Question

why does the hardness of water render it unfit for use in a (1) boilers (2) for washing purposes ?​

Answer 1

in case of boilers you can try and check that when you boil tap water in same utensil or container for a period of time its surface turns white that is due to some ca and mg salts present in hard water......these chemicals are not fit to consume.

in washing purpose due to insoluble salts of mg and ca no scum could be form hence cleaning action of soap is reduced.