This Scared Pepper experiment is brilliant fun – kids love watching the pepper flakes get ‘scared’ and scoot away! – and it’s also brilliantly educational.
It’s not only a mini lesson in the science of water-surface tension (see our Explainer below), it’s also a great, visual way to explain why washing your hands is so good at killing germs: tell your kids the pepper are the germs and that, when we wash our hands with soap, the germs run away!
The science behind the experiment: explained
Pepper is hydrophobic – which means water is not attracted to it and that’s why the pepper flakes don’t dissolve. But that doesn’t explain why the pepper flakes don’t sink. That’s all due to fact that water molecules like to cling together, lining up in a certain way that gives the top of the water a surface tension – which means something as light and as hydrophobic as pepper flakes will then just bob around on the top.
But, when you add your soap, it starts to break down the surface tension of water (as any good cleaner is designed to do). The water molecules still want to stick together and maintain that tension, so they move away from the soap, carrying the light pepper flakes along with them – creating the ‘scared’ pepper effect.
Here’s what you’ll need:
- A shallow bowl
- Black pepper
- Liquid soap (or washing-up liquid)
Here’s what to do:
- Put about 3cm of water in your bowl. Sprinkle some black pepper over the surface and watch them float, rather than sink. Now ask your child to put a finger in the middle of the bowl. See: nothing happens!
- Now squeeze a little soap onto your child’s finger, and ask them to put it back in the bowl. See what happens now! The black pepper flakes will dart to the side of the bowl, as it they’re scared of your child’s soapy finger!
If your child’s really intrigued, you could try repeating the experiment using olive oil instead of water: do you think the pepper flakes will act the same way?