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.
Acid strength
The usual measure of the strength of an acid is its acid dissociation constant ("K"), which can be determined experimentally by titration methods. Stronger acids have a larger "K" and a smaller logarithmic constant (p"K"=−log"K") than weaker acids. The stronger an acid is, the more easily it loses a proton, H. Two key factors that contribute to the ease of deprotonation are the polarity of the H—A bond and the size of atom A, which determine the strength of the H—A bond. Acid strengths also depend on the stability of the conjugate base.