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Capillary action

Ray Oliver explains how to investigate capillary action

The fact that something is odd about the behaviour of liquids in thin tubes has been known since Pierre Simon Laplace's studies in the 18th century.

He observed that water is drawn up into narrow tubes, the effect known as capillary action. The water surface is not flat, but curved at the edges (we call this a meniscus). The water molecules adhere to the glass more strongly than to each other. The human body is full of capillaries transporting fluids around. It has been estimated that the capillaries from one body, if joined together, would stretch right around the world. This should remain a conjecture.

There are lots of ways for children to investigate capillary action in a less invasive way. In an old-fashioned oil lamp, it is capillary action that delivers liquid fuel to the flame. The same thing happens in a candle, when the pool of liquid wax at the base of the flame soaks into the candle wick.

Colour some water with food dye and add a selection of straight tubes of different diameters, standing vertically. The narrower the tube, the higher the liquid will rise. Ask pupils to observe the shape of the meniscus, if necessary using a magnifying glass. Fix two microscope slides together using an elastic band, but push a card or knife blade into one edge to separate them. This produces a wedge-shaped gap between the glass plates.

Dip the lower edge into coloured water and watch the effect. Where the gap is narrowest, the liquid will rise the most.

Paper towels are generally made using two thin sheets of paper with air spaces in between. Cut a long strip of paper towel and dip one end into a cup of coloured water. Drape the paper over the edge of the cup and down to another cup below containing water of a different colour. Observe the movement of water in both directions by capillary attraction.

Try placing other materials in coloured water to observe capillary rise, for example car sponges, blotting paper or a disposable nappy.

Ray Oliver teaches science at St Albans Girls' School, Hertfordshire

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