Colloids and the Tyndall effect

Blood can be separated in a centrifuge, yet you can't see its cells with your eyes. So it is neither a solution nor an ordinary suspension. It is a colloid. Milk, tomato sauce and ice cream are colloids too.

"If blood were a suspension, what would happen?" Its cells would sink to your feet whenever you stood up. In a colloid, the particles do not settle; they stay evenly spread.

Particle size decides the type:

The Tyndall effect. In Activity 5.1, the laser beam was invisible in salt water but you could see its path in chalk water and in milk. The particles scatter the light sideways towards your eyes. This scattering is the Tyndall effect, first explained by John Tyndall.

You see it often:

• a thin beam of sunlight through a gap in a dark room, showing the dust;
• floodlights at a stadium at night;
• hazy skies over smoky, dusty cities.

Colloids and suspensions scatter light; clear solutions do not.

A colloid has two parts:

• the dispersed phase: the tiny particles that are spread out;
• the dispersion medium: what they are spread in.

In blood, the cells are the dispersed phase and the plasma is the dispersion medium.

When both parts are liquids, the colloid is an emulsion:

• Oil-in-water: milk, vanishing cream.
• Water-in-oil: butter, body lotions, cold cream.

An emulsifying agent, such as the proteins in milk, keeps it from separating. Shake oil and water with a little soap and you make an emulsion. (Milk only looks fully mixed: churning it separates out the butter.)