The Foundations of Chemistry

COLLOIDS

A solution is a homogeneous mixture in which no settling occurs and in which solute
particles are at the molecular or ionic state of subdivision. This represents one extreme
of mixtures. The other extreme is a suspension, a clearly heterogeneous mixture in which
solute-like particles settle out after mixing with a solvent-like phase. Such a situation results
when a handful of sand is stirred into water. Colloids (colloidal dispersions)represent
an intermediate kind of mixture in which the solute-like particles, or dispersed phase,
are suspended in the solvent-like phase, or dispersing medium.The particles of the
dispersed phase are so small that settling is negligible. They are large enough, however,
to make the mixture appear cloudy or even opaque, because light is scattered as it passes
through the colloid.
Table 14-4 indicates that all combinations of solids, liquids, and gases can form colloids
except mixtures of nonreacting gases (all of which are homogeneous and, therefore, true
solutions). Whether a given mixture forms a solution, a colloidal dispersion, or a suspen-
sion depends on the size of the solute-like particles (Table 14-5), as well as solubility and
miscibility.

Freshly made wines are often cloudy

because of colloidal particles.

Removing these colloidal particles

clarifies the wine.

14-15 Osmotic Pressure 575

TABLE 14-5 Approximate Sizes of Dispersed Particles

Mixture Example Approximate Particle Size

suspension sand in water larger than 10,000 Å
colloidal dispersion starch in water 10–10,000 Å
solution sugar in water 1–10 Å

TABLE 14-4 Types of Colloids

Dispersed Dispersing
(solute-like) (solvent-like) Common
Phase Medium Name Examples

solid in solid solid sol Many alloys (e.g., steel and

duralumin), some colored

gems, reinforced rubber,

porcelain, pigmented plastics

liquid in solid solid emulsion Cheese, butter, jellies

gas in solid solid foam Sponge, rubber, pumice,

Styrofoam

solid in liquid sols and gels Milk of magnesia, paints,

mud, puddings

liquid in liquid emulsion Milk, face cream, salad

dressings, mayonnaise

gas in liquid foam Shaving cream, whipped

cream, foam on beer

solid in gas solid aerosol Smoke, airborne viruses and

particulate matter, auto

exhaust

liquid in gas liquid aerosol Fog, mist, aerosol spray,

clouds

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 14.10, Colloids.