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?
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.
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.
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.
I can see why it may be misinterpreted that adding salt makes it boil 'faster'. One thing that the salt WILL do is introduce a surface (on the salt crystal) that helps the dissolved air to release from the water (looks like tiny bubbles). It can lead people to think that this is starting to boil. When water is actually boiling it is because liquid water is turned to water vapor, thus causing bubbles.
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/
I don't claim that it's true, but here's one more explanation, this one in favor of salt making water boil faster: http://www.swri.org/10light/water.htm
Briefly, they say that salt has lower heat capacity than water, and so water+salt will heat up more quickly than water alone. This overshadows the tiny increase in boiling point that the salt will also cause.
On the other hand, that same site says that adding salt to water will increase its volume, where I think the opposite is true (not 100% sure on that one).
Aside: It's fun how such an easy-to-evaluate experiment has so many different opinions, even with regard to the outcome of the experiment, let alone the explanation (:
Here's someone's video of doing the experiment (the water did boil faster with salt): https://www.youtube.com/watch?v=qcrDuc-XjRQ
Of course, you can poke plenty of holes in their methodology. We need a large government-funded study!
Here is one more link relating to the specific heat of water, when substances are dissolved in it: physics.stackexchange.com There, they did an experiment on cooling water, and found that water+salt cools faster. I think it's reasonable to believe that it heats faster, too.
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.