Glossary of Terms - H
A B C D E F G H I J K L M N O P Q R S T U V W Z
H2O - The chemical formula for water.
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.
hard water - Water containing total hardness in the amount of one grain per U.S. gallon (or more) measured as calcium carbonate equivalent.
heavy metals - Metallic elements with high atomic weights, e.g., mercury, chromium, cadmium, arsenic, and lead. They can damage living things at low concentrations and tend to accumulate in the food chain.
head - A measure of the pressure at a point in a water system, expressed in pounds per square inch (psi), 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".
heterotrophic plate count (HPC) - A procedure for estimating the total number of live non-photosynthetic bacteria in water.
high-test hypochlorite - A dry solid, largely calcium hypochlorite, used as a disinfecting agent; has excellent stability as long as kept dry.
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 weight to surface area ratio.
hydrogen cycle - The cation exchange cycle (H+ form) in which the cation medium is regenerated with acid and all cations in the water are removed by exchange with hydrogen ions.
hydrogen ion concentration - The concentration of hydrogen ions, in moles per liter of solution, used as a measure of the acidity of the solution. Expressed as pH. (See pH.)
hydrogen sulfide - A corrosive and flammable gas often found dissolved in well water and often accompanied by iron and low pH. It causes a black precipitate with many metals.
hydrologic cycle - The complete circuit pursued by water in nature, including 1. falling of precipitation (rain, hail, sleet, snow, dew); 2. The journal of fallen water over and through the earth's surface formations; and 3. Eventual evaporation of the water and its return to the atmosphere to again fall as precipitation.
hydrolysis - A chemical reaction in which a substance reacts with water and becomes a different substance. Example: 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.
hypochlorite - The OCl- anion, which is the dominant reaction product of the chlorine in water at a pH greater than 8. Calcium and sodium hypochlorites are commonly used as bleaches and disinfecting agents.
hypochlorous acid (HOCl) - The dominant reaction product of chlorine in water at pH less than 7.