Candle experiment a load of hot air

5th June 1998 at 01:00
From simple science to not-so simple Anglo-Saxon huts, Sarah Cassidy reveals some of the myths peddled by textbooks

A SIMPLE science experiment, taught in most secondary classrooms for decades, has misled generations of pupils because it is not as simple as it seems. Three Tasmanian schoolchildren have discovered that a straightforward experiment to reveal the amount of oxygen in air is a sham.

Millions of secondary pupils around the world have been taught to stand a candle in some water, cover it with a jar and wait for the flame to go out. The theory was that once all the oxygen was used the water would rise by exactly that amount. Many a science student has dutifully recorded the amount of water to produce a figure for the percentage of oxygen in air.

But when Emma, Andrew and Rebecca Fist, aged 9, 11 and 12, tried to burn the oxygen more quickly by repeating the experiment with three candles they found the water rose even further up the jar.

Scientists point out that the candle's heating effect contributes to the rise. When the candles go out the warm air cools and contracts so the water rises into the jar. Also carbon dioxide and water vapour are produced so the change in water level does not match the amount of oxygen used.

But that is just the tip of an iceberg of half-truths and textbook misconceptions.

John Stringer, of the Association for Science Education, said the simple model of the tongue showing particular areas sensitive to different taste could be another example. "I've never been able to get it to work, even with undergraduates doing careful trials. The reality must be far more complicated than is shown in the textbooks," he said.

History is peppered with colourful stories, some of which need to be taken with a pinch of salt. Becky Sullivan, of the Historical Association, said:

"The story of Alfred the Great and the cakes is a 12th-century construct, but the way it is presented in schools is rather mixed. He's also credited with founding the British navy, although most historians would dispute it."

The textbook presentation of Anglo-Saxon homes may also be a myth. Commonly pictured with a hole at the top to let out the smoke, modern reconstructions of the huts have shown that this simply wouldn't work. Publishers have had complaints from Anglo-Saxon enthusiasts who want the books revised.

In geography, much of the problem is due to out-dated textbooks.

Roger Carter, the Geographical Association's president-elect, complained that Britain's industrial areas are said to be in decline.

"Films like The Fully Monty echo what you get in textbooks about industrial decline, although Sheffield's chamber of commerce says the city made more wealth from steel in 1997 than in any one year in the 1960s," he said.

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