Scum

Scum is the insoluble solid formed when soap reacts with the calcium and magnesium ions dissolved in hard water. It wastes soap and reduces lather. Malay: kekat · Chinese: 皂垢.

Soap works because its sodium salt is soluble. Hard water, however, contains calcium and magnesium ions, and these ions swap places with the sodium in the soap to form calcium and magnesium salts of the fatty acid. Those salts do not dissolve, so instead of lathering, the soap collects as a grey solid that floats on the water or clings to the sides of the basin. Until enough soap has been used up to remove all the hardness ions, no lather forms, so soap is wasted in hard water.

Example. Washing with soap in a hard-water area leaves a ring of grey scum around the sink and a slippery residue on clothes. You have to add far more soap before it lathers, because the first portion of soap is spent turning the calcium and magnesium ions into insoluble scum.

Confusion to avoid. Do not think detergents form scum. Only soap does, because the calcium and magnesium salts of a detergent are soluble and stay in the water. This contrast is the whole point of the soap-versus-detergent comparison, so link “scum” firmly to “soap in hard water” and never to detergent.

Scum ties together soap, hard water and the advantage of detergents, so it appears in almost every SPM Chemistry question that compares the two cleaning agents. Naming the insoluble calcium and magnesium salts is what earns the mark. A full answer also explains the consequence: because the first soap added is used up making scum, more soap is needed before any lather appears, and the grey solid left behind marks clothes and basins. If a question asks why soap is unsuitable in a hard-water area, the word scum, together with the idea of wasted soap and poor lather, is the phrase the marking scheme is looking for.