Physics
| How does the level of atmospheric pressure affect the boiling point of water? Consider the process of boiling water over a stove. The average kinetic energy of the water molecules inside of the pot increases as the flames warm the metallic pot. Once the water reaches its boiling point the most energetic molecules within the water are the first to undergo a physical change. These molecules assume the form of a gas, and thus escape from the liquid as bubbles of water vapor. In normal atmospheric pressure (1 atm), the boiling point of water is 100 degrees Celsius (212 degrees Fahrenheit). At higher pressures, such as in a tea kettle or a pressure cooker, water vapor will accumulate within the airtight container and the contents will become highly pressurized. At such high pressure the water can be heated to temperatures higher than 100 degrees Celsius and the water still will not begin to boil. At high pressures the boiling point of water can be many degrees higher than the sea level boiling point. On the other hand, at higher altitudes the air is thinner, and there is less atmospheric pressure than there is at sea level. Because the atmosphere is not as dense the temperature required to make water boil is lower. This is why the preparation instructions on foods that require boiling water tell you to cook the food for longer if you are at a high altitude. -The water will never reach 100 degrees Celsius, and so the food must be cooked for longer at a lower temperature to be edible. The higher the altitude, the lower the boiling point. In Denver (1700 m above sea level), the boiling point of water is around 95 degrees Celsius (203 degrees Fahrenheit). At the top of Mt. Everest (9500 m above sea level), water boils at 75 degrees Celsius (167 degrees Fahrenheit). At extreme altitudes, and in the vacuum of space, water will boil at room temperature. If you place a beaker of cool water into a vacuum flask, and then you suck all of the air out of the flask the water will immediately begin to boil away even though its actual temperature remains close to room temperature. |
| Organization for the Advancement of Interdisciplinary Learning |