EXPLORE SCIENCE AT HOME Experiment

Ever tried to wash dishes without soap? Those greasy pots just won’t come clean! Here are some fun explorations you can try with your children while making or cleaning up after dinner.

Oil and water don’t mix.
If you’ve ever made homemade salad dressing with oil and vinegar, you know you need to shake it up to get the vinegar and the oil to mix. However, turn your back for just a few minutes, and the oil and vinegar separate. Why? And how can you prevent it?

To get started, you’ll need the following:
- 2 jars with lids
- canola or olive oil (we should ask Kathryn if vegetable oil is ok)
- food coloring
- water
- dish soap

Fill 1/3 of both jars with water. Add a few drops of food coloring to the water. Now add the same amount of oil to the jars as you did water. To the second jar, add a few drops of soap. Now put on the lids tightly, and shake. Put the jars back on the counter, and observe what happens over the next few minutes. You should see that the oil/water mixture begins to separate, with the oil on top. The jar containing the oil/water/soap mixture, however, will stay mixed together.

What happened?
Oil, or fat, is hydrophobic. Hydro means water, and phobic means fearing, so fat “fears” water.  Water is hydrophilic, or water-loving. Soap molecules are amphiphilic, which means they love both oil and water. One side of the soap molecule likes water, while the other likes fat. So the soap molecules arrange their hydrophobic ends around little droplets of fat molecules, while their hydrophilic ends point out towards the water. This arrangement allows the fat to coexist with the water. This demonstration can help explain how soap lifts all the greasy residue in your pans after you make lasagna – the soap’s hydrophobic ends help get the oil droplets into the water so that they can be washed down the drain.

The process described above is called emulsification, and soap acts as the emulsifier, bringing water-loving and water-fearing molecules together into a well-blended mixture. Look at the ingredients for salad dressings, chocolate, and other foods that may contain fats. You might find an ingredient called lecithin, which is a common emulsifier found in foods. Lecithin can come from eggs, or also from soybeans. We can probably all agree that lecithin is probably a much better tasting emulsifier than soap!

Courtesy of Dr. Kathryn Hollar, Director of Educational Programs, Harvard School of Engineering and Applied Sciences