Experiment:
DONE PEPPER
You need:
- A bowl or deep plate
- Pepper
- Dishwashing liquid
This is what you do:
Fill the bowl with water.
Sprinkle some pepper over the water.
Drip a few drops of washing-up liquid in the middle of the bowl.
Explore and observe:
- What happens to pepper when we drop soap in the bowl?
- What happens to pepper if we add more / less soap?
- Why do you think pepper moves when we put soap in the bowl?
- What happens to pepper if we use different types of soap (whirlpool, bar soap, hand soap, etc.)?
- What happens if you use a different seasoning such as salt, paprika or oregano?
What is going on?
Pepper is hydrophobic, meaning it doesn't like water or doesn't mix with water. Because of this, pepper cannot dissolve in the water. But why do the flakes float on top of the water? Water molecules like to stick together and create the surface tension on top. Because peppers are so light and hydrophobic, surface tension keeps them floating on top.
Why does pepper shoot to the sides when soap touches the water? The soap breaks down the surface tension of water - that's part of what makes soap a good cleaning agent. As the soap moves in the water, and the surface tension changes, the pepper no longer floats on top. But the water molecules will still keep the surface tension going, so they pull away from the soap and carry the pepper with them.
Do you think soap is the only substance that can break down water's surface tension? Try the same experiment, but with olive oil or hairspray. Do you think pepper will react the same way?
