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Density - Oil Floats (Robert Nairne)

Robert Nairne

Principles Illustrated

Most of my students have not been in tune with the dangers of oil spills in our oceans for animals and ecology. The most recent oil spill occurred on May 9, 2001 in the Galapagos Islands. The Galapagos Islands are located off the coast of Ecuador. The tanker, Jessica, tipped over pouring 240,000 gallons of oil into the ocean.  This is a small experiment for the students to see that il does in fact float on top of water.


7.7.A Select and use appropriate tools and technology (including calculators, computers, balances, spring scales, microscopes, and binoculars) to perform tests, collect data, and display data.

7.7.B Construct scale models, maps, and appropriately labeled diagrams to communicate scientific knowledge (e.g., motion of Earth’s plates and cell structure).

What you'll need:

  • Small soft drink bottle
  • Water
  • Food coloring
  • 2 tablespoons of cooking oil


  1. Add a few drops of food coloring to the water.
  2. Pour about 2 tablespoons of the colored water along with the 2 tablespoons of cooking oil into the small soft drink bottle.
  3. Screw the lid on tight and shake the bottle as hard as you can.
  4. Put the bottle back down and have a look, it may have seemed as though the liquids were mixing together but the oil will float back to the top.

What's happening?

While water often mixes with other liquids to form solutions, oil and water does not. Water molecules are strongly attracted to each other, this is the same for oil, because they are more attracted to their own molecules they just don't mix together. They separate and the oil floats above the water because it has a lower density.

If you really think oil and water belong together then try adding some dish washing liquid or detergent. Detergent is attracted to both water and oil helping them all join together and form something called an emulsion. This is extra handy when washing those greasy dishes, the detergent takes the oil and grime off the plates and into the water.


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