Soaps and detergents
- CBSE · NCERT Class 10, Ch 4 Carbon and its Compounds
Soaps and detergents are cleaning compounds with a long carbon chain that holds oil and a charged end that holds water.
Oily dirt does not wash off in plain water. A soap molecule has two ends. The long carbon tail mixes with oil. The charged head mixes with water. In water, the tails point inward around the oil and the heads point out. This cluster is called a micelle, and rinsing carries it away.
Soaps are sodium or potassium salts of long-chain carboxylic acids. They are made by saponification. Detergents are a different class, such as sodium salts of sulphonic acids.
A small example
Take 2 test tubes, each with about 10 mL of hard water. Add 5 drops of soap solution to one and 5 drops of detergent to the other. Shake both for the same time. The detergent tube foams well. The soap tube foams less, and a curdy white solid forms. That solid is scum.
Where marks go
Question 19 of CBSE's 2026-27 Science sample paper asks why soap forms scum in hard water but detergent lathers. The 2026-27 Science marking scheme marks option A. Calcium and magnesium ions, Ca²⁺ and Mg²⁺, react with soap to form insoluble compounds. Detergents do not.
One wrong option blames Na⁺ ions. So the mark rests on naming the right ions. It is 1 mark, with no part credit. See where the marks go in CBSE Science.
Sources
Facts last checked against these sources on 30 September 2026.
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