Hard water
Water's hardness is determined by the concentration of multivalent cations in the water. Multivalent cations are positively charged metal complexes with a charge greater than 1+. Usually, the cations have the charge of 2+. Common cations found in hard water include Ca and Mg. These ions enter a water supply by leaching from minerals within an aquifer. Common calcium-containing minerals are calcite and gypsum. A common magnesium mineral is dolomite (which also contains calcium). Rainwater and distilled water are soft, because they contain few ions.
Vickers hardness test
If HV is expressed in kgf/mm then the tensile strength (in MPa) of the material can be approximated as ≈ HV× ≈ HV/0.3, where is a constant determined by yield strength, Poisson's ratio, work-hardening exponent and geometrical factors - usually ranging between 2 and 4.. In other words, if HV is expressed in N/mm (i.e. in MPa) then the tensile strength (in MPa) ≈ HV/3. However it should be noted this empirical law depends variably on the work-hardening behavior of the material.