Oil, Water and Chemistry: Fun Science Experiments for Your Students
Although we might interact with oil just about everyday when we cook, did you know that it actually has many different properties? In fact, depending on what you mix it with, it can also react in various ways. One of the most fun ways to discover the properties of oil is by carrying out some science experiments. These experiments are safe to do, but make sure that you wear gloves and protective clothing in case of any spills! When you're done, don't forget to clean up.
Oil and Vinegar
The first experiment we'll start with is by mixing oil with vinegar. Get a glass or a transparent bowl and pour in some oil. When adding vinegar, it's best to try with balsamic vinegar, which is already quite dark. You'll see that the oil stays at the top and the vinegar sinks to the bottom. This is because the oil and vinegar particles are unable to stick together. We call this immiscible. The vinegar is heavier than oil, so it sinks down. You can temporarily force them to mix by stirring them with a fork, but after a little while, you'll see that they separate back again.
Now that we know what happens when oil is added to vinegar, let's think about how to make them combine permanently. We need to add a third ingredient. By mixing an egg yolk with some vinegar and then adding it to our previous oil and vinegar mixture, we find that everything is properly mixed without separating! The main reason is because the egg yolk provides a chemical called lecithin and it forms a chemical reaction called an emulsion. The fat molecules in the oil hang in the mixture of egg yolk and vinegar. When you mix it up, the fat molecules stick together. If you use the right amounts of these ingredients, you can create your very own mayonnaise!
Freezing Oil and Water
Let's make a new mixture now. This time we'll combine some oil and water together. At room temperature, the oil will float to the top of the water. This is because oil molecules are actually much lighter than water molecules. It's a bit like comparing a balloon and a brick. If you drop both of them into a room, the balloon will float up, while the brick falls down. This concept is known as density. However, the density of the water and oil is different if the temperature changes. Try putting your container of oil and water in the fridge. If you check back after a couple of hours, it will be reversed! The oil would have sunk to the bottom and the water would have risen to the top.
Soap and Oil
As far as we've seen, oil molecules love to be separate from other substances. However, when you add soap to oil, it acts as an emulsifier. Think about when your hands are greasy and you use soap to wash them. The soap helps the grease molecules on your hands mix with water molecules. If you try washing off grease using only water, it will stay immiscible. In other words, the oil and water molecules won't mix, so your hands will remain greasy. Another way to explain this is that the soap reduces the surface tension of the oil. It makes the oil molecules spread far apart instead of clinging together. This is a type of chemical reaction caused by mixing different substances together. Oil can sometimes be quite tricky to clean up properly. Think about when we accidentally have huge oil spills in the ocean. We could simply add soap to clean up the oil, but that would endanger the fish and other ocean creatures. Can you think of other ways to remove the oil?