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When we add a compound (salt) to a solvent, the boiling points rises. But, what could we say about the speed the solvent reaches the boiling point? It's better to add salt before or after boiling water while making pasta?

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Hello RM: Actually, I can't understand your question... – Waffle's Crazy Peanut Sep 24 '12 at 17:17
Could you please improve it..? – Waffle's Crazy Peanut Sep 24 '12 at 17:17
I kind of join Crazy Buddy. What is exactly the boiling speed? Do you mean how much time one actually has to boil it for the past to get soft? I suppose that at higher temperature, it gets softer more quickly. However, if you want to do it right, it's not just about the speed and every pasta has some instructions when you should add the salt, right? ;-) – Luboš Motl Sep 24 '12 at 17:34
@CrazyBuddy I mean the speed the solution reach boiling point. – R. M. Sep 24 '12 at 18:03
@R.M. it's still not entirely clear what you mean by the speed at which the solution reaches the boiling point. A boiling pot of water does not move and does not have a speed. – David Z Sep 24 '12 at 20:37
up vote 5 down vote accepted

You should add the salt befor you start heating the water.

For the technical details of why this is have a look at Why does salty water heat up quicker than pure water? and Why does adding solutes to pure water lower the the specific heat?. Salt lowers the specific heat of the water so for a given rate of heat input e.g. a given setting on your electric hob, salt water heats up faster than pure water. The reasons salt has this effect are a bit obscure, but are discussed in a paper I reference in my answer to the first question I've linked to.

There's a little more subtlety to the issue than this though. Adding salt increases the boiling point, so you need to get the salt solution hotter to make it boil. However for saturated salt solution the specific heat falls by about 22% while the boiling point only goes up by 8%, so the salt solution still boils faster even taking into account the elevated boiling point.

However salt solution is denser than pure water. Saturated salt solution is about 20% denser than water. So if you take a fixed volume of solution, rather than a fixed mass, the total heat capacity doesn't change much. For saturated salt solution the 22% decrease in specific heat is almost exactly compensated for by the 20% increase in density. Add the 8% elevation in boiling point, and for the same volumes saturated salt solution takes slightly longer to boil.

All of this is fun to calculate, but the small amount of salt used when cooking pasta is unlikely to have much effect.

See for various properties of salt solutions.

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protected by Qmechanic Oct 9 '13 at 21:47

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