Key concepts
Chemistry
Surfactants
Density
Polarity

Introduction
You may have heard people say, “Those two mix like oil and water,” when they’re describing two people who don’t get along. Maybe you’ve also noticed shiny oil floating on the surface of water puddles after it rains. In both cases you understand that water and oil don’t go well together—but have you ever wondered why? So many other things can dissolve in water—why not oil? In this activity we’ll explore what makes oil so special, and we’ll try making the impossible happen: mixing oil and water!

Background
Unlike many other substances such as fruit juice, food dyes or even sugar and salt, oils do not mix with water. The reason is related to the properties of oil and water. Water molecules are made up of one oxygen atom and two hydrogen atoms. In addition to having this very simple structure, water molecules are polar, which means there is an uneven distribution of charge across the water molecule. Water has a partial negative charge from its oxygen atom and partial positive charges on its hydrogen atoms. This polarity allows water molecules to form strong hydrogen bonds with each other, between the negatively charged oxygen atom on one water molecule and the positively charged hydrogen atoms of another. Other molecules such as salts and sugars are able to dissolve in water because of its polarity as well. The charges at either end of the water molecule help break up the chemical structures of other molecules.

Oils, by contrast, are nonpolar, and as a result they’re not attracted to the polarity of water molecules. In fact, oils are hydrophobic, or “water fearing.” Instead of being attracted to water molecules, oil molecules are repelled by them. As a result, when you add oil to a cup of water the two don’t mix with each other. Because oil is less dense than water, it will always float on top of water, creating a surface layer of oil. You might have seen this on streets after a heavy rain—some water puddles will have a coating of oil floating on them.

In this activity we will test the power of surfactants to help us mix oil and water. The surfactant we will use is dish detergent, which helps break up the surface tension between oil and water because it is amphiphilic: partly polar and partly nonpolar. As a result, detergents can bind to both water and oil molecules. We’ll see the results of this property in this activity!

Materials

  • 2 clear plastic water bottles with lids
  • 2 cups of water
  • One-half cup of oil (olive, cooking or vegetable oils will all work)
  • Liquid dishwashing soap
  • Clock or timer
  • Permanent marker
  • Measuring cup
  • Measuring spoon
  • Food coloring (optional)

Preparation

  • Remove any labels from your water bottles.
  • Use your marker to label the bottles: Label the first “Oil+Water” and the second “Oil+Water+Soap.” Write the labels as close to the tops of the bottles as possible.
  • Pour one cup of water into each bottle.
     

Procedure

  • Carefully measure and pour one-quarter cup of oil into the bottle labeled Oil+Water. Allow the bottle to sit on a countertop or flat surface while you observe the water and oil. Does the oil sink to the bottom of the bottle, sit on top of the water or mix with it?
  • Repeat this step, adding one-quarter cup oil to the bottle labeled Oil+Water+Soap. Does the oil sink to the bottom, sit on top of the water or mix with it?
  • Carefully add three tablespoons of dish soap to the bottle labeled Oil+Water+Soap. Try not to shake the bottle as you add the dish soap.
  • Make sure the bottle caps are screwed on tightly to each bottle.
  • Holding a bottle in each hand, vigorously shake the bottles for 20 seconds.
  • Set the bottles down on a flat surface with plenty of light.
  • Note the time on your clock or set a timer for 10 minutes.
  • Observe the contents of each bottle. Hold them up to a light one at time so you can clearly see what is happening inside the bottle. Did anything change when you shook the bottles? Do the mixtures look the same in the both? If not, what is different between them? How would you explain the differences that you observe?
  • After 10 minutes have passed look at the contents of the bottles and note the changes. What does the oil and water look like in each bottle? Has the oil mixed with the water, sink to the bottom or rise to the top?
  • Extra: Add food coloring to the water to get a lava lamp effect
  • Extra: Test other types of soap, such as toothpaste, hand soap and shampoo by mixing them with oil and water. 

Observations and results
In this activity you combined oil and water then observed how adding dish detergent changed the properties of this mixture. First you should have noticed that when you added the oil to the water they did not mix together. Instead the oil created a layer on the surface of the water. This is because oil is less dense than water and therefore it floats to the surface. When you shook the Oil+Water bottle you might have noticed the oil broke up into tiny beads. These beads, however, did not mix with the water. After you let the Oil+Water bottle sit for 10 minutes you should have observed the oil and water starting separating again almost immediately, and after another 10 minutes there was once again two distinct layers in your bottle.

In contrast you should have found shaking the Oil+Water+Soap bottle resulted in a lot of foam, but instead of immediately starting to separate, the mixture was a cloudy, yellow color. Eventually the oil and water should have separated into two layers again, but these layers should have appeared less distinct and cloudier than the layers in your Oil+Water bottle.

The difference between the two bottles results from adding dish detergent to the Oil+Water+Soap bottle. The detergent molecules can form bonds with both water and oil molecules. Therefore, although the oil and water aren’t technically mixing with each other, the dish detergent molecules are acting as a bridge between oil and water molecules. As a result, the oil and water molecules aren’t clearly separated in the bottle. Instead, you see a cloudy mixture, resulting from the oil, soap and water chains you’ve created by adding dish detergent.

More to explore
Goo-Be-Gone: Cleaning Up Oil Spills, from Science Buddies
Make Your Own Lava Lamp, from Scientific American
The Chemistry of Clean: Make Your Own Soap to Study Soap Synthesis, from Science Buddies
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies