Question

Can you use soaps and synthetic detergents to check the hardness of water ?

Answer 1

We can use soaps but not synthetic detergents to check the hardness of water.
Synthetic detergents do not get precipitated either in hard water or in soft water.
Soaps work in soft water. They do not work in hard water due to formation of insoluble calcium or magnesium salts of fatty acids.
Soaps are sodium or potassium salts of long chain fatty acids. Hard water contains calcium and magnesium ions.
During dissolution of soap in hard water, calcium and magnesium ions displace sodium or potassium ions from soaps to form insoluble calcium or magnesium salts.

Hence, soaps can be used to check the hardness of water.