + H2
Answer:
Correct equation is :
2Na + 2H20 → 2NaOH + H2

Question 2.
4P + 4O2 → 2P2O5
Answer:
Correct equation is :
4P + 5O2 → 2P2O5

Question 3.
FE2O3 + 2H2 → 2Fe + 3H20
Answer:
Correct equation is :
FE2O3 + 3H2 → 2Fe + 3H20

Question 4.
2A1 + 2H2SO→ A12(SO4)3 + 3H2
Answer:
Correct equation is :
2A1 + 3H2SO4 → A12(SO4)3 + 3H2

Question 5.

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Answer:
Correct equation is :

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Question 6.
ZnO + 3NaOH → NA2ZnO2 + H20
Answer:
Correct equation is :
ZnO + 2NaOH → NA2ZnO2 + H20

Question 7.
FeCl3 + 3NH4OH → 2NH4C1 + Fe(OH)3
Answer:
Correct equation is :

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Question 8.
FeS + 2HCl → 2FeCl2 + H2S
Answer:
Correct equation is :
FeS + 2HCl → FeCl2 + H2S

Question 9.
3NH3 + H2So4 → (NH4)2So4
Answer:
Correct equation is :
2NH3 + H2SO4 → (NH4)2SO4

Question 10.
PbO2 + 4HCl → PbCl2 + H20 + Cl2
Answer:
Correct equation is :
PbO2+ 4HCl → PbCl2 + 2H20 + Cl2