Dictionary of Chemistry [6th Ed.]

A

• The original paper by Raman and
  Krishnan
  Ramsay, Sir William (1852–1916)
  British chemist, born in Glasgow.
  After working under Robert Bun-
  sen, he returned to Glasgow before
  taking up professorships at Bristol
  (1880–87) and London (1887–1912). In
  the early 1890s he worked with Lord
  Rayleigh on the gases in air and in
  1894 they discovered argon. In 1898,
  with Morris Travers (1872–1961), he
  discovered neon, krypton, and
  xenon. Six years later he discovered
  the last of the noble gases, radon. He
  was awarded the Nobel Prize for
  chemistry in 1904, the year in which
  Rayleigh received the physics prize.
  random alloySee disordered
  solid.

random walkThe problem of de-

termining the distance from a start-

ing position made by a walker, who

can either move forward (toward +x)

or backwards (toward –x) with the

choice being made randomly (e.g. by

tossing a coin). The progress of the

walker is characterized by the net

distance DNtravelled in N steps. After

N steps the root mean square value

Drms, which is the average distance

away from the starting position, is

given by Drms= √N. Physical applica-

tions of the random walk include dif-

fusion and the related problem of

Brownian motion as well as prob-

lems involving the structures of poly-

mers and disordered solids.

Raney nickelA spongy form of

nickel made by the action of sodium

hydroxide on a nickel–aluminium

alloy. The sodium hydroxide dis-

solves the aluminium leaving a

highly active form of nickel with a

large surface area. The material is a

black pyrophoric powder saturated

with hydrogen. It is an extremely

Ramsay, Sir William 454

r

efÜcient catalyst, especially for hy-

drogenation reactions at room tem-

perature. It was discovered in 1927

by the American chemist M. Raney

(1885–1966).

ranksiteA mineral consisting of

a mixed sodium carbonate, sodium

sulphate, and potassium chloride,

2Na 2 CO 3 .9Na 2 SO 4 .KCl.

Raoult’s law The partial vapour

pressure of a solvent is proportional

to its mole fraction. If p is the vapour

pressure of the solvent (with a sub-

stance dissolved in it) and X the mole

fraction of solvent (number of moles

of solvent divided by total number of

moles) then p = p 0 X, where p 0 is the

vapour pressure of the pure solvent.

A solution that obeys Raoult’s law is

said to be an ideal solution. In gen-

eral the law holds only for dilute so-

lutions, although some mixtures of

liquids obey it over a whole range of

concentrations. Such solutions are

perfect solutions and occur when the

intermolecular forces between mol-

ecules of the pure substances are

similar to the forces between mol-

ecules of one and molecules of the

other. Deviations in Raoult’s law for

mixtures of liquids cause the forma-

tion of *azeotropes. The law was dis-

covered by the French chemist

François Raoult (1830–1901).

A

• Raoult’s original paper

rapeseed methyl esterSee bio-

fuel.

rare-earth elements See lan-

thanoids.

rarefaction A reduction in the

pressure of aÛuid and therefore of

its density.

rare gases See noble gases.

Raschig process An industrial

process for making chlorobenzene