Pepper Repellent – Kids Experiment

A simple yet satisfying way of showcasing the magic of chemistry, the ‘Pepper Repellent’ experiment is easy to do and only requires common household ingredients. The science behind it can be hard to get your head around, but we’re hoping you and your child find it interesting nonetheless:

You will need:

  • Black Pepper
  • Water
  • Washing Up Liquid
  • A Plate

Method:

  • Firstly, start by grabbing your plate and running it under a tap so that you have a pool of water resting on it. Steadily set it down on a hard surface and prepare your black pepper.
  • Sprinkle/spread out some black pepper on the surface of the water. What happens when you put your finger in the water afterwards? Give it a go!
  • Apologies if the result is disappointing, but not a lot is going to happen to the black pepper yet. Fortunately, the washing up liquid will change that! Put a small amount of it on the tip of your finger and try putting your finger in the water again. What happens this time? If you’ve done everything correctly, it’ll look like your finger is repelling the black pepper.
  • If you want to try it again, you can experiment to see how different amounts of black pepper effect the repelling nature of the washing up liquid. But why is the black pepper so scared of it?

The Science Bit:

It’s strange if you think about it, isn’t it? Not only is the black pepper floating on the water, but it is also repelled by washing up liquid. So what is going on? Well, pepper is a hydrophobic, meaning that it fails to mix with water. This is because hydrophobic molecules repel themselves from masses of water, which is why the pepper instead floats on top of the water. Furthermore, water molecules tend to cling to one another and arrange themselves in a way where light objects will float instead of sink. This is because of the surface tension that has been made from the water molecules clinging to each other and as a result, the top of the water keeps the pepper flakes from sinking.

But how does adding washing up liquid affect this? Interestingly enough, the soap in the washing up liquid breaks down the surface tension the water molecules made. As a result, the water molecules quickly spread out and carry the pepper with them. And that just about the breaks down the chemistry of the ‘Pepper Repellent’ experiment, we hope you found it interesting!