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Question

Explain the variation of boiling point with altitude and why are we using a pressure cooker at high altitude for cooking?

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Solution

The boiling point of a substance is the temperature at which the vapor pressure of the liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point decreases as the vapor pressure increases. The boiling point is inversely proportional to vapor pressure. So, a variation of boiling point mainly depends upon the vapor pressure.
High-altitude cooking is the opposite of pressure cooking in that the boiling point of water is lower at higher altitudes due to the decreased air pressure. A pressure cooker is often used to compensate for the low atmospheric pressure at a very high elevation. Under these circumstances, water boils at temperatures significantly below 100 C(212 F) and, without the use of a pressure cooker, may leave boiled foods undercooked. At High Altitude (3500-6500 ft) cooks really have to plan ahead to do some of the same things the rest of us take for granted. The reason foods have high altitude instructions is because of the boiling point of water changes with altitude. As you go higher, the boiling temperature decreases. At sea level, the boiling point of water is 212 F (100 C). As a general rule, the boiling point temperature decreases by 1 degree F for every 540 feet of altitude (0.56 C for every 165 meters). On top of the14,000 foot Pike's Peak, for example, the boiling point of water is 187 F(86 C).


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