Soaps and Detergents
This domain tests the difference between soap and synthetic detergent, the role of builders and the chemistry of hard water.
Core rules
- Soap is an alkali salt of a fatty acid, made by saponification. It cleans well in soft water but forms scum with calcium and magnesium in hard water.
- Detergents are formulated from surfactants plus builders. Anionic surfactants suit general laundry; nonionic surfactants suit oily soil and low-foam machines; cationic surfactants are used as softeners and germicides and are incompatible with anionic detergent.
- Builders soften water and raise pH. Common builders are sodium carbonate, sodium silicate and zeolites. Phosphate builders are restricted in many places because of eutrophication.
- Enzymes digest specific soils: protease for protein, amylase for starch, lipase for fat.
- Optical brighteners increase apparent whiteness; they do not remove soil.
Worked example
A laundry has water of 300 ppm hardness. Soap forms scum, wastes material and deposits grey residue on fabric. Reasoning: the calcium and magnesium ions consume the soap. Use a syndet with a builder, or soften the water first, and add detergent before the load so it can act before soil redeposits.
Traps that cost marks
- Mixing chlorine bleach with acid or ammonia, which releases toxic gas.
- Adding cationic softener in the same wash cycle as anionic detergent.
- Assuming more foam means better cleaning.
- Using soap in hard water and blaming the fabric for greyness.