Elements, Compounds and Mixtures

Exercise I

Question 1

Write the symbols of helium, silver, krypton, antimony, barium.

Answer

Helium (He)

Silver (Ag)

Krypton (Kr)

Antimony (Sb)

Barium (Ba)

Question 2

Write the names of the following elements

Na, C, Kr, U, Ra, Fe, Co

Answer

Na — Sodium

C — Carbon

Kr — Krypton

U — Uranium

Ra — Radium

Fe — Iron

Co — Cobalt

Question 3

Define:

(a) Elements

(b) Compounds

Answer

(a) An element is a pure substance that cannot be converted further 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 formed by the chemical combination of two or more elements in a fixed ratio by mass.

Question 4

Name the main metal present in the following:

(a) Haemoglobin

(b) Chalk

(c) Chlorophyll

(d) Chocolate wrappers

Answer

(a) Iron

(b) Calcium

(c) Magnesium

(d) Aluminium

Question 5

Give four examples of non-metallic elements.

Answer

Hydrogen(H), Chlorine(Cl), Sulphur(S), Phosphorus(P)

Question 6

What do you understand by:

(a) Metalloids

(b) Noble gases

Give two examples of each.

Answer

(a) Metalloids are those elements that show some properties of metals and some properties of non-metals. They are hard solids. Examples — Boron(B), Silicon(Si).

(b) Noble gases or inert gases are those elements that do not react chemically with other elements or compounds and they are found in air in traces. Examples — Helium(He), Neon(Ne).

Question 7

Select elements and compounds from the following list:

Iron, plaster of paris, chalk, common salt, copper, aluminium, calcium oxide, cane sugar, carbon, silica, sodium sulphate, uranium, potassium carbonate, silver, carbon dioxide.

Answer

Exercise II

Question 1

State four differences between compounds and mixtures.

Answer

Question 2

What are the characteristic properties of a pure substance? Why do we need them?

Answer

The characteristic properties of a pure substance are:

1. They have a definite composition and a definite set of properties like boiling point, melting point, density etc.
2. They are all homogeneous i.e. their composition is uniform throughout the bulk.

We need pure substances to:

1. Prepare medicines.
2. Maintain good health of human beings.
3. Prepare chemicals in laboratories.

Question 3

Give two examples for each of the following:

(a) Solid + Solid mixture

(b) Solid + Liquid mixture

(c) Liquid + Liquid mixture

Answer

(a) Sand and sugar, Brass

(b) Sand and water, salt in water

(c) Oil in water, Alcohol and water

Question 4

Define:

(a) Evaporation

(b) Filtration

(c) Sublimation

(d) Distillation

(e) Miscible liquids

(f) Immiscible liquids

Answer

(a) Evaporation is the process of converting a liquid into its vapour state either by exposing it to air or by heating.

(b) The process of separating insoluble solid particles from a liquid by allowing it to pass through a filter is called filtration.

(c) The process in which a solid substance directly changes into its vapour state on heating without passing through its liquid state is called sublimation.

(d) Distillation is the method of getting a pure liquid from a solution by evaporating and then condensing the vapours.

(e) Liquids which dissolve in each other completely in all proportions are called miscible liquids.

(f) Liquids which do not dissolve in each other are called immiscible liquids.

Question 5

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

(a) Iron and sulphur

(b) Ammonium chloride and sand

(c) Common salt from sea water

(d) Chaff and grain

(e) Water and mustard oil

(f) Sugar and water

(g) Cream from milk

Answer

(a) Magnetic separation

(b) Sublimation

(c) Evaporation

(d) Winnowing

(e) Separating funnel

(f) Crystallization

(g) Centrifugation

Question 6

How will you separate a mixture of common salt, chalk powder and powdered camphor? Explain.

Answer

On heating the mixture, camphor will evaporates as it is a sublime substance. Its vapours can be collected which will solidify on cooling. Common salt and chalk powder mixture is then mixed in water. Common salt gets dissolved in water while chalk powder does not. Then by filtration, chalk powder is removed from the solution as it gets deposited in filter paper. The filtrate contains water and common salt. Common salt can be separated from water by evaporation process.

Question 7

How is distillation more advantageous than evaporation?

Answer

Distillation is more advantageous than evaporation because both the components of the solid-liquid mixtures are obtained in distillation process but in evaporation process only solid is obtained.

Question 8

(a) What is chromatography?

(b) Why is it named so?

(c) What are the advantages of chromatography?

(d) Name the simplest type of chromatography?

(e) On what principle is this method based?

(f) What is meant by stationary phase and mobile phase in chromatography?

Answer

(a) The process of separating different dissolved constituents of a mixture by their adsorption(particles getting adhered to a surface) on an appropriate material is called chromatography.

(b) The word chromatography means colour writing. It is named so because earlier it was used to separate mixtures containing coloured components only.

(c) 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.

(d) Paper chromatography.

(e) Chromatography separates the components of a mixture on the basis of differences between two phases, one of which is stationary while the other is mobile.

(f) The stationary phase is made up of common adsorbents like filter paper, silica gel as they are stationary and the mobile phase is made up of common solvents like water, ethyl alcohol, acetic acid etc. which moves or spreads on the stationary phase.

Question 9

On what principle are the following methods of separation based? Give one example of a mixture for each of the methods mentioned in which they are used

(a) sublimation

(b) filtration

(c) sedimentation and decantation

(d) solvent extraction method

(e) magnetic separation

(f) by using a separating funnel

(g) fractional distillation

Answer

(a) This method is used for solid mixtures in which one of the components can sublime on heating. It is used to separate a mixture of common salt and ammonium chloride.

(b) This method is used for separating the components of a heterogeneous solid-liquid mixture in which solids are insoluble in liquids. A mixture of chalk and water can be separated by this method.

(c) This method is used when the solid component is insoluble and heavier than the liquid component. A mixture of sand and water can be separated by this method.

(d) This method is used when one component is soluble in a particular liquid while the other component is insoluble. A mixture of sand and salt can be separated by this method.

(e) This method is used when one of the components of the mixture is magnetic in nature. A mixture of iron and sulphur can be separated by this method.

(f) It is used to separate components of liquid-liquid heterogeneous mixture when one of the liquid components is heavier than the other. Kerosene oil and water can be separated by this method.

(g) It is used to separate components of homogeneous liquid-liquid mixture on the basis of their difference in boiling points. A mixture of water and alcohol can be separated by this method.

Objective Type Questions

Question 1

Fill in the blanks

(a) ............... are made up of same kind of atoms.

(b) ............... and ............... are pure substances.

(c) In a ..............., the substances are not combined chemically.

(d) Clay is separated from water by a method called ............... .

(e) ............... is a process to obtain a very pure form of a solid dissolved in a liquid.

(f) Camphor and ammonium chloride can ............... .

Answer

(a) Elements

(b) Elements, compounds

(c) mixture

(d) filtration

(e) Crystallization

(f) sublime

Question 2

Give one word answers for the following:

(a) The solid particles which remain on the filter paper after the filtration.

(b) The liquid which evaporates and then condenses during the process of distillation.

(c) The process of transferring the clean liquid after the solid settles at the bottom of the container.

(d) The process by which two miscible liquids are separated.

Answer

(a) residue

(b) distillate

(c) decantation

(d) fractional distillation

Question 3a

Choose the most appropriate answer from the options given below:

A pure liquid is obtained from a solution by:

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

Answer

distillation

Reason — Distillation is the method of getting a pure liquid from a solution by evaporating and then condensing the vapours.

Question 3b

Choose the most appropriate answer from the options given below:

Components of crude petroleum can be separated by:

1. distillation
2. evaporation
3. filtration
4. fractional distillation

Answer

fractional distillation

Reason — Petrol, kerosene, diesel etc. are obtained from crude petroleum by fractional distillation as they have different boiling points.

Question 3c

Choose the most appropriate answer from the options given below:

Example of a homogeneous mixture is:

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

Answer

tap water

Reason — Tap water is a homogeneous mixture because it contains small amounts of dissolved salts and air in it that we cannot see.

Question 3d

Choose the most appropriate answer from the options given below:

In chromatography, the filter paper is:

1. stationary phase
2. mobile phase
3. mixture
4. none of the above

Answer

stationary phase

Reason — The stationary phase in the process of chromatography is made of common adsorbents like filter paper.

Question 3e

Choose the most appropriate answer from the options given below:

A set of mixtures is:

1. ink, honey, ice cream, milk
2. tap water, gold, common salt, alloy
3. milk, brass, silver, honey
4. butter, petroleum, tap water, iron

Answer

ink, honey, ice cream, milk

Reason — They are homogeneous mixtures where constituents are uniformly distributed throughout its volume and cannot be recognized separately.