Oil and Water

Have you ever heard that oil and water don’t mix? Make enemies become friends in this colourful experiment.

Australian Curriculum links

  • Science > Chemical Sciences > Year 6 > ACSSU095
  • Science > Science as a Human Endeavour > Year 7 > ACSHE121

You'll need

  • clear jar or bottle with a tight fitting lid
  • detergent
  • cooking oil
  • water
  • food colouring

Try this

  1. Add 3 drops of food colouring to the jar.
  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.
  4. Let the liquids settle and observe them.
  5. Add a few drops of detergent to the jar and shake again.
  6. Observe the results.

What's happening?

Even when you mix oil and water together they still separate. The oil always floats to the top because it is less dense than water. Oil and water don’t mix because water molecules are more attracted to each other than to oil molecules. Detergent molecules are attracted to both water and oil. When you add the detergent, one end of each a detergent molecule attaches to a water molecule and the other attaches to an oil molecule. The detergent creates a mixture of water with droplets of oil spread through it.

Molecules of water are strongly attracted to each other because they are polar. Oil and water molecules are not attracted to each other because oil molecules are non-polar and hydrophobic or ‘water-fearing’. Oil has a lower density than water, so it always floats on top.

Detergent is attracted to both oil and water because one end is hydrophilic (or ‘water-loving’) and sticks to water and the other end is hydrophobic and 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.

Real world links

We use detergent when washing up because it attaches to the oil and grease on dirty dishes and lifts off into the water. Oil also helps animals that live in oceans stay warm because the oil on their feathers or coat keeps the cold water away from their skin.