Explain the cleaning action of soap. Why do soaps not work in hard water?

Solution -

Soap:

• Soaps are sodium and potassium salts of long chain fatty acids containing 12 to 18 carbon atoms.
• Soaps are denoted by the general formula RCOO^-Na⁺, where R denotes any long chain alkyl group containing 12 to 18 carbon atoms.

Some common examples of fatty acids that are used in soaps are"

1. Stearic acid which has the chemical formula C₁₇H₃₅COOH,
2. Palmitic acid which has the chemical formula C₁₅H₃₁COOH etc.

Cleansing action of soaps: 

A soap molecule consists of two parts:

A long chain hydrocarbon tail. It is hydrophobic in nature. Head: It is hydrophilic in nature.

When soap is added to an oily or greasy part of cloth, the hydrocarbon part of soap dissolves in oil, keeping the head away from the oil. Big molecules of oil and soap break by rubbing into small emulsified oil droplets in water, which are washed away by stream of water. The soap molecules which form micelles assist in dissolving the dirt in water, thus, we can wash our clothes.

Also, there are  calcium and magnesium ions in hard water. When soaps are dissolved in hard water, these ions displace sodium or potassium from their salts and form insoluble calcium or magnesium salts of fatty acids. These insoluble salts separate as scum. This is the reason why do not work in hard water.