ultraviolet light The portion of the spectrum that lies just beyond the violet;
therefore of short wavelength.
U.S.P. (United States Pharmacopeia) chemicals Chemicals certified as having a
standard of purity that demonstrates their fitness for use in medicine.
valence The combining power of an element; the number of electrons gained, lost,
or borrowed in a chemical reaction.
valence electrons The electrons in the outermost level or levels of an atom that
determine its chemical properties.
van der Waals forces Weak attractive forces existing between molecules.
vapor The gaseous phase of a substance that normally exists as a solid or liquid
at ordinary temperatures.
vapor pressure The pressure exerted by a vapor given off by a confined liquid or
solid when the vapor is in equilibrium with its liquid or solid form.
volatile Easily changed to a gas or a vapor at relatively low pressure.
volt A unit of electrical potential or voltage, equal to the difference of potential
between two points in a conducting wire carrying a constant current of 1
ampere when the power dissipated between these two points is equal to 1 watt
(abbreviation: V).
volume The amount of three-dimensional space occupied by a substance.
VSEPR The valence shell electron pair repulsion model. It expresses the non-90°
variations in bond angles for p orbitals in the outer energy levels of atoms in
molecules because of electron repulsions.
water of hydration Water that is held in chemical combination in a hydrate and
can be removed without essentially altering the composition of the substance.
See also Hydrate.
weak acid (or base) An acid (or base) capable of being only slightly ionized in
an aqueous solution.
Example: acetic acid (ammonium hydroxide).
weak electrolyte A substance that, when dissolved in water, ionizes only slightly
and hence is a poor conductor of electricity.
weight The measure of the force with which a body is attracted toward Earth by
gravity.
work The product of the force exerted on a body and the distance through which
the force acts; expressed mathematically by the equation W = Fs, where W =
work, F = force, and s = distance.
X-rays Penetrating radiations, of extremely short wavelength, emitted when a
stream of electrons strikes a solid target in a vacuum tube.