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I've always heard adding salt to water makes it boil faster. Is this true? If so, why? If not, why do people do it?

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2  
the best way to make water boil faster is to put on a tight fitting lid. Or move to a much higher altitude. which ever is easier. – Sam Holder Sep 3 '10 at 9:23
@Sam, I would avoid using altitude as a way to speed up boiling (not that it's even easy do). It boils faster because it's at a lower temperature, and for most uses (eg. Tea), the temperature of the water is an important factor, not just the fact that it's boiling. – dsample Sep 26 '12 at 13:15

7 Answers

up vote 18 down vote accepted

No. The amount of salt you would have to add to water to make an appreciable difference is enormous.

Salt is added to water for various reasons: to season whatever is being cooked in it; to maintain colour; to maintain structural integrity.

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And anyway the effect would be the opposite, making it boil slower – user2215 Sep 3 '10 at 1:52
Salt is added mainly to make food taste salty. Salty is better for organism and for taste. – Elzo Valugi Dec 8 '10 at 13:42
up vote 0 down vote

Some good answers here already, however, there are a couple of small effects to consider:

1) The solubility of gases in water decreases as the temperature is raised. So as you heat water to boiling, the gases dissolved in it become super-saturated. Adding salt to a supersaturated mixture provides nucleation sites for the gas to come out of solution (ie form bubbles). Those bubbles can make the water look cloudy or white, which can be mistaken for the start of boiling.

2) Solid NaCl actually releases heat when it is dissolved in water. Not very much heat, but if the water is on the narrow edge of boiling already, that added heat of dissolution can be enough to get things boiling a half a giffy sooner.

Again, these are both minor effects; the first merely looks a bit like near-boiling, the second probably can't be detected without a good stopwatch.

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The salt in the water is denser so it goes down on the bottom and it takes a lonnnnnnggggg time for the water to boil but on the other hand, no salted water boils water faster!

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You guys misunderstood. It doesn't make it Faster it makes it Hotter. This will help you understand. http://www.knowswhy.com/why-does-salt-make-water-boil-faster/

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Interesting point. But how much salt does one need to add to raise the temperature of boiling from 100 to 106 degrees? With sugar, you need 80% sugar concentration to reach 108° to 118° (the first stage given in candy charts). – rumtscho♦ Apr 17 '12 at 10:47
@rumtscho A lot. van.physics.illinois.edu/qa/listing.php?id=1457 gives ~0.5°C/3% salinity (and says we can scale that). So, 106°C would be around 36% salinity. Except not, as Wikipedia informs me you can't actually do that, 28% is as high as you can go. – derobert Jan 7 at 19:05
up vote -2 down vote

its because a lot of households have hard water which has many ions and a high boiling point. adding NaCl softens water and actually reduces ion content in the tap water making it easier to boil. everyone else is theoretically right to state that adding table salt to water increases boiling point but that is for pure water not tap water.

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How does dissolving an ionic substance in it reduce ion content? Is it making something precipitate out? – Peter Taylor Jan 28 '12 at 10:18
up vote 15 down vote

Actually adding salt to water makes it boil slower; it increases the boiling point so it takes a little longer to get there. It actually doesn't matter what you dissolve in water (or anything else). Adding a dissolved substance elevates the boiling point and lowers the freezing point.

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up vote 2 down vote

No. See: http://itotd.com/articles/521/water-freezing-and-boiling-myths/

Anecdotal however, I often observed that if you have water close to the boiling point adding salt can make it boil instantly. Not sure why.

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I suspect it is because as the salt is dissolved, that portion of water (salty) is now denser and remains at the bottom of the pan and gets more exposure to the heated bottom. Pure speculation though. – Jay Sep 3 '10 at 2:13
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I would assume the instant boiling is an effect of adding nucleation sites, though this too is speculative. – dmckee Sep 3 '10 at 2:17
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@dmckee - I think that is right as well; it isn't that you've suddenly reached the boiling point, you just release a bunch of bubbles of oxygen that are trying to come out of solution. – Michael at Herbivoracious Sep 3 '10 at 3:20
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Try this in the microwave: Use a pyrex measuring cup, and nuke a cup of water for about 2 minutes. Watch during the last minute, so that you can time when it starts to boil. Lets assume 1:45. Now, do it again, but only nuke it for 1:40. Take it out, and add a spoonful of salt (Stand back!!!!) It should explode into a boil. What happened is you actually raised the temperature of the water above 100 deg. But the boiling action requires nucleation points (salt crystals!) In the pot on a stove, the bottom is above 100, but the top isn't. Adding salt causes the bottom to nucleate. – Chris Cudmore Sep 3 '10 at 15:03
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Be very careful with the experiment that @chris suggests. Vibrations in the vessel can also nucleate the boiling, and people have been hurt when apparently quiescent liquids suddenly boil over. Ideally you would wear a full face shield, rubber apron and long rubber gloves. (I superheated half a cup of half-and-half and set it off in the middle of the kitchen, once. Splashes went five feet in every direction and the spousal unit and I both got multiple small burns.) That said, this is a fantastic demo. – dmckee Oct 6 '10 at 19:05
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