Only half of the science graduates training to be secondary teachers could provide an accurate description of how water boils, according to research due to be presented to the Association for Science Education's annual conference next week.
Only 26 of a 52-strong sample correctly identified the bubbles in boiling water as water vapour, steam or water itself. Most of the rest went for oxygen andor hydrogen, followed by air, and water and air.
Scientific knowledge was even shakier when it came to evaporation, with less than a third saying the temperature of an evaporating liquid would fall, the study found.
But study co-author Alan Goodwin, head of sciences education at Manchester Metropolitan University, doesn't believe the findings present a problem.
"It's educationally invalid to expect everyone to know the answers to all of those lists of things that teachers are supposed to know about science, " said Mr Goodwin. "Science teachers need to know as much science as possible but they must not think they have cracked it when they think they know it."
He notes how his own understanding has changed. Students asked to explain why shaken fizzy drinks explode when the can is opened said shaking the can transferred energy which raised the temperature and hence the pressure of the drink.
Almost all scientists tend towards this obvious but "wrong" explanation. Shaking does not transfer enough energy to produce such increases in temperature and pressure.
Instead, the "correct" explanation is based on the rate at which gas is enabled to escape from the solution - thanks to very small bubbles distributed throughout the liquid which act as nuclei for the formation of larger bubbles.
"I assumed fizzy drinks were not boiling, but now I believe quite passionately they are - although whether you can use the word is another matter," he said.
But Rebecca Edwards, the ASE's chair-elect and herself a secondary science teacher, suggested the study's results - based on a test of physical science knowledge - could reflect the predominance of biology graduates among student science teachers. Of the 52 graduates surveyed, 32 were biologists, 15 chemists and five were physicists.
"If you were to pick on one small detail in any subject, I wonder how many graduates in that area would know the answers," she said.
A Department for Education and Employment spokesman said the new teacher-training curriculum - mandatory from this September - would specify the core knowledge, understanding and skills required of all primary and secondary science trainees.
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