Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Rachael Edwards on 01/10/2010 19:30:03
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Rachael Edwards asked the Naked Scientists:
Why does water boil faster with salt?
What do you think?
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This question needs clarification. in general, adding pure common salt to boiling water will briefly stop it boiling because dissolving salt raises the boiling point of the water and extra heat is needed however salt crystals frequently contain air an the expansion of the air might give the impression of brief vigorous boiling. However different salts notably sodium bicarbonate and some additives to salt could have different results through chemical or physical action action notably the evolution of gas due to chemical breakdown and the introduction of nucleation centres for bubbles respectively. So could the questioner please describe the experiment more precisely.
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Some tests. (http://itotd.com/articles/521/water-freezing-and-boiling-myths/)
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Throwing a handful of salt - or sand - into a pot of boiling water will intensify the boiling effect (at least, in the case of salt, until the particles dissolve) because the surfaces of the salt crystals provide convenient sites (called nucleation centres) for the formation of bubbles of water vapour.
Bubbles normally find it hard to form underwater owing to surface tension - the natural tendency of water molecules to pull towards each other; but an introduced surface, usually containing small irregularities and also trapped bubbles of air, make the formation of bubbles more energetically favourable, so the water appears to boil more (not withstanding the point made above that, paradoxically, salt increases the boiling point of water and hence in the longer term the boiling rate will decrease, assuming the temperature doesn't rise futher).
The same physics is seen when sugar or salt are added to a glass of fizzy drink; the introduction of the crystals causes the drink to effervesce as dissolved carbon dioxide forms bubbles around the nucleation sites offered by the crystals.
Chris