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Density Mystery

In this activity, students make a ‘lava lamp’ that they can take home, to discover the relative densities of water and oil. 

The relative densities of liquid describes how closely packed together the molecules of a substance are. A more densely packed liquid will be "heavier" and sink under a less dense liquid.

For a variation of the plain oil & water Lava Lamp, have a look at this video from Science World's own Future Science Leaders.


  • Investigate the relative densities of oil and water.

  • Observe miscibility and immiscibility.


  • Per Student:
    large plastic test tubes with covers, OR small, empty clear pop or juice bottles (500mL or smaller), with lids
    vegetable oil
    food colouring
    funnel (or a paper cone with the end cut off)

    Optional Extensions: antacid tablet and /or table salt.

Key Questions

  • Is the water above or below the vegetable oil? Why?
  • Which layer does the food colouring mix with? The oil or water? Why?
  • What happens when you turn the lava lamp upside down? Why?

What To Do

  1. Give each student a test tube or bottle with a lid.
  2. Using a funnel, fill the test tube or bottle about 1/3–1/2 full of vegetable oil.
  3. Fill the test tube with about the same amount with water. Explain that the oil is less dense than the water and therefore sits as a layer on top of the water, they are immiscible.
  4. Add 1–2 drops of food colouring. Students will notice that the food colouring mixes with the water (they are miscible), not the vegetable oil. This is because food colouring is water based.
  5. Close the lid tightly and gently tip the lava lamp upside down. Watch the neat effect as the oil floats up to the top.


  • Give your lava lamp a boost by adding a small piece of an antacid tablet (about the size of a quarter will work). The tablet creates bubbles of carbon dioxide. The CO2 attaches to the water molecules and together they sink and rise, bringing and mixing the two layers in a neat lava-lamp-like effect.
  • Alternately, add 1 Tablespoon of table salt to the top of the mixture once students have observed it mixing and settling agian. Salt will carry oil into the bottom layer of water with it as it falls and create a lave lamp effect as the oil rises above the water layer again.

Other Resources

TedEd |Video | Why Don’t Oil and Water Mix?