Question

why D2O is hard water than H2O

Answer 1

Because it contains salts of magnesium. So it is harder by the presence of these type salrs

Answer 2

There are 3 isotopes of Hidrogen as Protium, Deuterium and Tritium.

In H2O, H is Protium and has 1 proton, 1eletrone but no neutrons. The atomic mass of H here is 1 therefore molecular mass of H2O is 18.

In D2O ,D is Deuterium. D has 1 p, 1e and 1 neutron. The a.m. of D is 2 therefore m.m. of D2O is 20.