To determine whether a given water sample is of hard water or soft water, one can use soap.
Soap produces lather in soft water whereas soap does not produce lather in hard water due to the formation of scum (insoluble precipitate). Scum is a result of reaction of soap with calcium and magnesium ions present in the hard water.
2RCOONa + Ca2+ → (RCOO)2Ca↓ (Scum) + 2Na+
