hardness - A characteristic of natural water due primarily to the presence of dissolved polyvalent (valence greater than 1) cations, such as calcium (Ca⁺²) and magnesium (Mg⁺²). Water hardness is responsible for most scale formation in pipes and water heaters, and forms insoluble "curd" when it reacts with soaps. Hardness is usually expressed in grains per gallon, parts per million, or milligrams per liter, all as calcium carbonate equivalent.

head - A measure of the pressure at a point in a water system, expressed in pounds per square inch, or in the height of a column of water which would produce the pressure. 1 psi equals 2.31 feet of head (water).

head loss - The same as "pressure drop".

high-test hypochlorite - A dry solid, largely calcium hypochlorite, used as a disinfecting agent; has excellent stability as long as kept dry.

home-owned - A slang term sometimes applied to permanently installed household water conditioning equipment, as opposed to rental or portable exchange equipment.

hydration - The chemical combination of water into a substance.

hydraulic - Referring to water or other fluids in motion.

hydraulic classification - A process in which particles of the same specific gravity may be graded according to size by backwashing or other relative upward flow of water, with the smallest particles tending to rise to the top of the bed, and largest particles tending to sink to the bottom, because of variations in weight to sur area ratios.

hydrogen cycle - The cation exchange cycle in which the cation exchanger is regenerated with acid, and cations are removed from the solution treated, in exchange for hydrogen ions.

hydrogen ion concentration - The concentration of hydrogen ions in moles per liter of solution; often expressed as pH. (See pH.)

hydrogen sulfide - This is not a routine test but is determined only upon request and on a separate special sample. It is a poisonous gas and will cause headache and nausea. It smells like "rotten eggs". It causes a black precipitate with many metals.

hydrologic cycle - The water cycle, including precipitation of water from the atmosphere as rain or snow, flow of water over or through the earth, and evaporation or transpiration to water vapor in the atmosphere. Water evaporates from the earth and rises into the atmosphere where it forms clouds. In nature, this is where water is in its purest form. However, it does not stay that way for long. Its stay in the air is short. Water droplets forming in clouds, absorb particles and impurities found floating in the air.

After reaching the ground, water continues to dissolve additional matter it contacts. As water runs over the surface, it can become cloudy, even muddy. Then, as water seeps down through the ground, it may dissolve a little bit of the minerals and substances that could be present, such as calcium, magnesium, iron, or radioactivity.

By the time water returns to rivers, lakes, reservoirs, or underground aquifers, it may have accumulated amounts of the elements it has contacted. Along the way bacteria, chemicals, agricultural byproducts, fertilizers, insecticides, and other man made wastes may also have entered the picture and become dissolved in the water. (See transpiration.)

hydrolysis - The reaction of a salt with water to form an acid and a base.

hydropneumatic system - A system utilizing both air and water in its operation, such as the pressure tank used with many well systems, which utilizes an air chamber to maintain pressure on the water when the pump is not operating.

hydrostatic test - A pressure test procedure in which a vessel or system is filled with water, purged of air, sealed, subjected to water pressure, and examined for leaks, distortion and/or mechanical failure.

hydroxide - A chemical compound containing hydroxyl (OH) ion. (See hydroxyl.)

hydroxyl - The OH anion which has a single negative charge, and provides the characteristics common to bases. (See base.)