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Chapter 3

Elements, Compounds and Mixtures

Class 8 - Concise Chemistry Selina

Exercise 3(A)

Question 1


(a) Elements

(b) Compounds


(a) An element is a pure substance which cannot be converted into anything simpler than itself by any physical or chemical process. It is made up of only one kind of atoms.

(b) A compound is a pure substance composed of two or more elements combined chemically in a definite proportion by mass.

Question 2

Give two examples for each of the following:

(a) Metals

(b) Non-metals

(c) Metalloids

(d) Inert gases


(a) Gold and Silver

(b) Oxygen and Sulphur

(c) Boron and Silicon

(d) Helium and Neon

Question 3

Differentiate between:

(a) Pure and impure substances

(b) Homogeneous and heterogeneous substances


(a) Difference between Pure and impure substances:

Pure substanceImpure substance
Pure substances have a definite chemical composition and definite physical and chemical properties.Impure substances are made up of two or more pure substances mixed together in any proportion. They do not have any definite set of properties.
Pure substances are all homogeneous, i.e. their composition is uniform throughout the bulk.Impure substances may be homogeneous or heterogeneous i.e. their composition is not uniform throughout the bulk.
Examples are gold, silver, water, sodium chloride etc.Examples are air, sea water, solution of sugar in water etc.

(b) Difference between Homogeneous and heterogeneous substances:

Homogeneous substanceHeterogeneous substance
In Homogeneous substance, the components are uniformly distributed throughout its volume and cannot be seen separately.In Heterogeneous substance, the components are not uniformly distributed throughout its volume and can be easily seen separately.
Examples are tap water, milk, brass, bronze etc.Examples are soil, mud and water, sand and stone etc.

Question 4

Write the chemical name of the following and also give their molecular formulae:

(a) Baking soda

(b) Vinegar

(c) Marble

(d) Sand


Common NameChemical NameMolecular Formula
(a)Baking sodaSodium bicarbonateNaHCO3
(b)VinegarAcetic acidCH3COOH
(c)MarbleCalcium carbonateCaCO3
(d)SandSilicon dioxideSiO2

Question 5

Why is iron sulphide a compound?


Iron sulphide is a compound because of the following reasons:

  1. Iron sulphide is a compound formed when iron and sulphur combine chemically, on heating, in 7 : 4 ratio by mass.
  2. Iron is a grey black metal and Sulphur is a yellow amorphous non-metallic solid whereas iron sulphide is a black solid.
  3. Iron is attracted by a magnet and Sulphur is soluble in carbon disulphide but iron sulphide is neither attracted by a magnet nor it is soluble in carbon disulphide.

From the above, we can say that properties of iron sulphide are entirely different from its constituent elements i.e., iron and sulphur. Hence, iron sulphide is a compound.

Question 6


(a) a soft metal

(b) a metal which is brittle

(c) a non-metal which is lustrous

(d) a liquid metal

(e) a metal which is a poor conductor of electricity

(f) a non-metal which is a good conductor of electricity

(g) a liquid non-metal

(h) the hardest naturally occurring substance

(i) an inert gas


(a) Sodium

(b) Zinc

(c) Graphite

(d) Mercury

(e) Tungsten

(f) Graphite

(g) Bromine

(h) Diamond

(i) Neon

Question 7

How does sodium chloride differ from its constituent elements? Explain.


The properties of sodium chloride are completely different from those of sodium and chlorine. Sodium is a soft, highly reactive metal. Chlorine is a poisonous greenish yellow gas with choking smell. But sodium chloride is a white crystalline solid which is non-poisonous and useful. It is added to our food as a mineral and also to add taste to it.

Exercise 3(B)

Question 1

Classify the following substances into compounds and mixtures:

Carbon dioxide, air, water, milk, common salt, blood, fruit juice, iron sulphide.


Compounds — Carbon dioxide, Iron sulphide, Water, Common Salt

Mixtures — Air, Milk, Fruit Juice

Question 2

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

(a) solid-solid homogeneous mixture

(b) solid-liquid heterogeneous mixture

(c) miscible liquids

(d) liquid-gas homogeneous mixture


(a) Brass is an example of solid-solid homogeneous mixture.

(b) Sand and water is an example of solid-liquid heterogeneous mixture.

(c) Mixture of Acetone and Water is a miscible liquid.

(d) Ammonia and water is a liquid-gas homogeneous mixture.

Question 3

Suggest a suitable technique to separate the constituents of the following mixtures. Also give the reason for selecting the particular method.

(a) Salt from sea water

(b) Ammonium chloride from sand

(c) Chalk powder from water

(d) Iron from sulphur

(e) Water and alcohol

(f) Sodium chloride and potassium nitrate

(g) Calcium carbonate and sodium chloride


(a) Salt from sea water is separated by evaporation method. Evaporation method is used to separate the components of a homogeneous solid-liquid mixture. The liquid is escaped in the form of vapour and the solid is left behind.

(b) Sublimation method is used to separate ammonium chloride from sand. This method is used when one of the component is sublime in nature. Ammonium chloride is sublime in nature so when it is heated, ammonium chloride directly changes into vapours and sand is left behind. The vapours of ammonium chloride on cooling changes directly into solid.

(c) Chalk powder and water can be separated by filtration method. Filtration method is used for separating insoluble solids from liquids. On filtration, chalk powder gets deposited on filter paper whereas water passes through the filter paper and gets collected in the beaker.

(d) Iron from sulphur can be separated by magnetic separation method. This method is used when one of the component is magnetic in nature. Iron gets attracted towards magnet and can easily be separated from sulphur.

(e) Water and alcohol can be separated by fractional distillation method using a fractionating column fitted over the distilling flask. Fractional distillation method is used to separate miscible mixtures in which the liquids have different boiling points. Boiling point of alcohol is 78°C and water is 100°C. Hence, this method is the most suitable for separating them. 95.5% pure alcohol is obtained by this method.

(f) Sodium chloride and potassium nitrate can be separated by fractional crystallization. This method is used when the solubility of solid components of a mixture is different in the same solvent. Both salts are soluble in water but solubility of potassium nitrate is more than that of sodium chloride in water.

(g) Solvent extraction method is used to separate calcium carbonate and sodium chloride mixture. This method is used when one component is soluble in a particular liquid while the other component is insoluble. Sodium chloride is soluble in water but calcium carbonate is insoluble which settles down.

Question 4a

Define mixture.


Mixtures can be defined as a kind of matter which is formed by mixing two or more pure substances (elements and compounds) in any proportion, such that they do not undergo any chemical change and retain their individual properties.

Question 4b

Why is it necessary to separate the constituents of a mixture?


It is necessary to separate the constituents of a mixture because

  1. To remove unwanted and harmful substances.
  2. To obtain pure and useful substances.

Question 4c

State four differences between compounds and mixtures.


A compound is a pure substance.A mixture is an impure substance.
Compounds are always homogeneous.Mixtures may be homogeneous or heterogeneous.
Formation of a compound involves a change in energy.Formation of a mixture does not involve any change in energy.
Components of compounds can be separated only by complex chemical processes.Components of mixtures can be separated by simple physical methods of separation.

Question 5a

What is chromatography? For which type of mixture is it used?


The process of separating different dissolved constituents of a mixture by their adsorption on an appropriate material is called chromatography. It is used to separate the components of a mixture when all the components are very similar in their properties. It is used to separate liquid-liquid mixtures.

Question 5b

What are the advantages of chromatography?


The advantages of chromatography are:

  1. A very small quantity of the substance can be separated.
  2. Components with very similar physical and chemical properties can be separated.
  3. It identifies the different constituents of a mixture.
  4. It also helps in quantitative estimation of the components of a mixture.

Question 5c

Give two applications of chromatography.


The two applications of chromatography are:

  1. It is used to purify many industrial products.
  2. It is used to separate pigments from natural colors.

Question 6a

Choose the most appropriate answer from the options given below:

A mixture of sand and ammonium chloride can be separated by

  1. filtration
  2. distillation
  3. sublimation
  4. crystallization



Reason — Sublimation method is used when one of the component is sublime in nature. Ammonium chloride is sublime in nature so on heating, it directly changes into vapour leaving behind the sand.

Question 6b

Choose the most appropriate answer from the options given below:

A pair of metalloids are:

  1. Na and Mg
  2. B and Si
  3. C and P
  4. He and Ar


B and Si

Reason — Boron(B) and Silicon(Si) are metalloids because they show some properties of metals and some properties of non-metals.

Question 6c

Choose the most appropriate answer from the options given below:

Which of the following property is not shown by compounds?

  1. They are heterogeneous.
  2. They are homogeneous.
  3. They have definite molecular formulae.
  4. They have fixed melting and boiling points.


They are heterogeneous.

Reason — Compounds are always homogeneous because the composition and properties is same throughout.

Question 6d

Choose the most appropriate answer from the options given below:

A solvent of iodine is

  1. Water
  2. Kerosene oil
  3. Alcohol
  4. Petrol



Reason — Iodine gets dissolved in ethyl alcohol.

Question 6e

Choose the most appropriate answer from the options given below:

This gas is highly soluble in water

  1. Ammonia
  2. Nitrogen
  3. Carbon monoxide
  4. Oxygen



Reason — Ammonia is highly soluble in water.