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Posted by on Aug 4, 2017 in Tell Me Why |

Where Will Water Boil at a Low Temperature?

Where Will Water Boil at a Low Temperature?

Water will boil anywhere, but it boils at different temperatures in different places. For example, it will boil at a lower temperature up a mountain than at sea level. The boiling point of water is the temperature at which its vapor pressure becomes equal to the outside atmospheric pressure. As the atmospheric pressure is always changing so the boiling point of water will vary from day to day. Water boils at 100 degree Celsius only when the atmospheric pressure outside is at the “standard “value.

At Quito in Ecuador, which is about 2,700 meters (or 8,800 feet) above sea level, water boils at about 90 degree Celsius. People who explore in mountainous regions find a pressure cooker very useful. The time required to cook food can be greatly reduced if the boiling point of the water is raised. The pressure cooker does this, since it is an aluminum container filled with a sealing ring but with a loaded pin valve which allows steam to escape. The valve can be set at varying pressures, enabling the food to be cooked at a temperature of about 120 degree Celsius.

pressure cooker cooking at high altitude

All liquids, at any temperature, exert a certain vapor pressure. The vapor pressure can be thought of as the degree to which the liquid molecules are escaping into the vapor phase. The vapor pressure increases with temperature, because at higher temperature the molecules are moving faster and more able to overcome the attractive intermolecular forces that tend to bind them together. Boiling occurs when the vapor pressure reaches or exceeds the surrounding pressure from the atmosphere or whatever else is in contact with the liquid.

At standard atmospheric pressure (1 atmosphere = 0.101325 MPa), water boils at approximately 100 degrees Celsius. That is simply another way of saying that the vapor pressure of water at that temperature is 1 atmosphere. At higher pressures (such as the pressure generated in a pressure cooker), the temperature must be higher before the vapor pressure reaches the surrounding pressure, so water under pressure boils at a higher temperature. Similarly, when the surrounding pressure is lower (such as at high altitudes), the vapor pressure reaches that pressure at a lower temperature. For example, in the Denver, Colorado area of the U.S. where the elevation above sea level is approximately one mile (1600 meters), the atmospheric pressure is about 83% of a standard atmosphere, and water boils at approximately 95 degrees Celsius.

The relationship between vapor pressure and temperature (or, looked at in the reverse direction, between boiling temperature and pressure) is called the vapor pressure curve. Water’s vapor pressure curve is of great importance, since it determines the relationship between temperature and pressure in any process where water is going from a liquid to a vapor state or vice-versa. Such processes are important both in industry (for example in steam power generation where water is made into steam in boilers and eventually condensed after running through turbines) and in nature (for example, in rainfall and evaporation from bodies of water).

Everyone knows that the boiling point of water is 212º Fahrenheit or 100º Celsius. However, that fact is true at sea level and is subject to change under different circumstances. One of those circumstances is a change in altitude. At higher altitudes, air pressure is lower. There’s simply less air pushing down on us the higher we ascend into the atmosphere. So why does this affect the boiling point of water?

To boil water, you need to apply energy in the form of heat. As energy is transferred to the water molecules, they begin to break the bonds holding them together. The water will boil or turn to vapor as soon as its internal vapor pressure equals the pressure exerted on it by the atmosphere. When that occurs, bubbles begin to form and the water boils.

When atmospheric pressure is lower, such as at a higher altitude, it takes less energy to bring water to the boiling point. Less energy means less heat, which means water will boil at a lower temperature at a higher altitude. Some people think that a lower boiling point means that foods will cook more quickly at higher altitudes. However, the opposite is true. If you want to boil an egg, it will take a bit longer at altitude.

That’s because cooking involves heating a food to a certain temperature for a certain length of time. If the temperature is lowered because of altitude (as in the case of boiling water), cooking time will have to be extended to complete the cooking process!

Content for this question contributed by Richy Chilson, resident of Carlsbad, San Diego County, California, USA