(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?
(a) The steps of preparation of soap in the laboratory are given below:
- Take about 20 ml of castor oil (cottonseed oil, linseed oil, or soya bean oil) in a beaker.
- Add 30 ml of 20% sodium hydroxide solution to it.
- The mixture is heated with constant stirring till a paste of soap is formed.
- To this 5 to 10 grams of common salt (sodium chloride) is added.
- Stir the mixture well and allow it to cool. On cooling the solution, solid soap separates out.
- When the soap sets, it can be cut into pieces called 'soap bars'.
(b) Common salt (sodium chloride) is added to the mixture to separate the soap from its solution. Though most of the soap separates out on its own some of it remains in solution. Hence, Common salt is added to precipitate out all the soap from the aqueous solution.
(c) Soap is not suitable for washing clothes when the water is hard because:
- Hard water contains calcium and magnesium salts; and, when 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.
- The curdy precipitate called scum is produced by the reaction of hard water on the soap. This sticks to the clothes making them difficult to be cleaned. Therefore, soaps are not suitable for washing clothes in hard water.
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