Surface Tension of Water

These super simple investigations are great for demonstrating the surface tension of water.

What is surface tension?

Surface tension is a force which causes a layer of liquid to behave like an elastic sheet or skin.

It is the high surface tension of water which allows insects to walk over it. These pond skaters have long hairy legs which allow them to spread their weight over a wide area. They press very gently on the surface of the water so as not to break through it.

In a container of water, molecules below the surface are pulled together ( or attracted to each other ) equally in all directions, but those on top are pulled together more tightly, as they don’t have water molecules above them, this draws them together to form a ‘skin’. It is this skin ( surface tension ) that stops items on the surface sinking.

Surface Tension Holes Experiment

You’ll need

A big bowl of water

Some ground pepper (black so you can see it) or any other ground product with colour

Washing up liquid ( dish soap )

Method

Once the water settles, sprinkle the ground pepper over the top.

In the middle of the bowl drip some washing up liquid and watch what happens.

You should see a hole appear in the centre as the pepper moves outwards. This is your surface tension hole!

Why does this happen?

The surface tension hole is caused by the washing up liquid reducing the surface tension of the water. This allows the particles of water at the surface to spread out, starting from where the washing up liquid was added.

More Surface Tension Experiments

Frugal Fun for Boys has a great surface tension investigation using a coin and different liquids!

You can use washing up liquid’s power of disrupting the surface tension of water to race lolly sticks.

In a magic milk experiment the washing up liquid disrupts the surface tension of the milk which makes food colouring spread out just like the pepper and water.

Another surface tension experiment is where you make a shape on the surface of water with cocktail sticks and drop some washing up liquid in the centre to force the sticks apart.

Try filling a bowl half full with water and carefully place a paperclip on the top so it floats. Mix a little washing up liquid in a cup with water and gently pour into the bowl, the paperclip will sink as the water can no longer support the weight of the paper clip after the washing up liquid disrupts the surface tension of the water.