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.
Roughness (psychophysics)
Roughness is studied by examining how textures are perceived and encoded by an individual's somatosensory system. In an experiment to measure and compare the roughness of different sounds, listeners are presented with different sounds and asked to rate their roughness, for example on a rating scale. Recent research has displayed that there are two different codes, at least, for roughness: a vibrotactile code used for fine surfaces, and a spatial code used for coarse to medium surfaces.