An official website of the United States government.

We've made some changes to If the information you are looking for is not here, you may be able to find it on the EPA Web Archive or the January 19, 2017 Web Snapshot.

Polluted Runoff: Nonpoint Source Pollution

Darby Duck, the Aquatic Crusader - Shake it up, Baby!

When you pour two liquids together, will they always mix? Different liquids have different densities, which means that some liquids are heavier than others.

In this experiment, you will see which liquid is heavier: oil or water. You will also learn how detergents work to get greasy, dirty clothes clean.


  • glass jar with a screw-on lid
  • water
  • cooking oil
  • dish washing detergent


  1. In a glass jar, pour one cup of water and a thin layer of cooking oil. What happens?
  2. Screw on the top and shake the bottle to mix the oil and water. Can you get the oil and water to mix together? NO! The two liquids always separate into two layers, no matter how hard or long you shake the bottle. The oil is less dense, or lighter, than the water so it floats on top. Also, the water molecules are more attracted to each other than they are to the oil, so the liquids don't mix.
  3. Next, add a squirt of dish washing detergent to the jar. Put on the lid and shake the jar. Are the oil and water still separated? As the foam clears, you should see a milky liquid and no layers.

How it Works

Detergent molecules are attracted to both water and oil. One end of the detergent molecule loves water and the other end loves oil (and dirt). The detergent breaks up the surface tension that usually keeps water molecules stuck together and allows the water to be free to mix with the oil. This is how detergent and water can get dirty clothes clean.