• Soaps are the sodium or potassium salts of long chain carboxylic acids while detergents are the ammonium of sulphonate salts of long chain carboxylic acids.
• The dirt is oily in nature and when soap is added to water, its molecules form structures called micelles in which carbon chain of the molecules dissolves in the oil while the ionic end dissolves in water and faces outside. The micelles thus help in dissolving the dirt in the water. 
• Ca²⁺ and Mg²⁺ present in hard water form insoluble substance (scum) with soap.
•  Two problems :

(i) Non-biodegradable

(ii) Water pollution/soil pollution

Cleansing Action of  Soap : When soap is dissolved in water, it forms a colloidal suspension. In this colloidal suspension, the soap molecules cluster together to form micelles and remain radially suspended in water with the hydrocarbon end towards the centre and the ionic end directed outward. The dirt particles always adhere to the oily or greasy layer present on the skin or clothes. When a dirty cloth is dipped into a soap solution, its nonpolar hydrocarbon end of micelles gets attached to the grease or oil present in dirt and polar end remains in the water layer.

The mechanical action of rubbing subsequently dislodges the oily layer from the dirty surface shaping it into small globules. A stable emulsion of oil in water is formed. The emulsified oil or grease globules bearing the dirt can now be readily washed with water.

Soaps do not form lather when the water is hard. When soap is added to hard water, calcium and magnesium salts present in water displace sodium or potassium ions from the soap molecules forming an insoluble substance called scum.

Problems that arise due to use of detergents instead of soap :

(i) Detergents being non-biodegradable, they accumulate in the environment causing pollution.

(ii) In soil, the presence of detergents leads to pH changes making soil infertile.

(iii) The entry of detergents into food chain leads to bioaccumulation in living beings and tends toward serious health issues.