Course 2.1Lesson 3 of 3

Suspensions and colloids

A suspension has particles that can be seen, that settle and that can be filtered. A colloid has particles that are too small to be seen or to settle, but large enough to scatter light, which is the Tyndall effect.

about 25 min1 simulation2 practice questions1 Python program4 questions to check yourself

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Is it a solution?

Stir a spoon of sand in water. It looks cloudy, but after some time the sand settles at the bottom. Stir a spoon of milk in water. It stays cloudy and does not settle. A spoon of salt makes a clear solution. These are three different kinds of mixture.

Suspensions

A suspension is a heterogeneous mixture in which the solute does not dissolve, but remains suspended (spread) in the medium.

The particles are large and can be seen with the eye.
The particles settle when the suspension is left alone. Muddy water is a suspension.
It can be filtered. The particles stay on the filter paper.
It scatters light, but we mostly see the particles themselves.

Other examples: chalk powder in water, flour in water, and some medicines that we must shake before use.

Colloids

A colloid is a mixture that looks homogeneous, but it is heterogeneous. Its particles are bigger than those of a solution, but smaller than those of a suspension.

The particles are too small to be seen with the eye.
The particles do not settle, however long the colloid is left.
They cannot be separated by ordinary filtering.
They are big enough to scatter a beam of light.

The Tyndall effect

When a beam of light passes through a true solution, we cannot see the beam. When it passes through a colloid, the particles scatter the light in all directions, and the beam becomes visible. This is the Tyndall effect. We see it when sunlight comes through the mist in a forest, or when the light of a torch passes through a smoky room, or through milk and water.

Three beakers with a beam of light shining into each. In the solution of salt in water, the beam cannot be seen. In the colloid of milk and water, the beam is clearly visible. In the suspension of muddy water, the beam is visible and mud has settled at the bottom. The caption says that the beam is seen in the colloid because its particles scatter the light, which is the Tyndall effect.

The parts of a colloid

A colloid has two parts. The dispersed phase is the substance that is spread out as the tiny particles. The dispersion medium is the substance in which the particles are spread. In milk, tiny drops of fat (the dispersed phase) are spread in water (the dispersion medium).

Examples of colloids

Dispersion medium	Dispersed phase	Example
Gas	Liquid	Fog, cloud, mist
Gas	Solid	Smoke
Liquid	Liquid	Milk
Liquid	Gas	Foam, shaving cream
Solid	Liquid	Cheese, butter, jelly

The three kinds compared

Property	Solution	Colloid	Suspension
Particles seen?	No	No	Yes
Settle on standing?	No	No	Yes
Separated by filtering?	No	No	Yes
Scatter light?	No	Yes	Yes (but we see the particles)

You should find that the beam appears at $1\ \text{nm}$ , and that only particles bigger than about $1000\ \text{nm}$ settle in a sensible time.

Practise with new numbers

Try it in Python

The program names a mixture from the size of its particles.

Remember these

A suspension has large particles that settle and can be filtered.
A colloid has small particles that do not settle and scatter light (the Tyndall effect).
A true solution has very small particles, does not scatter light and does not settle.