maintain moisture levels. Humec-
tants are generally *hygroscopic. For
example, glycerol is employed as a
humectant in confectionery, food-
stuffs, and tobacco. Other polyhydric
alcohols, such as mannitol and sor-
bitol, are also used as humectant ad-
ditives in the foodstuffs industry.Humphreys series A series of
lines in the *hydrogen spectrum
with the form
1/λ= R(1/6^2 – 1/n^2 ), n = 7,8,9...,
where λis the wavelength associated
with the lines and R is the Rydberg
constant. The Humphreys series was
discovered by C. J. Humphreys in
1953 and lies in the far infrared.Hund coupling casesSee cou-
pling.Hund’s rules Empirical rules for in-
terpreting atomic *spectra used to
determine the lowest energy level for
a conÜguration of two equivalent
electrons (i.e. electrons with the
same n and l quantum numbers) in a
many-electron *atom. (1) The lowest
energy state has the maximum *mul-
tiplicity consistent with the *Pauli
exclusion principle. (2) The lowest
energy state has the maximum total
electron orbital angular momentum
quantum number, consistent with
rule (1). These rules were put forward
by the German physicist Friedrich
Hund (1896–1993) in 1925. Hund’s
rules are explained by quantum
theory involving the repulsion be-
tween two electrons.hyaluronic acid A *glycosamino-
glycan (mucopolysaccharide) that is
part of the matrix of connective tis-
sue. Hyaluronic acid binds cells to-
gether and helps to lubricate joints.
It may play a role in the migration of
cells at wounds; this activity ceases
when hyaluronidase breaks down
hyaluronic acid.hybrid orbitalSee orbital.
hydracidSee binary acid.hydrate A substance formed by
combination of a compound with
water. See water of crystallization.
hydrated aluminaSee aluminium
hydroxide.
hydrated aluminium hydroxide
See aluminium hydroxide.
hydration See solvation.hydrazineA colourless liquid or
white crystalline solid, N 2 H 4 ; r.d. 1.01
(liquid); m.p. 1.4°C; b.p. 113.5°C. It is
very soluble in water and soluble in
ethanol. Hydrazine is prepared by
the Raschig synthesis in which am-
monia reacts with sodium(I) chlorate
(sodium hypochlorite) to give NH 2 Cl,
which then undergoes further reac-
tion with ammonia to give N 2 H 4. In-
dustrial production must be carefully
controlled to avoid a side reaction
leading to NH 4 Cl. The compound is a
weak base giving rise to two series of
salts, those based on N 2 H 5 +, which
are stable in water (sometimes writ-
ten in the form N 2 H 4 .HCl rather than
N 2 H 5 +Cl–), and a less stable and ex-
tensively hydrolysed series based on
N 2 H 6 2+. Hydrazine is a powerful re-
ducing agent and reacts violently
with many oxidizing agents, hence
its use as a rocket propellant. It re-
acts with aldehydes and ketones to
give *hydrazones.hydrazoic acidSee hydrogen
azide.hydrazonesOrganic compounds
containing the group =C:NNH 2 ,
formed by condensation of substi-
tuted hydrazines with with aldehy-
des and ketones (see illustration).
Phenylhydrazonescontain the group
=C:NNHC 6 H 5.
A- Information about IUPAC nomenclature
Humphreys series 274h