Encyclopedia of Chemistry

LAR. With a few exceptions, usually involving fluorine,
the associated energies are less than 20–25 kJ mol–1
(5–6 kcal mol–1).
A type of bond formed when the partially positive
hydrogen atom of a polar covalent bond in one
molecule is attracted to the partially negative atom of a
polar covalent bond in another.

hydrogen ion A single proton with a charge of +1.
Also called a HYDRON.

hydrogen-oxygen fuel cell A cell in which hydrogen
is the fuel (reducing agent) and oxygen is the oxidizing
agent. Francis Thomas Bacon (1904–92), a British
engineer, developed the first practical hydrogen-oxygen
fuel cell.

hydrolase An ENZYMEof EC Class 3, also known as
a hydro-LYASE, that catalyzes the HYDROLYSISof a SUB-
STRATE.

hydrolysis SOLVOLYSISby water.

hydrolysis constant An equilibrium constant for a

hydrolysis reaction (reaction of a substance with water

or its ions).

hydrometallurgy The extraction of metals and

metallic compounds such as gold, alumina, nickel, cop-

per, and zinc from minerals by solutions.

hydrometer A device used to measure the densities

of liquids and solutions; determines specific gravity of

the solution. Invented by the French scientist Antoine

Baumé, although Leonardo da Vinci is also given

credit.

hydron General name for the ion H+either in natural

abundance, or where it is not desired to distinguish

between the isotopes, as opposed to proton for^1 H+,

deuteron for^2 H+, and triton for^3 H+. Not in general use.

hydronium ion A water molecule (H 2 O) with an

added hydrogen ion, making it H 3 O+.

hydrophilic Water loving. The capacity of a MOLEC-

ULAR ENTITYor of a SUBSTITUENTto interact with polar

solvents, in particular with water, or with other polar

groups. Hydrophilic molecules dissolve easily in water,

but not in fats or oils.

hydrophilic colloids Macromolecules that interact

strongly with water and form colloids.

hydrophilicity The tendency of a molecule to be sol-

vated by water.

See alsoSOLVATION.

hydrophobic Fear of water. The tendency to repel

water.

See alsoHYDROPHILIC.

hydrophobic 133

The hydrogen bond is a form of association between an elec-
tronegative atom and a hydrogen atom attached to a second,
relatively electronegative atom.