cellulose acetate is made and dis-
solved in a solvent. The solution is
forced through nozzles into air,
where the solvent quickly evaporates
leaving aÜlament of acetate rayon.RBSSee rutherford backscatter-
ing spectrometry.RDXSee cyclonite.reactant See chemical reaction.
reaction See chemical reaction.reactive dye See dyes.reagent A substance reacting with
another substance. Laboratory
reagents are compounds, such as sul-
phuric acid, hydrochloric acid,
sodium hydroxide, etc., used in
chemical analysis or experiments.realgar A red mineral form of ar-
senic(II) sulphide, As 2 S 2.real gas A gas that does not have
the properties assigned to an *ideal
gas. Its molecules have aÜnite size
and there are forces between them
(see equation of state).rearrangement A type of chemical
reaction in which the atoms in a
molecule rearrange to form a new
molecule. Examples are the *pinacol
rearrangement and the *Wagner–
Meerwein rearrangement.reciprocal lattice A lattice for a
crystal that can be deÜned from the
lattice in real space. If the primitive
translation vectors of the lattice in
real space are denoted by a, b, c then
the primitive translations a′, b′, c′in
the reciprocal lattice are deÜned by
a′= b×c[abc], b′= c×a[abc], c′=
a×b[abc], where [abc] denotes the
scalar triple producta.(b×c).
The reciprocal lattice is a funda-
mental concept in the theory of *X-
ray diffraction and energy bands
with a diffraction pattern being
much more closely related to the rec-iprocal lattice than the real-space lat-
tice.
reciprocal proportions See chemi-
cal combination.
recombination process The
process in which a neutral atom or
molecule is formed by the combina-
tion of a positive ion and a negative
ion or electron; i.e. a process of the
type:
A++ B–→AB
or
A++ e–→A
In recombination, the neutral
species formed is usually in an ex-
cited state, from which it can decay
with emission of light or other elec-
tromagnetic radiation.recreational drug A drug used for
recreation, as opposed to medical
use. Strictly, recreational drugs in-
clude alcohol and nicotine, but the
term is often understood to mean
substances such as marijuana, co-
caine, and amphetamines.
recrystallization A process of re-
peated crystallization in order to pu-
rify a substance or to obtain more
regular crystals of a puriÜed sub-
stance.
rectiÜcation The process of purify-
ing a liquid by *distillation. See frac-
tional distillation.rectiÜed spiritA constant-boiling
mixture of *ethanol (95.6°) and
water; it is obtained by distillation.red lead See dilead(ii) lead(iv)
oxide.redox See oxidation–reduction.
reducing agent (reductant) A sub-
stance that brings about reduction in
other substances. It achieves this by
being itself oxidized. Reducing
agents contain atoms with low oxida-
tion numbers; that is the atoms haveRBS 456r