Pure water is an odorless, tasteless liquid. It has a bluish tint, which may be detected, however, only in layers of considerable depth. Supercooled water will freeze if it is disturbed, if the temperature is lowered further, or if an ice crystal or other particle is added to it. Its physical properties are used as standards to define the calorie and specific and latent heat and in the metric system for the original definition of the unit of mass , the gram . Because most substances are somewhat soluble in water, it is frequently called the universal solvent .
Water combines with certain salts to form hydrates.
As a solid, or ice, it is found as glaciers and ice caps, on water surfaces in winter, as snow, hail, and frost, and as clouds formed of ice crystals. As gas, or water vapor , it occurs as fog, steam, and clouds. Atmospheric vapor is measured in terms of relative humidity, which is the ratio of the quantity of vapor actually present to the greatest amount possible at a given temperature. Water occurs as moisture in the upper portion of the soil profile, in which it is held by capillary action to the particles of soil. In this state, it is called bound water and has different characteristics from free water.