Posted by Eric Troy on 13 Jan 2014 17:47
Water boils at 212° F (100° C). That is, pure water being heated at sea level boils at this temperature. This temperature drops 1° F for every 500 foot increase in altitude (960 feet for 1° C). So, at an altitude of 1000 feet, water boils at 210° F. I'll explain the reason for that at the end of this article, once we've covered the question in the title.
Now, once water is boiling, it will not get any hotter, so adding more heat will not decrease the cooking time of foods. However, it is possible to raise the temperature at which water boils. Will this make foods, such as pasta, cook faster?
Well, many people claim that one reason to put salt in your pasta water is because salt increases the boiling temperature, and therefore the pasta will cook faster. Is there anything to this claim?
As mentioned, pure water heated at sea level boils at 212° F, but most water is not pure. Water, unless distilled, usually has things like mineral salts, organic matter, and dissolved gases in it. You may have heard the terms hard water and soft water. Hard water has more calcium and magnesium dissolved in it, whereas soft water has more sodium. Hard water, by the way, leaves a ring in your bathtub, a grayish residue on the bottom of pans, and can even make white clothes a bit grayish when washed, so if you notice these things, you probably have hard water. If water has enough mineral dissolved in it, the boiling temperature can increase a little bit. In fact, when almost any solid is added to water, the boiling point increases and the freezing point lowers.1,2,3
Therefore, yes, adding salt to water will increase its boiling point. But the question is by how much? Well, if you add one tablespoon of salt to five quarts of boiling water (an amount sufficient to cook some pasta), it will raise the boiling point by seven hundredths of 1° F.2 In other words, it won't make a bit of difference to how fast your pasta cooks. To raise the boiling point enough to make a difference in cooking time, you'd have to add so much salt your pasta would be inedible.
The only real reason to add salt to pasta water is to make the pasta taste better. Some claim it makes the pasta tougher. Depending on who's talking, this is a good thing, as it keeps the pasta from being mushy, or a bad thing, as it keeps the pasta from being tender enough. There is almost certainly nothing to this claim.
Does it Matter When You Add the Salt?
Have you ever heard, or read in a cookbook, that the precise timing of when you add the salt to your pasta water is very important? If you ever fell for this, I'm afraid you had on your gullible hat. There is no way that when you add salt could make any difference to your pasta, as long as the salt is dissolved, which it will do quite easily unless the water is very cold (like during the winter). Once water is boiling (a little stir might help), it is certainly not going to have any trouble. Yet, food lore has it that if you add the salt to the water before it boils, it could make the pasta taste bad.
Salt dissolved in water is salt dissolved in water. It could not possibly matter whether the salt was added before or after the water boiled.
However, I will warn you, if you're new to cooking: Sometimes, when you add a lot of salt to water that is already at a rolling boil, the water can react a bit violently and begin immediately boiling much more actively. Some people believe this is because the salt "adds heat" to the water, but this is not true at all. The violent bubbling is due to many new nucleation sites being added to the water, which lets large bubbles form very quickly. This can sometimes result in a splatter which could get you burned. So add it carefully, don't just dump it in all of a sudden. This isn't only true of salt, it's true of sugar or any other similar thing you add to boiling water. I have been burned by adding instant coffee too quickly to boiling water (it isn't necessary to boil it), causing the water to virtually explode out of the cup onto my face and hands.
Why Does Boiling Temperature Decrease at Higher Altitudes?
Above, I pointed out that the boiling temperature of water increases 1° F for every 500 feet climb in altitude. This means, incredibly, that if you boil water on top of Mt. Everest, which is around 29,000 feet, water would boil at around 154° F.
As water or any liquid heats, there is a point at which it begins to change to a gas. This point is the boiling point. When water reaches 212°F or 100°C, it begins to turn to steam or vapor. Once this point is reached, the water will not get any hotter. This process of the water being changed to steam is different than evaporation, because it does not just take place at the surface of the water, but takes place within the volume of water. The vapor being produced in the water causes pressure called vapor pressure. This pressure has the pressure of the atmosphere pushing on it, so the water cannot "boil" and begin to release the vapor until the vapor pressure is strong enough to overcome atmospheric pressure. This is the boiling point.
As altitude increases, the atmospheric pressure decreases. Thus, the vapor pressure has less and less pressure acting against it causing the boiling point to decrease as the vapor can be released more easily.1 When you first begin to heat water, some air bubbles begin to rise to the top, long before it boils, but water vapor bubbles may actually form on the bottom and the sides of the pan, where the heating action is quicker. Once the vapor begins being released as steam, however, it is invisible, and only becomes visible as 'steam' as it cools down. This is the visible cloud that we associate with water vapor. To see this for yourself, bring your eyes level with the top of a pan of boiling water. You'll notice that you can't see the steam until a certain point above the pan (after it cools a bit).
Boiling Point of Water at Various Altitudes
For reference, below is listed some of the boiling points of water at various altitudes. However, you can figure out the boiling point at any altitude by simply subtracting 1° F for every 500 additional feet above sea level, or 2° F per 1000 feet climb, or 1° C for every 960 feet of elevation.
|Altitude in Feet||Boiling Point in Degrees Fahrenheit||Boiling Point in Degrees Celsius|
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