Oil and water

Science concepts

Oil and water don’t mix.
Detergent mixes with both oil and water.

Special instructions

Thirsty? This shows the oil and water as separate layers, before adding the detergant.

Teachers should conduct their own risk assessment of this activity.

Class time required: 15 minutes

Materials

• A clear jar or bottle with a tight fitting lid
• Detergent
• Cooking oil
• Water
• Food colouring

What to do

1. Mix some food colouring with the water.
2. Add equal amounts of oil and water to the jar, each about one third of the jar’s volume.
3. Screw the lid on tightly and shake the jar vigorously.
4. Let the liquids settle and observe them.
5. Add a few drops of detergent to the jar and shake again.
6. Note the results.

What’s happening?

Molecules of water are strongly attracted to each other (hydrophilic) but oil and water are not attracted to each other (oil is hydrophobic) so they do not mix. Oil has a lower density than water, so it always floats on top.

Detergent is attracted to both oil and water. One end of a detergent molecule sticks to water, and the other end sticks to oil. When we add detergent to oil and water and shake the mixture, the liquids are held to each other by the detergent molecules and form an emulsion. An emulsion is a stable mixture of two or more liquids that would not normally mix.

Did you know? – We use detergent for washing up because it attracts the oil and grease on dirty dishes and lifts it off into the water.