Question

What change will you observe if you test soap with litmus paper (red and blue)?

Answer 1

Hint: Soaps are the potassium or sodium salts of fatty acids. Thus, we can say that they are salts of weak acid and strong base. Litmus paper shows red colour in pH range 0 to 7 and shows blue colour in pH range 7 to 14.

Complete step by step solution:
We will see the composition of soap and how litmus paper changes its colour upon the nature of the solution.
- We know that soaps are sodium or potassium salts of fatty acids. The fatty acids are not as strong acids as mineral acids. Thus, we can call them the salt of weak acid and strong base.
- We know that the salt of weak acid and strong base gives an alkaline solution when dissolved in water. Soaps give $NaOH$ and $KOH$ upon its dissolution in water. Alkaline solution has the pH range from 7 to 14.
- Litmus paper has some dyes on the paper which are sensitive to the pH of the solution. It has its characteristic colour in different solutions. Litmus paper is available in two colours; red and blue.
- When we dip a red litmus paper in the solution of soap, it will change its colour from red to blue. It shows that the solution is alkaline in nature.
- When we dip a blue litmus paper in the solution of soap, then it remains blue in colour.

Note: Note that we must require aqueous medium in order to check the pH of the solution by litmus paper. If we put dry litmus paper on solid soap without any presence of water on it, the red litmus paper will remain red.