Page 62 - Ripples SCIENCE 7 - TEJPUR Edition 2024 Answer Key

Principle: On increasing the temperature, the solubility of a substance in a solution

also increases. Hence, when this supersaturated solution is cooled down, the excess
salt turns into crystals.

Material required: Beaker, water, dilute
sulphuric acid, copper sulphate powder

and burner
Procedure:

• Take a beaker half-filled with water
and add a few drops of dilute

sulphuric acid.
• Heat the water till it starts boiling.

• Slowly start adding copper sulphate
powder while continuously stirring.

• Keep adding copper sulphate powder till the solution becomes saturated, i.e.
no more powder can be dissolved in it.

• Remove the impurities by filtering the solution and allow the solution to cool
down. Ensure that it remains undisturbed while cooling.

• Observe the solution after a few hours and record your observations.

Observations: You will see the crystals of copper sulphate. After some more time,
you will see that the size of the crystals has grown.

3. Give an example of a chemical reaction for each of the following.
(a) Change in colour

Ans. Change in Colour : When potassium permanganate (KMnO4) crystals are dissolved
in water, they have a deep purple colour. However when hydrogen peroxide is added

to this solution, which is a colourless liquid, manganese dioxide which is brown in

colour, is formed.
(b) Evolution of a gas

Ans. Evolution of gas : The reaction between calcium carbonate & hydrochloric acid
results in the evolution of carbon dioxide gas.

(c) Heat is released
Ans. Heat is Released: The reaction between water and calcium chloride to form

hydrochloric acid & calcium hydroxide results in the evolution of heat.

4. Why do iron filings acquire a brown colour when dipped in copper sulphate solution?
Ans. When iron filings are dipped into a solution of copper sulphate they turn brown

because of a chemical reaction that happens between iron & copper in the solution.

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