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Author Topic: Why does adding salt to hot water make it boil harder?  (Read 12406 times)

stephen sommerhalter

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stephen sommerhalter  asked the Naked Scientists:

When I drop ordinary table salt into a pot of boiling water, the boil suddenly and sharply increases (sometimes boiling over the pot) even though the flame remains constant. After a moment, the boil returns to normal. Does this have something to do with the chemical composition of salt, or the granular nature of table salt, or other factors?

Thanks. I listen to the podcast every week, please keep up the good work.
 
Stephen Sommerhalter
San Francisco CA

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Offline dentstudent

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Why does adding salt to hot water make it boil harder?
« Reply #1 on: 11/11/2008 09:42:25 »
It's all about nucleation sites. When you put the salt into the boiling water, the gasses in suspension are able to form around the rough edges of the salt crystals and because the gasses in boiling water expand to something like 20 times the size they were in cold water, the bubbling increases profusely. As the salt dissolves into the boiling water, so the rough edges of the salt crystals disappear, and so too do the nucleation sites. Therefore, the bubbling reduces again.
 

lyner

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Why does adding salt to hot water make it boil harder?
« Reply #2 on: 11/11/2008 14:24:24 »
Plus, the dissolved salt will take the boiling point above where it was with the pure water.
 

Offline Bored chemist

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Why does adding salt to hot water make it boil harder?
« Reply #3 on: 11/11/2008 19:44:22 »
Plus, the dissolved salt will take the boiling point above where it was with the pure water.
Which, for a given flame or hotplate, will make it boil slightly more slowly.
 

Offline usererror

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Why does adding salt to hot water make it boil harder?
« Reply #4 on: 12/11/2008 19:44:34 »
Would any similarly granular non-salt material produce the same effect? thanks
 

Offline chris

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Why does adding salt to hot water make it boil harder?
« Reply #5 on: 12/11/2008 21:49:59 »
Yes it would. Adding sand would do exactly the same thing to the boil (but without affecting the boiling point, of course).

Chris
 

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Why does adding salt to hot water make it boil harder?
« Reply #5 on: 12/11/2008 21:49:59 »

 

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