Why do you add salt to boiling water? There are a couple of answers to this common cooking question.

Salting Water for Cooking

Usually, you add salt to water in order to boil the water to cook rice or pasta. Adding salt to water adds flavor to the water, which is absorbed by the food. Salt enhances the ability of chemoreceptors in the tongue to detect molecules that are perceived through the sense of taste. This is really the only valid reason, as you'll see.

Another reason salt is added to water is because it increases the boiling point of the water, meaning your water will have a higher temperature when you add the pasta, so it will cook better.

That's how it works in theory. In reality, you would need to add 230 grams of table salt to a liter of water just to raise the boiling point by 2° C. That is 58 grams per half degree Celsius for each liter or kilogram of water. That is much more salt than anyone would care to have in their food. We're talking saltier than the ocean levels of salt.

Although adding salt to water raises its boiling point, it's worth noting the salted water actually boils more quickly. That seems counter-intuitive, but you can easily test it yourself. Put two containers on a stove or hot plate to boil -- one with pure water and the other with 20% salt in water. Why does the salted water boil more quickly, even though it has a higher boiling point? It's because adding the salt lowered the heat capacity of the water. The heat capacity is the amount of energy needed to raise the temperature of water by 1°C. Pure water has an incredibly high heat capacity. When heating up salt water, you've got a solution of a solute (salt, which has a very low heat capacity) in water. Essentially, in a 20% salt solution, you lose so much resistance to heating that the salted water boils much more quickly.

Some people prefer to add salt to water after it has boiled. Obviously, this doesn't speed the rate of boiling at all because the salt is added after the fact. However, it may help protect metal pots from corrosion, since the sodium and chloride ions in salt water have less time to react with the metal. Really, the effect is negligible compared with the damage you can do your pots and pans by letting them wait around for hours or days until you wash them, so whether you add your salt at the beginning or the end isn't a big deal.