Soap 

Soap is substance used for cleaning purposes. Chemically, it is a salt of fatty acids that is capable of carrying away dirt along with water. Soaps are obtained by treating vegetable or animal oils and fats with a strong base such as sodium hydroxide or potassium hydroxide in an aqueous solution. A few drops of sodium chloride are added in order to increase precipitation. The process of making soap is known as saponification. 

An example of soap is Sodium Stearate: NaOOCR or NaCOOC₁₇H₃₅. 

fats/oils                                          alkali                                                             soap                                glycerol 

Here, R represents alkyl radical of long chain hydrocarbons. 

R = C₁₁H₃₅ = Stearate 

R = C₁₆H₃₃ = Oleate 

R = C₁₅H₃₁ = Palmitate 

When soap is used for cleaning purposes in hard water, it reacts with the salt of calcium and magnesium present in hard water to form a brown layer on the clothes known as scum. So, soaps are not very good for cleaning purpose in hard water. But since soaps are biodegradable, they do not cause any pollution. 

Detergents 

Detergents are synthetic organic compounds obtained from hydrocarbon extracted from petroleum such as alkyl benzene sulphonate. Therefore, detergents are the sodium salt of long chain benzene sulphonic acid. Detergents are suitable for cleaning purposes in hard water, they do not react with calcium and magnesium salt of hard water so do not form any scum. Detergents are cheaper than soaps because they are formed from the byproduct of petroleum. Therefore, detergents are also known as soapless soap as they have cleansing action like soap but are formed by the chemicals that are different from soap. One example of detergent is Sodium lauryl sulphate (C₁₂H₂₅SO₄-Na+). 

But most of the detergents are non-biodegradable so they are not broken down into simpler and harmless form by the microorganism. Thus, they cause chemical pollution.