There is nothing to sneeze about in this activity. Learn why some animals can walk on water by investigating the surface tension of water.

Australian Curriculum links

• Science > Chemical Science > Year 6 > ACSSU095

You'll need

• take-away food container or other clear container
• water
• pepper
• detergent in a small cup
• matchstick

Try this

1. Make sure the container is clean and completely free of detergents and soaps before starting. (If it isn’t clean this activity won’t work!)
2. Cover the bottom of the container with at least 3 cm of water.
3. Sprinkle some pepper on top of the water and watch what happens to the pepper. (The pepper should float.)
4. Dip the matchstick in the detergent and place a drop of detergent in the middle of the water. Watch what happens to the pepper.

What's happening?

Water has high surface tension which is why the pepper sits on top of the water when you sprinkle the pepper into the container. High surface tension means that all of the little pieces of water called ‘water molecules’ are holding on and pulling against each other really strongly. This makes the surface of the water strong enough to hold up the pepper.

When you drop the detergent on the surface of the water the surface tension on that part of the surface of the water is reduced, but not over the entire surface of the water. Some of the water further away from the detergent still has high surface tension meaning the water molecules further away from the detergent are still pulling against each other strongly; this leads to a force that pulls the pepper away from the detergent.

Real world links

Some small insects such as mosquitoes, water skippers and water striders use the high surface tension of water to walk on water. Even spiders can do this. The water spider (Dolomedes sp.)—which is commonly found in gardens all around Sydney—uses the fine hairs on its legs to increase its surface area. This lets it walk on the surface of ponds or gentle streams without breaking the surface tension of the water. This amazing spider can even survive underwater for up to an hour by using the surface tension of water to trap small bubbles of air on its body.