Detergent

A detergent is the sodium salt of a sulphonic acid derivative, made from petroleum. Unlike soap, it does not form scum in hard water. Malay: detergen · Chinese: 洗涤剂.

A detergent particle cleans in exactly the same way as soap: it has a hydrophilic head, here the sulphonate group –SO₃⁻, and a long hydrophobic tail. The tail dissolves in grease, the head dissolves in water, and together they lift oily dirt away as an emulsion that rinses off. The important advantage is what happens in hard water. The calcium and magnesium salts of a detergent are soluble, so a detergent keeps lathering and cleaning in hard water instead of wasting itself as an insoluble scum. This is why detergents are preferred in areas where the water supply is hard.

Example. Washing powders and dishwashing liquids are detergents. They are usually sold as mixtures that also contain a builder to soften the water, a suspension agent to stop dirt settling back onto the cloth, a whitening agent, and sometimes a biological enzyme to break down protein or fat stains. Each additive is included to make the detergent clean better under everyday conditions.

Confusion to avoid. Do not confuse a detergent with soap. Both have the same head-and-tail design and clean the same way, but soap comes from oils and fats while a detergent comes from petroleum, and only soap forms scum with the ions in hard water. That single difference is the usual reason a compare question is set, so learn it precisely rather than vaguely.

The soap-versus-detergent comparison is one of the most reliable SPM Chemistry questions in this chapter, and knowing that the detergent advantage is “no scum in hard water” is what turns a partial answer into a full one. We drill this contrast until students can state it in one clean sentence.