vacuum breaker - A mechanical device which automatically vents a water line to the atmosphere when subjected to a partial vacuum, thus preventing back-flow. (See back-flow, air gap, back-flow preventer.)

valence - A small positive or negative whole number, also called oxidation number, which indicates the net number of electrons gained or lost in the formation of an ion, or the number of electrons the substance can donate or accept in a chemical reaction, and thus the numbers of each kind of ion necessary for a balanced chemical reaction. For example, two hydrogen ions (each with a valence of +1) must be present for each ion of oxygen (-2) to form a molecule of water (H₂O).

viable - Alive and capable of continued life.

virus - The smallest form of life known to be capable of producing disease or infection, usually considered to be of large molecular size. They multiply by assembly of component fragments in living cells, rather than by cell division, as do most bacteria.

viscosity - The resistance of fluids to flow, due to internal forces and friction between molecules, which increases as temperature decreases.

void volume - The volume of the spaces between particles of ion exchanger, filter media, or other granular material; often expressed as a percentage of the total volume occupied by the material.

volatile - Capable of vaporization at a relatively low temperature.

volatile solids - Matter which remains as a residue after evaporation at 105 or 180^oC, but which is lost after ignition at 600^oC. Includes most forms of organic matter.

volumetric - Referring to measurement by volume rather than weight. (See gravimetric.)