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Why do oil and water not mix?

Explore the role of density in the mixing of liquids.

Why do oil and water not mix?





Developing Skills

Mathematics, Observing



What you need

  • Cooking oil (125 mL)
  • Blue food colouring
  • Small objects of various materials and sizes
  • Clear container or drinking glass
  • Water (125 mL)

Safety first! Food colouring can cause stains. Handle it with care and wipe up spills immediately.

What to do

  1. Add the water and blue food colouring to the clear container.
  2. Gently add the cooking oil to the container. What happens to the two liquids?
  3. Add the small objects, one at a time, to the container and observe what happens. Where do the objects end up in relation to the water and the oil? What do you think is happening here?


What's happening?

Oil and water are two liquids that are immiscible, meaning they will not mix together. Liquids tend to be immiscible when the force of attraction between the molecules of the same liquid is greater than the force of attraction between the two different liquids.

Although this experiment uses the same volume of oil as water, the two liquids have different masses and therefore, different densities. Density is a measure of how much of a substance is contained in a specific volume of liquid. A liquid that is less dense than water will float on the water; a liquid that has a greater density will sink.

To gain an understanding of density, think of two zippered plastic bags of the same size (same volume). Imagine that one bag contains 10 marbles and the other 20 marbles. The bag containing 20 marbles is denser than the bag containing 10 marbles because it contains more material – even though it is the same material. This analogy describes the relative densities of different concentrations of the same substance.

Also imagine that there is a third bag containing 10 very large marbles. The volume of the material is the same, the number of molecules (marbles) are the same but again, the bag of large marbles contains more material - it has a greater mass and so a greater density. This analogy represents the experiment above since two different materials are used.

The objects added to the container will float at different levels according to their density. If the density of the object is similar to that of water, the object will float in the water. If similar to the oil, the object will float in the oil.

What does it matter?

Getting oil and water to mix is at the very heart of cleaning dishes and clothes. A lot of agents that make dishes and clothes dirty are greasy or contain oil. Water alone is not attracted to these compounds. However, because a detergent has one end that is attracted to oil-like molecules, detergents tend to bind to dirt, grease and oil. The other half of the detergent binds to water molecules, allowing the soiling agent to be washed away.

Investigate further!

You can also try layering oil, water, pancake syrup and dish detergent in a narrow, cylindrical drinking glass or vase. Pour the liquids into the container slowly. Let the container sit still for a few minute before observing.

In what order do the liquids form layers in the container? What does this tell you about the density of each liquid?