Can you use soaps and synthetic detergents to check the hardness of water?
Soaps get precipitated in hard water, but not in soft water. Therefore, soaps can be used for checking the hardness of water.
However, synthetic detergents do not get precipitated either in hard water or in soft water. Therefore, synthetic detergents cannot be used for checking the hardness of water.
Soaps get precipitated as insoluble calcium and magnesium soaps in hard water but detergents do not. Therefore, soaps but not synthetic detergents can be used to check the hardness of water.