Question

Explain why soaps are not effective cleansing agents in hard water.

Answer 1

Soap is less effective in hard water since it reacts to form a calcium or magnesium salt of the organic acid in the soap and forms a grayish soapscum with no lather. Detergents, on the other hand, lather in both hard and soft water.

Answer 2

Although soap is a good cleaning agent, its cleaning capacity is reduced when used in hard water. Hardness of water is due to the presence of sulphates, chlorides or bicarbonate salts of Ca2+ or Mg2+ ions. Soaps are sodium or potassium salts of long chain fatty acids. When soap is added to hard water, the Ca2+ and Mg2+ ions present in hard water react with soap. The sodium salts present in soaps are converted to their corresponding calcium and magnesium salts which are precipitated as scum. The insoluble scum sticks on the clothes and so the cleaning capacity of soap is reduced.





The cleaning action of soap is very effective in soft water because it contains negligible calcium and magnesium ions.

Synthetic detergents are used in the case of hard water also because the calcium and magnesium salts of detergents are soluble in water. Detergents are more soluble than soaps and hence form more lather than soaps.