CSIRO student activity: Clingy cork

CSIRO student activity: Clingy cork

In this CSIRO student activity, kids can experiment with corks and water to learn about water molecules.

A cork floating in a glass of water.

You will need

  • Large glass
  • Small cork
  • Small water jug (optional)
  • Towels to mop up any spilled water

What to do

1. Half fill the glass with water.

2. Put the cork in the glass.

3. Watch the cork. Where does the cork sit in the glass? Does it stick to the side of the glass, or float to the centre?

4. Slowly add more water to the glass, right to the very top. Keep adding water until it is raised just above the top of the glass, but doesn't spill.

5. Look at the cork. Where does the cork sit in the glass now?

What's happening?

You might think that the surface of water is flat, but take a closer look. The water only curves a millimetre or two, but if you look carefully you can see it. You might find it easier to notice when you take the cork out.

A cork will float near the highest point of the water. When the glass is half full, that's near the edge. When it's almost overflowing, the highest point is in the middle of the glass. The shape of the water's surface is known as the meniscus, and it is usually curved. The curve happens because water molecules stick to some things and not to others.

Water molecules stick to each other, but they stick to glass even more. In a half filled glass, the water pulls itself up the sides of the glass, and the water surface forms a curve that's low in the middle and high at the edge.

When you overfill a glass, the water level reaches just above the top of the container. Why doesn't it overflow? Once again, it's sticky water. But this time it's because water sticks to itself. Water near the top is pulled back towards the rest of the water. But this force isn't unbeatable - fill the glass too far, and the glass will eventually overflow.

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This article first appeared in CSIRO's Science by Email (written by David Shaw and edited by Jasmine Fellows). Sign up to Science by Email to receive science news, an activity and a quiz each fortnight. It's free!

Find more resources about atoms and molecules on Splash, including videos and games.

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