Is Matter Around Us Pure 

Q97. An element is sonorous and highly ductile. Under which category would you classify this element? What other characteristics do you expect the element to possess? ‘

Ans. It is a metal.

Metals usually show some or all of the following properties:

• They have a lustre (shine).

• They have silvery-grey or golden-yellow colour.

• They conduct heat and electricity.

• They are ductile (can be drawn into wires).

• They are malleable (can be hammered into thin sheets).

• They are sonorous (make a ringing sound when hit).

Q98. Classify each of the following, as a physical or a chemical change. Give reasons.

(a) Drying of a shirt in the sun.

(b) Rising of hot air over a radiator.

(c) Burning of kerosene in a lantern.

(d) Change in the colour of black tea on adding lemon juice to it.

(e) Churning of milk cream to get butter.

Ans. Physical Change

(a) Drying of a shirt in the sun.

(b) Rising of hot air over a radiator.

(e) Churning of milk cream to get butter.

There is no change in chemical composition.

Chemical Change

(c) Burning of kerosene in a lantern.

(d) Change in the colour of black tea on adding lemon juice to it.

New substances are formed.

Q99. How can we obtain pure copper sulphate from an impure sample?

Ans. Steps

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

• Dissolve it in minimum amount of water.

• Filter the impurities out.

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

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

• You will obtain the crystals of copper sulphate in the china dish.

• This process is called crystallisation.

Q100. What is a suspension? Write its properties.

Or

What are the properties of suspension?

Ans. A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. Particles of a suspension are visible to the naked eye.

Properties of a Suspension

1. Suspension is a heterogeneous mixture.

2. The particles of a suspension can be seen by the naked eye.

3. The particles of a suspension scatter a beam of light passing through it and make its path visible.

4. The solute particles settle down when a suspension is left undisturbed, that is, a suspension is unstable. They can be separated from the mixture by the process of filtration.

Q101. What is the principle of centrifugation write its application?

Ans. Sometimes the solid particles in a liquid are very small and pass through a filter paper. For such particles the filtration technique cannot be used for separation. Such mixtures are separated by centrifugation. The principle is that the denser particles are forced to the bottom and the lighter particles stay at the top when spun rapidly.

Applications

• Used in diagnostic laboratories for blood and urine tests.

• Used in dairies and home to separate butter from cream.

• Used in washing machines to squeeze out water from wet clothes.

Q102. How can we obtain coloured component (dye) from blue/black ink? 

Ans. Steps

1. Fill half a beaker with water.

2. Put a watch glass on the mouth of the beaker.

3. Put few drops of ink on the watch glass.

4. Now start heating the beaker. 

5. We will see that evaporation is taking place from the watch glass.

6. Continue heating as the evaporation goes on and stop heating when we do not see any further change on the watch glass.

7. Now we will observe that the water get evaporated from the watch glass and residue is left in the watch glass. This residue is the coloured component (Dye).

Image from NCERT

We find that ink is a mixture of a dye in water. Thus, we can separate the volatile component (solvent) from its non-volatile solute by the method of evaporation.

Q103. How can we separate a mixture of two immiscible liquids?

Or

How will you separate a mixture of kerosene oil and water?

Ans. We can separate kerosene oil from water using a separating funnel.

Steps

• Pour the mixture of kerosene oil and water in a separating funnel.

• Let it stand undisturbed for some time so that separate layers of oil and water are formed.

• Open the stopcock of the separating funnel and pour out the lower layer of water carefully.

• Close the stopcock of the separating funnel as the oil reaches the stop-cock.

 Image from NCERT

Q104. Name the process associated with the following:

(a) Dry ice is kept at room temperature and at one atmospheric pressure.

(b) A drop of ink placed on the surface of water contained in a glass spreads throughout the water.

(c) A potassium permanganate crystal is in a beaker and water is poured into the beaker with stirring.

(d) An acetone bottle is left open and the bottle becomes empty.

(e) Milk is churned to separate cream from it.

(f) Settling of sand when a mixture of sand and water is left undisturbed for some time.

(g) Fine beam of light entering through a small hole in a dark room, illuminates the particles in its paths.

Ans. (a) Sublimation of dry ice (solid) to C0₂ (gas)

(b) Diffusion of ink into water

(c) Diffusion or dissolution of solid into liquid

(d) Evaporation, diffusion of acetone in air

(e) Centrifugation

(f) Sedimentation

(g) Tyndall effect – Scattering of light

Q105. What do you mean by solute and solvent? Explain with the help of examples.

Ans. A solution has a solvent and a solute as its components. The component of the solution that dissolves the other component in it (usually the component present in larger amount) is called the solvent. The component of the solution that is dissolved in the solvent (usually present in lesser quantity) is called the solute.

Examples:

(i) A solution of sugar in water is a solid in liquid solution. In this solution, sugar is the solute and water is the solvent.

(ii) A solution of iodine in alcohol known as ‘tincture of iodine’, has iodine (solid) as the solute and alcohol (liquid) as the solvent.

Q106. Classify the following as chemical or physical changes:

• cutting of trees,

• melting of butter in a pan,

• rusting of almirah,

• boiling of water to form steam,

• passing of electric current, through water and the water breaking down into hydrogen and oxygen gases,

• dissolving common salt in water,

• making a fruit salad with raw fruits, and

• burning of paper and wood.

Ans.

• cutting of trees – physical change

• melting of butter in a pan - physical change

• rusting of almirah – chemical change

• boiling of water to form steam - physical change

• passing of electric current, through water and the water breaking down into hydrogen and oxygen gases - chemical change

• dissolving common salt in water - physical change

• making a fruit salad with raw fruits - physical change

• burning of paper and wood - chemical change