Science corner

21st October 2005 at 01:00
Sophie Duncan breaks the surface tension

Having a stressful week? Here are some ways to break the tension.

There are lots of experiments you can do to explore surface tension.

Take a small plastic basket and place it in a bowl of water. You should find that it floats. Ask your students to guess what would happen if you added a tissue to the basket - would it still float? Carefully lay the tissue at the bottom of the basket and watch what happens. The basket should sink.

Water molecules are strongly attracted to each other and form a skin on the surface of the water. When the tissue is added it absorbs water and breaks the skin. This means the basket sinks.

Repeat the experiment. This time, instead of adding a tissue, add some washing-up liquid. The basket will sink because the surface tension is broken by the washing-up liquid.

Take three colours of food colouring and a cereal bowl of skimmed milk. Put a few drops of each colour onto the surface of the milk, making sure the colours are separate. Add a drop of washing-up liquid to the centre of the bowl and watch what happens. Experiment by putting the washing-up liquid in different places. Once again the surface tension is broken by the washing-up liquid.

Take a bowl of clean water and float a loop of string on the surface.

Carefully touch the surface of the water in the centre of the loop with a bar of soap. This time, the surface tension is broken in the middle of the loop, but not on the outside. This means there is an imbalance and the string is pulled into a circle.

Finally, take a small metal strainer and coat it with cooking oil. Now pour water down the side of the strainer and watch what happens.

Touch the bottom of the strainer with your finger. Initially the water will stay in the strainer. This is because the molecules of oil attract each other to make a watertight skin. When you touch this with your finger, the tension is broken and the water drains out.

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