How does soap change how molecules interact?
Time to celebrate! Fireworks are a major hallmark of warmer seasons, but the weather is still cool and conditions might not be right anyway. How can we get the amazing effects of fireworks without using, well, fire? In this experiment, we can create bursts of color using nothing more than simple kitchen supplies.
The structure of soap molecules enables them to remove dirt with ease, as well as move around oily particles. They consist of a hydrocarbon chain with an atom at the end. The hydrocarbon end is attracted to oil and repels water, whereas the other end attracts water. This can create all sorts of interesting effects in practice. Using the materials below, see if you can create a pop of color on your kitchen table.
Why did the food colouring push away from the cotton swab? Milk has fat and water in it, and fat does not dissolve in water. Soap is made up of special molecules that can interact with both fat and water, so when the soap starts to connect to the fat molecules in the milk, it allows them to dissolve in water and be moved around. The soap molecules are looking for fat molecules to join with, and as they move through the milk, they push the food colouring around. This eventually stops happening because all of the soap molecules find fat molecules to connect to, and they don’t need to move around anymore. This is why dish soap is so effective at getting grease and oil off dishes. The soap molecules attach to grease molecules and allow them to be more easily dissolved in water, so they can wash right down the drain.
If the fat in the milk is what makes this happen, do the results change if you try milk with different amounts of fat? For example, instead of whole milk, can you use skim milk, or 2%, or 1%? What about cream? What else do you have that is made with milk and can be a liquid?
Milk is great because the white background makes it easy to see the food colouring, but does it have to be milk, or can you use other liquids, or even combine other liquids? Keep in mind that not all liquids around the house are safe, so always check with an adult before using. Be especially careful if you want to use a liquid you wouldn't drink.
Do you think there might be anything other than dish soap you could use to create the same effect? If you find options that work better or worse, can you come up with or research reasons why they are better or worse?
Lastly, what about the cotton swab? See if you can think of other ways to add the dish soap. What if you add it in two or more spots at the same time?
Bonus: food colouring works great as a visual display, but feel free to get creative. You can probably come up with other ways to show that the milk is moving.
This experiment was selected for Science at Home because it teaches NGSS Disciplinary Core Ideas, which have broad importance within or across multiple science or engineering disciplines.
Learn more about how this experiment is based in NGSS Disciplinary Core Ideas.