The Boiling Point of Water

At what Temperature does Water Boil?

In order to getting your food cooked perfectly you have to understand the physics of boiling water. Cooking times depend on the temperature of your boiling water (among other factors). The temperature of boiling water is always the same, you say? That is almost true, but not quite. 

Altitude

Altitude is one factor; the higher up you are, the lower the air pressure and the lower the boiling point of water. Weather-related high or low pressure zones do not make much of a difference, but if you live high up in the mountains, you may have to vector this effect in.

Altitude (m)	Altitude (feet)	Air Pressure (Pascal)	Boiling point (Celsius)	Boiling point (Fahrenheit)
0	0	101325	100	212
500	1640	95461	98	209
1000	3281	89875	97	206
1500	4921	84556	95	203
2000	6562	79455	93	200
2500	8202	74683	92	197
3000	9843	70109	90	194
3500	11483	65764	89	191
4000	13123	56640	87	188

Denver: at an altitude of 1609 m the boiling point of pure water here is 202 Degrees Fahrenheit, a full 10 degrees Fahrenheit less than at sea level. 

The central plateau of Tibet has an altitude of 4,500 m. Here the boiling point of water is about 85 degrees Celsius or 185 degrees Fahrenheit.

And if you are a mountaineer planning to climb Mount Everest, be prepared to see boiling points of 74 degrees Celsius at your highest camp (at 8,000 m altitude). You would not be able to boil an egg at these altitudes, but under the circumstances eggs would not be on your diet anyway.

So if altitude reduces the boiling point, what increases it? One answer is simple: pressure cookers. The pressure inside a pressure cooker can be about double of the original surrounding (atmospheric) air pressure, thus increasing the boiling point to 120 degrees Celsius or slightly higher. 

Minerals Content of the Water

Boiling point elevation is the name of the phenomenon we will look into next. By adding increasing amounts of a solute, such as kitchen salt (NaCl or sodium chloride), to water you will see an elevation of the boiling point (which is the highest temperature you can reach while boiling at sea level). 

Regular tap water already has minute fractions of solutes, but their effect on the boiling point is insignificant. However, if you use lots and lots of salt, you will see an increase of the boiling point. The rule of thumb is: every 58 grams of kitchen salt per kg of water will increase the boiling point by 0.5 degree Celsius. 

Some cooking processes require reaching a certain temperature. At altitude these temperatures may not be reached by just boiling plain water.

Other processes require a certain cooking time. At altitude, you will have to increase the duration.

The United States Department of Agriculture Food Safety and Inspection Service published this interesting paper about high altitude cooking.