Question

In three test tubes A, B and C, three different liquids namely, distilled water, underground water and distilled water in which a pinch of Calcium sulphate is dissolved, respectively are taken. An equal amount of soap solution is added to each test tube and the contents are shaken. In which test tube will the length of the foam (lather) be longest? Justify your answer.

Answer 1

Soap lather:-

1. It is actually a foam or froth which is formed when a detergent such as soap is agitated in water.
2. Test tube $\mathrm{A}$ contains distilled water which is considered soft water and has no free ions, so the length of the foam in the test tube $\mathrm{A}$ will be the longest.
3. Test tube B contains underground water which consists of ions such as ${\mathrm{Mg}}^{2+}$, ${\mathrm{Ca}}^{2+}$, etc. It will react with soaps to form salts of the fatty acids known as scum and it is insoluble in water, so the length of foam will be less in test tube $\mathrm{B}$ as compared to the test tube $\mathrm{A}$.
4. Test tube $\mathrm{C}$ consists of distilled water with Calcium sulphate ${\mathrm{CaSO}}_4$ and it consists of ${\mathrm{Ca}}^{2+}$ ions which will react with soaps to form salts of fatty acid which is scum hence, the length of foam in test tube $\mathrm{C}$ will be less compared to that of test tube $\mathrm{A}$.

Therefore, the length of the foam will be the longest in test tube A, which consists only of distilled water because lather is formed easily in the soft water and distilled water is soft water.