Spicy Surface Tension

Did you know that water has a skin? In this experiment we're going to put stuff on its skin, and then split the skin to see what happens! No, really!


  1. Fill a shallow saucer or bowl with water.
  2. Sprinkle ground black pepper over the water. You'll see it floating on top - it's actually sat on the water's skin!
  3. Get your finger soapy with washing up liquid or ordinary soap.
  4. Dip your finger slowly into the middle of the water - what happens?

You will see the black pepper immediately shoot away from your finger. If it hasn't all gone to the edges, try dipping your finger in other places and you'll see it do it again and again.

So what's happening?

The "skin" of water and other liquids is actually called surface tension, which is caused when water molecules line up with each other at the edges of the liquid. When you add soap it weakens the strength of the surface tension and with the rest of the surface under tension, appears to rip a hole in the surface. Washing up liquid and soaps are known as detergents, which are made up of molecules that have two ends - one end that loves water (hydrophilic) and one that hates water (hydrophobic).

When you add a detergent to water, one end of each of the soapy molecules is attracted to the water and the other end wants to push it away, and this is what causes the pepper to spread away from your soapy finger as quickly as it can!

What if?

What would happen if you:

  • Used milk instead of water?
  • Used salt instead of pepper?
  • Used ketchup instead of soap?