Question

What is the advantage of using detergents over soap in hard water?
a.) Can be used with both hard and soft water
b.) Used in acidic solutions
c.) Prepared from hydrocarbons of petroleum
d.) All the above

Answer 1

Hint: Detergents are mostly sodium salt of alkyl hydrogen sulphate or long chain alkyl benzene sulphonic acid, while soaps are metallic salt of higher fatty acid, and both have cleansing activity.

Complete step by step solution:
- Soaps are sodium or potassium salt of long-chain fatty acids. Hard water contains calcium and magnesium ions. When soaps are dissolved in hard water, these ions displace sodium and potassium from their salt and form insoluble calcium and magnesium salts of fatty acids. These insoluble salts get separated as scum.
\[2{{\text{C}}_{17}}{{\text{H}}_{\text{35}}}\text{COONa}\,\,\text{+}\,\,\text{CaC}{{\text{l}}_{\text{12}}}\,\,\,\to \,\,\text{2NaCl}\,\,\text{+}\,\,{{\text{(}{{\text{C}}_{\text{17}}}{{\text{H}}_{\text{35}}}\text{COO)}}_{\text{2}}}\text{Ca}\]

(A) Synthetic detergent can be used in both soft water and hard water because synthetic detergent does not get precipitated either in hard water or soft water. However soaps get precipitated in hard water and check the hardness of the water. So that they do not produce more lather and cause wastes of soaps, but not in soft water.
(B) Detergents are cationic, anionic and non-ionic in nature. Synthetic detergent will show cleaning activity in all types of chemical medium. However soaps are basic in nature, in acidic water they get precipitated. So, synthetic detergent is used in acidic water.
(C) Synthetic detergents are mainly prepared from petroleum hydrocarbon fraction and converted into organic acid. This hydrocarbon later on is neutralized with sodium hydroxide to form a detergent.
So, the correct answer is “Option D”.

Note: When synthetic detergents are dissolved in hard water containing calcium and magnesium ions, these ions form a soluble that acts as a cleansing agent. However, when soaps are dissolved in hard water forms insoluble salt that are of no further use.