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Water course

Surface tension explains why bubbles form and how our lungs breathe. Jackie Hardie investigates some of the properties of water

Water is amazing stuff. Not only does solid ice float, giving us icebergs and ice cubes, but liquid water has surface tension. If you drop paper clips gently on to water in a bowl, they seem to be supported by a "skin". This skin is a result of surface tension, which is caused by the molecules at the surface being pulled together and down by the cohesion of molecules in the liquid below. This produces a force of tension across the surface that makes it behave like a stretched elastic skin. Surface tension also pulls a drop of liquid into a sphere (this is what makes bubbles round). And if you dip a clean, dry paint brush into water and then lift it out gently, the skin of water molecules will move the bristles into a fine shape that is easier to dip into paint

If your paper clips are still floating on the water, add a drop of washing-up liquid and see the effect. Detergents damage the surface tension and the clips sink. Because soap and detergents reduce surface tension, a soapy liquid gives you foam and bubbles when you shake it. No matter how much you shake pure water, you won't get a lasting foam.

If you add food colouring to a glass of water, you'll see the water apparently "creeping" up the sides of the glass. This is because there is a strong attraction between the water molecules and those in glass. The liquid surface thus acquires a slightly curved shape, called the meniscus.

Aquatic bugs exploit surface tension. Some, such as stick-like pond-skaters, have water-repellent bodies and long thin legs which spread widely across the water and are also water-repellent. Surface tension supports the animal. Water boatmen bugs, on the other hand, switch beneath the surface and take a supply of air down with them in the form of a bubble. Surface tension helps again, in forming the air bubble at the end of the abdomen, from where they breathe. Detergents can pollute water by destroying surface tension, but not all toxic substances work in this way.

Understanding surface tension may also help babies born as much as 17 weeks premature to survive. Our lungs have lots of minute bubble-like air sacs in them and these must be kept open. If the sides of the air sacs touch, surface tension would stick them together. Our lungs make a surfactant (wetting agent) which affects the surface tension and keeps the air sacs open. In an unborn baby, surfactant starts to be made at about 21 weeks but it's only by the 34th week that there is enough to enable the baby to survive by breathing air. A baby born before this time will have to make a massive effort to breathe and could die from exhaustion and suffocation (the so-called respiratory distress syndrome). Scientists can make the surfactant artificially and if only as little as 0.5ml is introduced into the lungs, it will line all the air sacs, prevent sticking and give the baby a chance of life.

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