The Foundations of Chemistry

THE TYNDALL EFFECT

The scattering of light by colloidal particles is called the Tyndall effect(Figure 14-18).

Particles cannot scatter light if they are too small. Solute particles in solutions are below

this limit. The maximum dimension of colloidal particles is about 10,000 Å.

The scattering of light from automobile headlights by fogs and mists is an example of

the Tyndall effect, as is the scattering of a light beam in a laser show by dust particles in

the air in a darkened room.

THE ADSORPTION PHENOMENON

Much of the chemistry of everyday life is the chemistry of colloids, as one can tell from

the examples in Table 14-4. Because colloidal particles are so finely divided, they have

tremendously high total surface area in relation to their volume. It is not surprising, there-

fore, that an understanding of colloidal behavior requires an understanding of surface

phenomena.

Atoms on the surface of a colloidal particle are bonded only to other atoms of the

particle on and below the surface. These atoms interact with whatever comes in contact

with the surface. Colloidal particles often adsorb ions or other charged particles, as well

as gases and liquids. The process of adsorptioninvolves adhesion of any such species onto

the surfaces of particles. For example, a bright red sol(solid dispersed in liquid) is formed

by mixing hot water with a concentrated aqueous solution of iron(III) chloride (Figure

14-19).

2 x[Fe^3 
(aq)
3Cl(aq)]
x(3
y)H 2 O88n[Fe 2 O 3 yH 2 O]x(s)
 6 x[H

Cl]

yellow solution bright red sol

Each colloidal particle of this sol is a cluster of many formula units of hydrated Fe 2 O 3.

Each attracts positively charged Fe^3 
ions to its surface. Because each particle is then

surrounded by a shell of positively charged ions, the particles repel one another and cannot

combine to the extent necessary to cause precipitation.

14-17

14-16

Figure 14-18 The dispersion of a beam of light by colloidal particles is called the Tyndall

effect. The presence of colloidal particles is easily detected with the aid of a light beam.

578 CHAPTER 14: Solutions

Light source

Beam of light not visible

Colloidal gold

A gold sol

Light beam

visible

NaCl solution

Light beam

not visible

Tobacco smoke

An aerosol

Light beam

visible