Chemistry, Third edition

11 · SOLUTIONS AND SOLUBILITY

Colloids

What is a colloid?

A colloid is a mixture of solute and solvent in which the particles of solute are inter-

mediate in size between those found in true solutions (such as sugar in water) and

suspensions (such as chalk or sand in water).

Examples of colloids include tea, starch and milk in water, gelatine (a protein) in

water, aluminium hydroxide ‘gel’, and soapy water. Colloids are sometimes called

colloidal solutions,colloidal suspensionsorcolloidal dispersions.

Key points about colloids

1.The particles in a colloid have diameters in the range 1–1000 nm. Such particles

are too small to be seen by optical microscopes. The particles are either very large

molecules (such as proteins) or else aggregates of molecules or ions.

2.The particles of colloids do not settle at the bottom of the container in the same

way that suspensions do, nor can they be filtered off using ordinary filter papers.

However, colloids can be separated from true solutions using a special membrane

• such a separation is called dialysis.

3.Colloids can be distinguished from true solutions by shining light through the

solution. The relatively large particles of a colloid scatter the light. This is known

as the Tyndall effect(Fig. 11.12). Car headlights in fog is a familiar example of

this effect.

11.7

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Fig. 11.12The Tyndall effect.

Example 11.3 (continued)

Comment

The osmotic pressures developed by these solutions are surprisingly high. This

means that the osmotic pressure of very dilute solutions may be measured. This

is useful in determining the molecular mass of polymers and large biologically

important molecules which are not usually very soluble in any solvent.

Osmotic pressure

Calculate the average molecular mass of a protein given that a solution containing 0.25 g of

the protein in 1.0 dm^3 of water produced an osmotic pressure (taken against pure water) of

91 Pa at exactly 300 K.

Exercise 11I