Laboratory Methods of Inorganic Chemistry, 2nd English Ed. 1928

CHAPTER II.

CHANGES OF CONDITION.

CHANGES in the state of aggregation of substances have already been men-

tioned in the preceding chapter; but under changes in condition are included

also polymerization and dissociation, as well as the formation of allotropic,

passive; amorphous, and colloidal modifications. These phenomena are, with

few exceptions, not confined to any particular class of substances.

POLYMERIZATION AND DISSOCIATION.

The phenomena of polymerization and dissociation, as far as they

occur with gaseous or dissolved bodies, are capable of a complete theoret-

ical interpretation. By polymerization is understood the adding together of

particles of material of the same kind to form larger aggregates; the opposite

of this process is dissociation. The occurrence of either may be demon-

strated by determining the molecular weight of the substance in the vaporized

or dissolved state. For example, the diatomic iodine molecules dissociate

at high temperatures into atoms; on cooling, the atoms again poly-

merize into molecules of the original sort:

Dissociation

I, = 21

Polymerization

In a similar way ferric chloride forms simple molecules at high temperatures,

and double molecules at lower temperatures:

Dissociation

Fe 2 Cl 6 = 2 FeCl 3

Polymerization
The enumeration of experiments illustrating these phenomena may be
dispensed with at this point; the conception of dissociation, however, is
developed more fully in the general section of Chapter III, and that of elec-
trolytic dissociation in the sections on acids, bases, and salts, and in No. 34.

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