Is Matter Around Us Pure

Activity: 2.3 (Chapter: Is matter around us pure)

• Take approximately 50 mL of water each in two separate beakers.

• Add salt in one beaker and sugar or barium chloride in the second beaker with continuous stirring.

• When no more solute can be dissolved, heat the contents of the beaker to raise the temperature by about 5°C.

• Start adding the solute again.

Question : Is the amount of salt and sugar or barium chloride that can be dissolved in water at a given temperature, the same?

Answer

No. The amount of salt, sugar, and barium chloride that can be dissolved in water at a given temperature is not the same.

Explanation

This happens because the solubility of all salt is not the same in a solvent.

The different substance has a different solubility in a given solvent at a particular temperature.

Solubility The ability of solute to be dissolved in a solvent at a particular temperature, is called Solubility.

Solute The component which is present in less amount in a mixture of two or more than two components, is called solute.

Solvent The component which is present in more compare to other component(s) in a mixture is called the Solvent.

In the given activity, salt, sugar, and barium chloride are examples of solutes. And water is an example of a solvent.

Activity: 2.4 (Chapter: Is matter around us pure)

• Fill half a beaker with water.

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

• Put a few drops of ink on the watch glass.

• Now start heating the beaker. We do not want to heat the ink directly. You will see that evaporation is taking place from the watch glass.

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

• Observe carefully and record your observations.

Now answer

• What do you think has got evaporated from the watch glass?

• Is there a residue on the watch glass?

• What is your interpretation? Is ink a single substance (pure) or is it a mixture?

Answer

Observation

(a) After some time of starting heating, the evaporation has started taking place on the watch glass.

(b) After some time, a coloured residue was found on the watch glass.

Question (a) What do you think has got evaporated from the watch glass?

Answer

The liquid solvent from the ink has got evaporated leaving a solid component of ink on the watch glass.

Question (b) Is there a residue on the watch glass?

Answer

Yes. A residue was left on the watch glass.

Question (c) What is your interpretation? Is ink a single substance (pure) or is it a mixture?

Answer

Ink is a mixture of two or more components.

Activity: 2.5 (Chapter: Is matter around us pure)

• Take some full-cream milk in a test tube.

• Centrifuge it by using a centrifuging machine for two minutes. If a centrifuging machine is not available in the school, you can do this activity at home by using a milk churner, used in the kitchen.

• If you have a milk dairy nearby, visit it and ask (i) how they separate cream from milk and (ii) how they make cheese (paneer) from milk.

Now answer

• What do you observe on churning the milk?

• Explain how the separation of cream from milk takes place.

Question : What do you observe on churning the milk?

Answer

After churning milk is separated into two layers. These layers are fats and milk without fats. Fats present in the milk start floating at the upper layer and milk without fat forms the lower layer.

Question : Explain how the separation of cream from milk takes place.

Answer

In the churning machine milk is poured and moved in a circular direction with a speed continuously for some time.

Cream, which is fat and solid and has less density than water present in the milk, is separated out and starts floating above the milk. This happens because of centrifugal and centripetal forces applied while moving the milk in the churning machine.

The fats (cream) are separated out and milk without fat is drained out of the churning container.

Question : If you have a milk dairy nearby, visit it and ask (i) how they separate cream from milk and (ii) how they make cheese (paneer) from milk.

(i) How they separate cream from milk?

Answer

To separate cream from milk a machine called a cream separator or milk churning is used.

Milk is poured into the container and moved rapidly. By moving the milk kept in the container, fats, which are colloids, suspended in the milk start separating from the milk.

The cream is drained out from an outlet attached to the churning machine and fat free milk (toned milk) is drained through another opening of the machine.

Cream separated from the milk is processed further for obtaining ghee and toned milk (fat free milk) is sold in the market.

(ii) How they make cheese (paneer) from milk?

Answer

Heat is supplied to the milk to make it Luke warm. After that vinegar or other lemon or paneer water is added to this Luke warm milk by stirring it. After some time, the paneer starts separating from the milk. After about one-hour paneer is separated out using a strainer and sent for further processing, like making sweets (ras-gulla a type of sweet) or sent it to the market for selling.

Activity: 2.6 (Chapter: Is matter around us pure)

• Let us try to separate kerosene oil from water using a separating funnel.

• Pour the mixture of kerosene oil and water into 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.

Answer and Explanation

Kerosene is lighter than water.

When kerosene and water are mixed kerosene starts floating on the water.

The mixture of kerosene and water can be separated out using a separating funnel.

After pouring the mixture of kerosene and water into the separating funnel and leaving it for some time, kerosene starts floating on the upper layer, and water forms the lower layer.

The water in the mixture is drained out slowly and carefully by opening the stop cork present at the bottom of the separating funnel.

This method is used to separate a mixture of two or more immiscible liquids.