Q1. What is meant by crystallisation? How is impure copper sulphate purified by the process?


Crystallization is a process that separates a pure solid in the form of its crystals from a solution. The various steps of purification used for impure copper sulphate are:

1.Take some (approximately 5 g) impure sample of copper sulphate in a china dish.

2.Dissolve it in minimum amount of water and filter out the impurities.

3.Evaporate water from the copper sulphate solution so as to get a saturated solution.

4.Cover the solution with a filter paper and leave it undisturbed at room temperature to cool slowly for a day.

5.Then crystals of copper sulphate in the china dish are obtained.

Q2. Why is the mixture of sand and sugar called a heterogeneous mixture?


The mixture of sand and sugar is called a heterogeneous mixture because: a. Different parts of this mixture will have different sand-sugar compositions. b. There is a visible boundary of separation between sand and sugar particles.

Q3. Explain the process by which we can separate cream from milk?


Milk is a suspension of tiny droplets of oil (cream) in a watery liquid. The process of centrifugation is used to separate cream from milk. The milk is put in a close container in big centrifuge machine. When the centrifuge machine is switched on, the milk is rotated (or spun) at a very high speed in its container. The centrifugal force acts on the milk and due to this, the milk separates into cream and skimmed milk. The cream, being lighter, floats over the skimmed milk and can then be removed.

Q4. If you are provided with two liquids, how will you find out which one is a pure compound and which one is a mixture (solution)?


In order to distinguish between the given two liquids, we should evaporate them separately. The liquid which leaves behind a residue on evaporation is a mixture (solution) whereas the liquid which evaporates completely, leaving behind no residue is a compound.  

Q5. Smoke and fog are aerosols. How do they differ from each other?


The dispersed phase in smoke is solid while it is liquid in fog. The dispersion medium is gas in both.

Q6. Which method will you use to separate the following mixtures?

(i) Sand and water

(ii) Clay and water

(iii) Cream and milk

(iv) Used tea leaves and prepared tea


  • Sand and water: Filtration
  • Clay and water: Centrifugation
  • Cream and milk: Centrifugation

(iv) Used tea leaves and prepared tea: Filtration

Q7. How is scrap iron separated from the waste materials in factories?


In factories, scrap iron is separated from the heap of waste materials by using big electromagnets fitted to a crane. When a such a crane is lowered on to the heap of waste materials, then the scrap iron objects present in the heap, cling to the electromagnet. The crane is then moved up and away to drop these scrap iron objects at a separate place.

Q8. What happens when a few drops of lemon juice are added to milk?


When a few drops of lemon juice are added to milk, chemical change takes place resulting in the formation of lactic acid which turns the milk sour and changes it into ‘paneer’. The properties of paneer are different from those of milk.

Q9. Describe what happens when we burn a piece of paper? What type of a change is this?


When a piece of paper is burnt, entirely new substances like carbon dioxide, water vapour, smoke and ash are formed. Hence, burning of paper is a chemical change.

Q10. (a) What is meant by concentration of a solution? (b) The concentration of a salt solution in terms of mass percentage is 20% and the mass of the solution is 550 g. Determine the mass of solute present in the solution.


(a) The concentration of a solution is the amount of solute present in a given quantity of the solution. (b) Given: Mass percentage of solution = 20% Mass of solution = 550 g Mass of solute = m Since mass percentage of solution = (mass of solute/ mass of solution) X 100 Therefore, Mass of Solute = (Mass percentage of Solution x Mass of Solution) / 100 Substituting the values we get m = (20 x 550)/100    = 110 g

Q11. How will you differentiate between true solution and colloidal solution by passing light through them?


True solution Colloidal solution In a true solution, the light will pass through the solution relatively unimpeded and invisible from side view. Particles in a colloid scatter light. If a beam of light is passed through a colloid, the light is scattered by colloidal particles and the path of light is illuminated as a blue cone called Tyndall cone and is known as Tyndall effect

Q12. State any two differences between a mixture and a compound.


1. Mixture has a variable composition whereas a compound has a fixed composition. 2. A mixture shows the properties of the constituent elements whereas in a compound, the new substance has totally different properties from its constituents.

Q13. What is Tyndall effect?


Particles in a colloid scatter light. If a beam of light is passed through a colloid, the light is scattered by colloidal particles and the path of light is illuminated as a blue cone called Tyndall cone and is known as Tyndall effect.

Q14. Why do we need to separate different components of a mixture?


We need to separate different components of a mixture to remove unwanted substances which are harmful for us.

Q15. What is meant by a saturated solution? What happens when a saturated solution is heated?


When no more solute can be dissolved in a solution at a given temperature, it is called a saturated solution. On heating, a saturated solution becomes unsaturated.

Q16. Name any four solid substances which do not undergo sublimation.


Common salt, sand, iron filings and sulphur are the substances which do not undergo sublimation.

Q17. Classify the following into (a) solid sol (b) Aerosol (c) Emulsion. Mist, Colored glass, Hair cream, Gem stones, Fog, Milk


(a) Solid sol: Colored glass, Gem stones (b) Aerosol: Fog, Mist (c) Emulsion: Milk, Hair cream

Q18. (a) What is Tyndall Effect? (b) “Tyndall Effect can be observed when sunlight passes through the canopy of dense forest.” Explain how this occurs.


(a) Scattering of beam of light by particles (suspension/colloids) is called Tyndall Effect. (b) Forest contains mist which contains tiny droplets of water which act as particles of colloid dispersed in air and scatter the light falling on them hence show tyndall effect.

Q19. What type of change is ‘cutting of trees’?


Cutting of trees is a physical change because only the shape and size of the trees change and no new products are formed during this process.

Q20. Elements are classified as metals, non-metals and metalloids. Give any one property of each. Also give one examples of each.


(i) Metals:Iron They are lustrous and good conductors of electricity.   (ii) Non – Metals: Oxygen They are non – lustrous and poor conductors of electricity.   (iii) Metalloids: Germanium  They show intermediate properties between those of metals and non-metals and are semiconductors.

Q21. Why is salt solution considered a mixture?


A salt solution is considered a mixture because of the following reasons:

1. Salt solution shows the properties of both its constituents, salt as well as water.

2. Salt solution can be separated into salt and water by physical process like distillation.

3. The composition of salt solution is variable.

4. Salt solution does not have a fixed boiling point.

Q22. What is meant by the term unsaturated solution?


A solution in which more quantity of solute can be dissolved without raising its temperature is called an unsaturated solution.

Q23. Describe the filtration tank used in the process of water purification in water works.


During the process of water purification, water which comes to the filtration tank has impurities and suspended particles. Filtration tank which has three layers: fine sand layer at the top, a coarse sand layer in the middle and gravel at the bottom. In this process, the layers of sand and gravel act as filters. When water passes though these layers, even the smallest of suspended particles are removed. Water from a river or a lake is brought through canals or long pipes to the water work where it is mixed with required quantities of alum and soda lime solutions. The substances react with one another to form aluminium hydroxide, a jelly-like, sticky solid. It is then pumped into big settling tanks, where most of the suspended impurities settle down in two or three days. The clear water still containing some suspended matter is passed through successive filters of boulders, gravel, coarse sand and fine sand. The clear water from the filters is chlorinated and then passed to the reservoirs for distribution in the city.  

Q24. Simple distillation is used for what types of mixtures?


Simple distillation is used for the separation of components of mixture containing two miscible liquids that boil without decomposition and have sufficient difference in their boiling points.

Q25. How is centrifugation done? Describe the process of separation by centrifugation.


Centrifugation is done by using a machine called centrifuge. In the method of centrifugation, the mixture of fine suspended particles in a liquid is taken in a test-tube. The test-tube is placed in a centrifuge machine and rotated rapidly for some time. As the mixture rotates round rapidly, centrifugal force acts on the heavier suspended particles in it and forces them down to the bottom of the test-tube. The clear liquid, being lighter, remains on top.

Q26. Among the substances given below choose the element, mixture and compound. (a)Air (b)Lead (c)Diamond (d)Calcium Carbonate


Element: Diamond, Lead Mixture: Air Compound: Calcium Carbonate

Q27. On what basis does the separation of different components of black ink takes place in chromatography?


As water moves up the filter paper strip having a small drop of black ink, it takes along the dyes present in ink. The dye which is more soluble in water dissolves first, rises faster and produces a coloured spot on the paper at a higher position. The less soluble dyes dissolve a little later, rise slower and form coloured spots at lower heights.

Q28. Explain how during the burning of a candle, both physical and chemical changes take place.


Wax of the candle is made of carbon and hydrogen which reacts with oxygen on burning and changes into carbon dioxide gas and water which escapes into the air. This is a chemical change. As a result, candle melts and reduces in size. This is a physical change. So, during the burning of a candle, both physical and chemical changes take place.

Q29. Give difference between dilute and concentrated solution.


Dilute Solution Concentrated Solution A solution is said to be dilute if there is less amount of solute dissolved in it. A solution is said to be concentrated if it has more amount of solute dissolved in it.

Q30. What do you mean by the terms: solute and solvent? Identify them in the following solutions: (i) Sugar solution (ii) Soft drinks.


A solution is a homogeneous mixture of two or more non reacting Substances. The major component, which is used in larger quantity, is called solvent and the substance which is used in lesser quantity is called solute. (i) Sugar water is a solution of sugar in water. So it has sugar as solute and water as solvent. (ii) Soft drinks contain carbon dioxide as solute and water as solvent.