Question

On washing with soap, a turmeric stain on the cloth turns to red because
A) Soap solution is alkaline
B) Soap solution is acidic
C) Turmeric contains a natural indicator
D) Turmeric contain litmus
Select the correct alternative

1. A and C
2. B and D
3. C and B
4. A and D

Answer 1

When the alkali solution is dissolved in water, the solution formed is called alkaline solution. This can be formed in the laboratory by the process of erosion. The solution which contains higher number of hydrogen ions in the solution than water, it is called acidic solution and the solution with lower number hydrogen ions than water, it is called alkaline solution. The indicator which can determine the acidic and alkaline solution, it is called a natural indicator. Examples of natural indicators are turmeric, curry powder and grape juice. The dye which is obtained from lichens and changes its colour to blue under alkaline conditions and red under acidic conditions, is called litmus.

Complete solution:
Soap solution is alkaline in nature which is manufactured by the process of saponification known as base catalyzed hydrolysis of animal fat. The pH for alkaline soaps are between 8-9 due to hydrolysis. If the basicity of soap solution gets lowered on the addition of acidic contaminants, the precipitate of insoluble fatty acids takes place and scum is formed. Hence soap solution is alkaline.
Turmeric is a natural acid base indicator. It used to determine the nature of the substance just by changing colour. If the solution or the substance is basic then it changes its colour to red or reddish brown and if the solution is acidic in nature then it changes its colour to yellow. Here soap solution is basic, so the turmeric stain turns red in colour.

Therefore,the correct option is (3).

Note: When the soap solution is removed from the turmeric stain on washing with water, the stains turn to yellow again. Saponification is the process of converting lipids, fats, oil into soap and alcohol by heating them in the presence of aqueous alkali. It is an exothermic chemical reaction. It is a hydration reaction in which the free hydroxides break the ester bonds present in fatty acids.