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.
Types of cheese
Categorizing cheeses by moisture content or firmness is a common but inexact practice. The lines between soft, semi-soft, semi-hard and hard are arbitrary, and many types of cheese are made in softer or firmer variants. The factor that controls cheese hardness is moisture content, which depends on the pressure with which it is packed into moulds, and upon aging time.