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KS1 Science at Home: week 8

Key Stage 1 Experiment of the Week! – Germs!

How does it work?


Pepper is hydrophobic, meaning the water is not attracted to it. However, when the soap is added (on your finger) to the water, the surface tension changes, and the pepper no longer floats on the top.  The water molecules still want to keep the surface tension going, so they pull back away from the soap, carrying the pepper with them. 

Materials you need

  • Shallow bowl or dish
  • Water
  • Pepper
  • Liquid dish soap

    Pepper soap ros ex 1 ros


1. Cover the bottom of your shallow dish with water.

2. Sprinkle black pepper across the surface of the water. Note how the surface tension of the water causes the pepper flakes float.

Pepper soap plate ex ros

3. Stick your finger in the centre of the dish; did anything happen? Not much right? You probably just got some pepper flakes stuck to your finger. Now imagine that the pepper flakes are germs.

4. Now dip the tip of your finger into the liquid dish soap—you don’t need much.

5. Now stick that finger into the centre of the dish. What happens? Your soapy finger chased those pepper flakes to the edges of the plate! Dish soap is formulated to break the surface tension of water, which is why it is so effective on greasy, dirty dishes. And it wasn’t until you added soap to the bowl that those "germs" were chased away. This is the reason grown-ups are always nagging you to wash your hands with soap!

Questions to think about

Does it work with different things other than pepper? What about seeds?

Some research for next week: How can you make a walking rainbow?

We will answer this question with next week’s science experiment!