Surface Tension

We had a bowl of water (with yellow food dye in it, just so we can see the water more easily).

Teacher sprinkled pepper on top of the water.  We expected the pepper to dissolve like salt does, but instead it just sits on top of the water.  Teacher told us that it sits on a kind of a skin (molecules holding on to one another).  This is called surface tension.

We predicted what would happen if we put a piece of tissue in the middle of the bowl.  We guessed that the water (and some of the pepper) would run to the tissue and be absorbed.  We were right!

Now teacher put a drop of hand sanitiser on her finger and touched it to the middle of the bowl.  Some of the water (and pepper) ran away from teacher’s finger.  This is because the surface tension was broken by the sanitiser.

Then teacher used wash-up-liquid on her finger instead of sanitiser.  This was much better.

The water and pepper moved really quickly away from teacher’s finger.

Germs are like this.  Soap and wash-up-liquid are really really good at repelling germs in exactly the same way.

We could even make a little mini-card boat and float it on the water (surface tension again).  If we drop a drop of wash-up-liquid at the back of the boat, the boat will power forward in the water, moving forward as the surface tension is broken.  In our video we show you what happens if the drop of liquid is dropped on the side of the boat!