Question

Explain briefly
the action of soap on hard water

Answer 1

the action of soap on hard water


When soap is added to hard water, the Ca2+ and Mg2+ ions present in hard waterreact 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.
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Answer 2

Ordinary water does not remove dirt from clothes or skin because the dirt present is oily or greasy in nature. Soaps are one of the most commonly used cleansing agents and are capable of reacting with water to remove  dirt. They are either animal origin or plant in origin.
Chemically, they are water soluble sodium or potassium salts of higher fatty acids such as stearic acid, palmitic acid or oleic acid. Fatty acids are found in fats and oils. When fat or oil is treated with alkali such as caustic soda or caustic potash soap and glycerin are produced.
The soap molecule is generally represented as RCOONa. In solution, it ionizes to form RCOO- and Na+. Each soap molecule has a polar head group (carboxylate ion, COO- group) and a long non-polar hydrocarbon tail (R group from long chain fatty acid). The polar head attracts the polar water molecule and is called hydrophilic end and the non-polar tail attracts the water insoluble oily or greasy dirt particles.

When a dirty cloth is placed in soap solution, the long non-polar hydrocarbon tail of soap molecules points towards the oily dirt particles and the polar heads point towards the water. This forms a spherical structure with polar parts of the molecule on the surface and non-polar parts in the center. This spherical structure is called micelle. This micelle is attracted towards water and carries the oily dirt particles along with it. This causes the dirt particles to detach from the fibres of the cloth. In this manner, clothes become free from dirt or dust.

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