Thursday, June 15, 2006

Why Can't Oil and Water Mix?

Have you ever wondered why oil and water do not mix? Chemistry gives us a clue, saying that liquids which are made of similar molecules tend to mix with each other easily. Molecules are some of the fundamental building blocks of matter.

Oil and water are made of different kinds of molecules which interact differently with each other so they do not tend to mix. The molecules of water, for instance has a more "electronic interaction" whereas oil has "non-electronic interactions."

Further, a drop of oil is usually lighter than an equally sized drop of water, so usually oil floats on top of water when the two are present in the same place. In other words, water is more dense than oil. You might have seen this when you place a drop of butter on top of a hot soup. Soup is mostly water and butter has a lot of oil in it, so as the butter melts it floats as a thin layer on the soup.

This is the same thing that happens when an oil spill occurs if a ship carrying petroleum (crude oil) breaks while on the sea. The oil spills and floats on top of the sea water causing lot of pollution and problems to aquatic life. Of course, oil spills occur very rarely and the more common reason for oil in the ocean water is from cleaning and rinsing of ships.

Just like cleaning oil spills can be a problem for environmental scientists, removing grease from clothes is a problem for all of us. Fortunately, although oil and water do not mix, there are chemicals like detergents which are attracted to both oil and water and can aid in their mixing.

When a detergent is added and mixed up between the oil and the water, it holds hands with oil and water molecules and helps in getting the oil rinsed off with excess water. Such a mixture where oil and water can finally be together with the help of another substance is called emulsion.

Some emulsions which are actually edible happen to be milk, butter, mayonnaise, etc. Of course these are not made with detergents! In summary, oil and water will not mix by themselves because of many different properties, but they can be made to mix with the help of things like detergents or emulsifiers. Since oil floats on water, a lot of fun (but messy) experiments can be done using oil and water.

For fun experiments about oil and water, see

Little Lion Experiment:

We will learn how oil and water interact . Caution: These experiments can all get pretty messy, so do NOT attempt them on carpeted floors at all. Also, it is advised to not do it on a wooden floor either, as any spill can be slippery and dangerous. These are best done on a garage floors, preferably with a lot of paper towels around and a small box of sand nearby.

You will need:

  • Water
  • Vegetable oil
  • Glass jar or clear drinking glass (an old pasta sauce bottle will do the job)
  • Food coloring (optional)
  • Salt
  • Detergent powder or dishwashing liquid.
  • A wide glass bowl.

Steps: Experiment 1:

  1. Pour water halfway into the glass jar.
  2. Pour quarter cup oil on top of the water.
  3. Let the liquids settle and observe what happened, which layer is on top, etc.
  4. If you have food coloring add a drop or two to the top surface and wait and see what happens.
  5. Another thing you can do is, sprinkle some salt to the top of the oil and see what happens.
  6. You can also now try pouring a small amount of water using a table spoon to the top of the oil layer and see what happens to this new water.
  7. Add some detergent powder or a few drops of dishwash liquid and mix things up with a spoon. Allow mixture to settle and see what it looks like now. Do you still see two clear layers?

Steps: Experiment 2:

  1. Pour water halfway into the glass bowl.
  2. Take a table spoon of oil and try to form a small region of oil film on the water.
  3. Try to see if you can break the oil film into several small regions with your spoon.
  4. Then try putting them back together into one film.
  5. Sprinkle some detergent on top of the oil and mix it up.
  6. Now see what has happened to the shiny oil film.