Dictionary of Chemistry [6th Ed.]

Spin–orbit coupling is large in many

nuclei, particularly heavy nuclei.

spirits of saltA name formerly

given to hydrogen chloride because

this compound can be made by

adding sulphuric acid to common

salt (sodium chloride).

spiro compound A type of com-

pound in which there are two rings

linked through a single atom (the

spiro atom). In most cases, the spiro

atom is a carbon atom, and the rings

are not in the same plane. Some

spiro compounds with substituents

on both rings may have a chirality

element and consequently show opti-

cal activity.

A

• Information about IUPAC nomenclature


• Alternative naming system for spiro
  compounds

spirits of salt 500

s

Spiro compound

spontaneous combustionCom-

bustion in which a substance pro-

duces sufÜcient heat within itself,

usually by a slow oxidation process,

for ignition to take place without

the need for an external high-

temperature energy source.

spontaneous emissionThe emis-

sion of a photon by an atom as it

makes a transition from an excited

state to the ground state. Sponta-

neous emission occurs independently

of any external electromagnetic radi-

ation; the transition is caused by in-

teractions between atoms and

vacuumÛuctuations of the quantized

electromagneticÜeld. The process of

spontaneous emission, which cannot

be described by nonrelativistic

*quantum mechanics, as given by

formulations such as the *Schrö-

dinger equation, is responsible for

the limited lifetime of an excited

state of an atom before it emits a

photon.

spot testA simple test for a given

substance using a reagent that

changes colour when mixed with the

substance. In forensic science spot

tests are often called *presumptive

tests.

sputteringThe process by which

some of the atoms of an electrode

(usually a cathode) are ejected as a re-

sult of bombardment by heavy posi-

tive ions. Although the process is

generally unwanted, it can be used to

produce a clean surface or to deposit

a uniformÜlm of a metal on an ob-

ject in an evacuated enclosure.

squaleneAn intermediate com-

pound formed in the synthesis of

cholesterol; it is a hydrocarbon con-

taining 30 carbon atoms. The im-

mediate oxidation of squalene to

squalene 2,3-epoxide is the last com-

mon step in the synthesis of *sterols

in animals, plants, and fungi.

square-planar Describing a coordi-

nation compound in which four lig-

ands positioned at the corners of a

square coordinate to a metal ion at

the centre of the square. See complex.

stabilization energyThe amount

by which the energy of a delocalized

chemical structure is less than the

theoretical energy of a structure with

localized bonds. It is obtained by sub-

tracting the experimental heat of for-

mation of the compound (in kJ mol–1)

from that calculated on the basis of a

classical structure with localized

bonds.

stabilizer1.A substance used to in-

hibit a chemical reaction, i.e. a nega-

tive catalyst. 2.A substance used to

prevent a colloid from coagulating.