Practical work

Solution, colloid or suspension?

Statement of the problem

The aim of this practice is to classify mixtures of everyday substances into solutions, colloids and suspensions.

Your proposal

How can we distinguish a suspension from a solution? And a solution from a colloid? What tests can we use to determine whether we are dealing with one of these types of mixture?

Our proposal

We will look at some properties that allow us to differentiate between suspensions, solutions and colloids:

• If the mixture separates into layers when not stirred for a certain time.

• If the complete mixture passes through filter paper, or instead part of it remains unfiltered.

• Whether it is transparent to the light of a laser pointer or whether, on the contrary, it causes this light to be scattered and can then be observed.

Materials

• Water

• Flour

• Cylinder

• Coffee

• Beakers

• Milk

• Filter paper

• Laser pointer or torch.

• Funnel

• Balance

Preparation of the mixtures

Prepare three different mixtures: one of water and flour, one of water and coffee, and one of water and milk. For the latter two, use only a few drops of coffee or milk.

Sequence of tests

• Filtration. First, pass the mixtures through the filter paper, using the funnel (set-up shown in section 4). Observe whether there are differences between the appearance of the original mixture, the filtrate, and the part retained on the filter paper.

• Sedimentation. Then allow the samples to stand, each in a different container, for several hours. Observe if there are any differences before and after the standing time.

• Tyndall effect. Shine the light of a laser pointer through the mixtures. To see it better, turn off the lights in the laboratory. Observe whether or not the light beam is visible as it passes through the mixture.

Were the components separated by filtration?

Did we observe the Tyndall effect?

1 Using the diagram above, draw conclusions about the experiments you performed.

2 Explain why it is possible to see the light beam through a colloid and a suspension.