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Surface tension

Sophie Duncan describes some sink-or-swim experiments

Everyone likes water - and there are lots of ways of experimenting with it.

Take a small rectangular piece of cardboard (about 3cm by 5cm). Shape one end into the front of a boat, and cut a small notch out of the middle of the opposite end. Place your boat in a roasting tin one third full of clean water. Drop a small amount of washing-up liquid in the notch at the back of the boat. Watch what happens. You can repeat the experiment, but replace the water each time.

Take a cotton handkerchief, and rinse it in water. Wring it out. Take a jam jar full of water and place the handkerchief taut across the top, holding it in position with a rubber band. Quickly turn the jar over, and see what happens. If you end up getting soaked then you haven't turned the jar over quickly enough.

Finally, take a glass of water that is full. Ask your students to work out how many 5p pieces you can fit into the glass. Write down their guesses.

Now carefully slide a 5p piece down the side of the glass. Continue until the glass overflows. You will be surprised how many will fit in.

These experiments rely on surface tension. Water molecules are strongly attracted to each other and this attraction causes water to form an invisible skin over its surface. When soap is added to the water the soap gets in between the water molecules and forces them apart.

In the first experiment, the movement of the soap molecules causes the boat to move forwards.

In the second, the surface tension of the water prevents water from squeezing through the gaps in the fabric, and the water remains in the jar.

This can be disrupted by touching the surface of the handkerchief with some washing-up liquid.

In the final experiment the surface of the water rises, forming a convex surface above the top of the glass. The surface tension stops the water spilling out of the glass. This is broken only after many 5p pieces have been put into the glass.

Sophie Duncan is project manager for science at the BBC

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