Mix It Up with Oil and Water

Video Why does oil float on waterGist Chemistry Surfactants PolarityIntroduce You may have heard people say, “Those two mix like oil and water,” when they describe two people who don’t get along. You’ve probably also noticed shiny oil floating on the surface of puddles after it rains. In either case, you understand that water and oil don’t go hand in hand – but have you ever wondered why? Lots of other things can dissolve in water – why not oil? In this activity we will discover what makes oil so special and we will try to make the impossible possible: mixing oil and water! Reading: Why does oil float on water?Story Unlike many other substances such as juices, food dyes or even sugar and salt, oils do not mix with water. The reason is related to the characteristics of oil and water. The water molecule is made up of one oxygen atom and two hydrogen atoms. In addition to having this very simple structure, water molecules are also polar, which means that there is an uneven distribution of charge across the water molecule. Water has a partial negative charge from its oxygen atom and a positive charge on its hydrogen atom. This polarization 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 atom on the other. Other molecules such as salts and sugars can also dissolve in water due to its polarity. The charges at either end of the water molecule help to disrupt the chemical structure of the other molecules, in contrast, oils are nonpolar and as a result they are not attracted to the polarity of the water molecule. In fact, oil is hydrophobic, or “afraid of water.” Instead of being attracted to the water molecules, the oil molecules are repelled by them. As a result, when you add oil to a cup of water, the two don’t mix. Because oil is less dense than water, it will always float on top of the water, creating a layer of oil on the surface. You may have seen this on the street after a heavy rain – some puddles will have a layer of oil floating on them. The surfactant we will be using is a dish detergent, which helps to break the surface tension between oil and water because it is amphoteric: partly polar and partly non-polar. As a result, detergents can bind to both water and oil molecules. We will see the results of this property in this activity!Material

  • 2 clear plastic water bottles with lids
  • 2 cups of water
  • Half a cup of oil (olive oil, cooking oil, or vegetable oil all work)
  • Dish soap
  • Clock or timer
  • Permanent marker
  • Measuring cup
  • Measuring spoon
  • Food coloring (optional)
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Preparation

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

Procedure

  • Measure carefully and pour 1/4 cup of the oil into the bottle labeled Oil + Water. Leave the bottle on a table or flat surface while you watch the water and the oil. Does the oil sink to the bottom of the bottle, stay on the water, or mix with it?
  • Repeat this step, adding a quarter cup of oil to the bottle labeled Oil + Water + Soap. Does the oil sink to the bottom, stay on 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 when you add dish soap.
  • Make sure that the bottle caps are securely screwed onto each bottle.
  • Holding a bottle in each hand, shake the bottles vigorously for 20 seconds.
  • Place the bottle on a flat surface with plenty of light.
  • Record the time on your watch or set a timer for 10 minutes.
  • Observe the ingredients inside each bottle. Hold them up to the light for a while so you can clearly see what’s going on inside the bottle. Does anything change when you shake the bottles? Are the mixtures the same in both? If not, what is the difference between them? How would you explain the difference you observed?
  • After 10 minutes have passed, look at the composition of the bottles and note the changes. What does the oil and water in each bottle look like? Does oil mix with water, sink to the bottom or float to the top?
  • More: Add food coloring to the water for a lava lamp effect
  • More: Try other soaps, such as toothpaste, hand soap, and shampoo by mixing them with oil and water.
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Observations and results In this activity, you combined oil and water, then watched how adding dish detergent changed the properties of the mixture. First, you should notice that when you add oils to water, they don’t mix. Instead, the oil creates a layer on the surface of the water. This is because oil is less dense than water and so it floats to the surface. When shaking the Oil + Water bottle, you may notice the oil breaking into small particles. However, these particles do not mix with water. After you leave the Oil + Water bottle for 10 minutes, you should have observed that the oil and water begin to separate again almost immediately, and after another 10 minutes there are two distinct layers in your bottle. In contrast, you may find that shaking the Oil + Water + Soap bottle results in a lot of foam, but instead of immediately starting to separate, the mixture is opaque yellow. In the end, the oil and water should have separated into two layers again, but these should be less distinct and have more residue than the layers in your Oil + Water bottle. Soap bottle. Detergent molecules can form bonds with both water and oil molecules. So even though oil and water don’t technically mix, the dishwashing detergent molecules are acting as a bridge between the oil and water molecules. Therefore, the oil and water molecules are not clearly separated in the bottle. Instead, you will see a cloudy mixture, caused by oil, soap, and water chains that you created by adding dishwashing detergent.More to discover Goo-Be-Gone: Clean up spills, from Science Buddies Make your own lava lamp, from Scientific American. Clean Chemistry: Make Your Own Soap to Study Soap Synthesis, from Science Buddies Science Activities for All Ages!, from Science BuddiesThis is brought to you in partnership with Science BuddiesRead more: why baseball players wear necklaces | Top Q&A

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