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Question

(a) Explain the process of preparation of soap in laboratory.
(b) Why is common salt (sodium chloride) added during the preparation of soap?
(c) Why is soap not suitable for washing clothes when the water is hard?


Solution

(a) Following is the process of preparing soap in the laboratory:
Raw materials required are:

(i) Vegetable oil like castor oil
(ii) Sodium hydroxide (caustic soda), and
(iii) Sodium chloride (common salt)

Steps involved in the preparation of soap are:

1. Take a beaker with about 20 ml of castor oil in it.
2. Also, add 30 ml of 20% sodium hydroxide to the beaker.
3. Heat the mixture and stir it constantly until a soap is formed.
4. Add 5-10 grams of common salt to it and stir the mixture. On cooling the solution, we will see the solid soap getting separated.
5. The solid soap, once separated, is made to set, and then it is cut into pieces called soap bars.

(b) Common salt (sodium hydroxide) is added during the preparation of soap to separate the soap from the aqueous solution. On adding common salt, solubility of the soap, present in the solution, decreases. This allows the soap to be taken out from the solution in its solid form.

(c) (i) Hard water contains calcium and magnesium salts; and, when a soap is used for washing clothes with hard water, a good amount of it gets utilised in reacting with the calcium and magnesium ions to form an insoluble scum. So, a large amount of soap gets wasted in this process.

(ii) The curdy precipitate called scum, produced by the reaction of hard water on soap, sticks to the clothes making them difficult to be cleaned. Therefore, soaps are not suitable for washing clothes in hard water.


Science
Lakhmir Chemistry 2013
Standard X

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