Elements, Compounds and Mixtures

Introduction

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

The below chart shows the classification of matter:

Elements

Question 2

Define the terms elements, compounds & mixtures with a view to show their basic difference.

Answer

Elements — It is made up of one kind of atoms only. Elements cannot be broken down into two or more simpler substances by any means. It is a pure substance.

Compounds — It is formed by combination of two or more different elements. The elements are combined chemically in a fixed proportion. It can be broken down into its elements by chemical means. It is a pure substance.

Mixtures — It is formed by combination of two or more elements, compounds or both. The substances are mechanically mixed in any proportion. It is an impure substance.

Question 3

'An atom is the basic unit of an element'. Draw a diagram of an atom — divisible as seen today.

Answer

The structure of an atom is shown in the below diagram:

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

Elements with atomic numbers 1 to 20 are classified into metallic elements, metalloids, non-metals and noble gases in the table below:

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

Six general differences in physical properties of metals & non-metals:

Two metals that contradict with the general physical properties:

1. Mercury — It is liquid at room temperature.
2. Zinc — It is non-malleable [beyond certain temperatures].

Two non-metals that contradict with the general physical properties:

1. Iodine — It is lustrous.
2. Graphite — It is lustrous and conducts electricity.

One difference between metalloids & noble gases:

Metalloids are chemically reactive whereas noble gases are chemically inert or inactive.

Molecules

Question 6

With reference to elements — define the term 'molecule'. Give two examples each of a monoatomic, diatomic & polyatomic molecule.

Answer

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 & can retain, the physical & chemical properties of the substance.

Examples :

Monoatomic molecule — Sodium [Na] and Magnesium [Mg]

Diatomic molecule — Hydrogen [H₂] and Oxygen [O₂]

Polyatomic molecule — Ozone [O₃] and Tetra phosphorous [P₄]

Compounds

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.

In carbon dioxide, 1 atom of carbon combines with 2 atoms of oxygen to form one molecule of carbon dioxide [CO₂], hence, we say that carbon and oxygen combine in a fixed ratio of 1:2 to form carbon dioxide.

Question 8

State five different characteristics of compounds. Give three differences between elements & compounds with relevant examples.

Answer

Five characteristics of compounds are:

(i) Components in a compound are in a definite proportion.

(ii) Particles in a compound are of one kind

(iii) Compounds have a definite set of properties

(iv) Elements in the compound do not retain their original properties.

(v) Components in a compound can be separated by chemical means only.

Three differences between elements & compounds are:

Mixtures

Question 9

Explain the term 'mixture'. Differentiate between homogeneous & heterogeneous mixtures. State why brass is considered as a homogeneous mixture while a mixture of iron & sulphur - heterogeneous. Give an example of two liquids which form (a) homogeneous (b) heterogeneous mixtures.

Answer

A mixture is made up of two or more substances — elements or compounds or both — mechanically mixed together in any proportion.

Difference between homogeneous and heterogeneous mixtures:

Brass is considered as a homogeneous mixture because:

1. The constituents are uniformly mixed and
2. Properties and composition are same throughout the mixture.

A mixture of iron & sulphur is considered a heterogeneous mixture because:

1. The constituents are not uniformly mixed and
2. Properties and composition vary throughout the mixture.

(a) Two liquids forming homogeneous mixture : Alcohol and Water

(b) Two liquids forming heterogeneous mixture : 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

Separation of mixtures

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 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) Two solid mixtures one of which - directly changes into vapour on heating — Sublimation

(b) Two solid mixtures one of which — dissolves in a particular solvent and other does not — Solvent extraction

(c) A solid-liquid mixture containing - an insoluble solid in the liquid component — Filtration

(d) A solid-liquid mixture containing - a soluble solid in liquid component — Evaporation

(e) A liquid-liquid mixture containing - two immiscible liquids having different densities — Separating Funnel

(f) A liquid-liquid mixture containing - two miscible liquids having different boiling points — Fractional Distillation

(g) A liquid-gas mixture containing - a gas dissolved in a liquid component — Boiling the liquid gas mixture

(h) A gas-gas mixture containing - two gases with different densities — Diffusion

(i) A mixture of different solid constituents - in a liquid constituent — 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 and 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 i.e lead nitrate solution.

(g) Carbon tetrachloride from a mixture of carbon tetrachloride [heavier component] & water.

(h) Benzene from a mixture of benzene [b.p. 80°C] & toluene [b.p. 110°C].

(i) Different dyes in their liquid constituent ink.

Answer

(a) Sublimation can be used to separate Ammonium chloride from a mixture of ammonium chloride and potassium chloride.

Process — Take a mixture of ammonium chloride and potassium chloride in an evaporating dish, covered with a funnel plugged with cotton. Heat it slowly as shown below:

Ammonium chloride collects on the inner side of the funnel and is scrapped out easily.

Potassium chloride remains in the evaporating dish and is separated out easily.

(b) Magnetic separation can be used to separate iron from a mixture of iron & copper.

Process — Take a mixture of iron & copper in a flat dish. Move a magnet over the mixture as shown below:

Magnetic solid 'Iron' is attracted to the magnet.

Non-magnetic solid 'copper' remains in the dish.

(c) Solvent extraction can be used to separate Sulphur from a mixture of sulphur & copper

Process — Add the mixture to a beaker containing the solvent carbon disulphide. Stir well till sulphur dissolves. Filter the contents.

Soluble component [sulphur] is obtained on slow evaporation of the filtered solution.

Insoluble component [copper] remains on the filter paper.

(d) Fractional crystallization can be used to separate potassium nitrate from a mixture of potassium nitrate & potassium chlorate.

Process — Add the mixture of potassium nitrate & potassium chlorate to a beaker of water & heat to obtain, hot saturated soln. Cool the hot saturated soln. when crystals of less soluble 'potassium chlorate' crystallize out.

Less soluble [potassium chlorate] is filtered out from the hot saturated soln. recrystallized from hot water and dried.

More soluble [potassium nitrate] is obtained from the filtrate by heating the filtrate followed by cooling. The crystals of the more soluble solid crystallize out.

(e) Filtration can be used to separate insoluble lead carbonate from a mixture of lead carbonate & water.

Process — Place a wet folded filter paper in a funnel, clamped above a beaker. Pour the mixture carefully through it.

Insoluble lead carbonate remains on filter paper, it is dried and recovered.

Water remains in the beaker.

(f) Distillation can be used to separate lead nitrate [soluble] from a mixture of lead nitrate & water i.e lead nitrate solution.

Process — Add the mixture to the flask 'X' of the distillation apparatus. The flask is then carefully heated and the mixture evaporates and later condenses.

Soluble lead nitrate remains in the flask 'X' after evaporation.

Water collects in receiver after condensation.

(g) Separating funnel can be used to separate carbon tetrachloride from a mixture of carbon tetrachloride [heavier component] & water.

Process — Pour the mixture from the top of a separating funnel and allow the mixture to stand till the heavier layer [carbon tetrachloride] and lighter layer [water] separate.

Water remains in the separating funnel and carbon tetrachloride collects in the flask on opening the tap.

(h) Fractional distillation can be used to separate benzene from a mixture of benzene [b.p. 80°C] & toluene [b.p. 110°C]

Process — Mixture is kept in a distillation flask 'X' with a fractionating column. The flask is then heated as shown and the mixture evaporates and later condenses.

Higher boiling point liquid [toluene] remains in the flask X, after condensation.

Lower boiling point liquid [benzene] collects in the receiver Y, after condensation in Leibigs condenser.

(i) Chromatography can be used to separate different dyes - in their liquid constituent ink.

Process — Place the ink spot containing different dyes on the filter paper.

The filter paper is hung with its lower end completely dipped in the solvent.

The solvent flows over the ink spot and the dyes separate out as shown below.

Objective Type Questions

Question 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

1. A diatomic molecule

2. A metalloid

3. A non-metal which is lustrous

4. A mixture consisting of elements & a compound

5. A noble gas

Answer

1. A diatomic molecule — E: Bromine

2. A metalloid — C: Boron

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

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

5. A noble gas — D: Helium

Question 2

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

Answer

Question 3

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

1. Can two immiscible liquids be separated by this process.

2. Separation of liquids by this process is based on which physical property.

3. If methyl alcohol & water are to be separated, which liquid would remain in flask 'X' after condensation.

4. Give a reason for the above answer.

5. State the purpose of the fractionating column in the apparatus.

Answer

1. No, immiscible liquids are separated by separating funnel.

2. Separation of liquids by this process is based on difference in boiling points of the two liquids.

3. Water remains in flask 'X' after condensation.

4. As water has a higher boiling point so it remains in flask 'X' after condensation and methyl alcohol having lower boiling point boils and is collected in receiver after condensation.

5. Fractionating column helps in condensation & separation of vapours.

Question 4

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

1. Dust in air is an example of ............... [heterogeneous/homogeneous] mixture.

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

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

4. Compounds are ............... [homogeneous or heterogeneous/ always homogeneous] in nature.

5. An example of a monoatomic molecule is ............... [hydrogen/helium].

Answer

1. Dust in air is an example of homogeneous mixture.

2. A soluble solid is separated from an insoluble solid by solvent extraction.

3. The reactive element from the two monoatomic elements is silicon.

4. Compounds are always homogeneous in nature.

5. An example of a monoatomic molecule is helium.

Question 5

Give reasons for the following statements:

1. Components in a mixture can be separated by physical methods only.

2. Centrifugation can be used for separating an insoluble heavier solid, present in an insoluble solid-liquid mixture.

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.

4. Brass & bronze are examples of mixtures, while copper sulphate & lead nitrate are examples of compound.

5. Zinc is considered an element, while zinc sulphide is considered a compound.

Answer

1. A mixture is made by mixing two or more substances mixed mechanically and the components retain their original properties, hence, components in a mixture can be separated by physical methods only.

2. Centrifugation is used for separating an insoluble heavier solid, present in an insoluble solid-liquid mixture as more dense component migrates away from the axis of the centrifuge and less dense component migrates towards the axis of the centrifuge. Hence, the two components can be separated.

3. On moistening the filter paper cone, it sticks to the walls of the funnel and makes filtration easy.

4. Brass & bronze are examples of mixtures as the components are mixed together mechanically and the components retains their original properties. Copper sulphate & lead nitrate are examples of compounds as the components are combined chemically in fixed proportions and they do not retain their original property.

5. Zinc is considered an element as it is made up of one kind of atoms only, while zinc sulphide is considered a compound as it is made up of two elements [zinc and sulphur] combined in a fixed proportion.