Why does altitude increase air pressure

Internal energy - heat capacity

Graph of air pressure and boiling temperature as a function of altitude

Place the course of air pressure p and boiling temperature ϑ in one diagram eachs depending on the height.

h in m09031804300041915382656577418910
ϑs in ° C1009794908682787470
p in hPa1013909814701601513436370312

Take the boiling temperature of water on the 6273 m high Chimborasso (highest mountain in Ecuador) from the diagram.

Mr. Schlaumeier would like to cook his mountaineering menu on the summit of Chimborasso. From the description he takes a cooking time of 15 minutes. When Mr S. tried his meal after 15 minutes, he was not entirely satisfied. I wonder why?

The boiling temperature of the boiling water is only about 80 ° C on the Chimborasso. The cooking time stated on the packaging, however, refers to a boiling point of approx. 100 ° C. Mr. Schlaumeier is not satisfied with his food because it has not yet been completely cooked (besides, it doesn’t taste good at such high altitudes anyway).

Explanation of the lowering of the boiling point when the air pressure decreases
If vapor bubbles form in the water during boiling, the pressure in the water vapor bubble (vapor pressure pD.) be about the same as the air pressure b (due to the hydrostatic pressure of the water, pD. even be slightly larger than b).
If the external air pressure decreases, a lower vapor pressure is sufficient for the formation of bubbles and thus for boiling. This means that the water vapor molecules no longer have to patter so intensely on the vapor-liquid interface. The mean kinetic energy of the molecules and thus the temperature can be lower.

Experiment: How to bring water to a boil by cooling it

Water boils at approx. 100 ° C



The water is taken from the hotplate. It stops boiling quickly.


If you close the flask with a cork and pour cold water over it, the water begins to boil again.

Interpret the result of the experiment.

After removing the heat source, boiling stops. If you close the flask with the cork, neither water vapor can escape nor air from outside can penetrate the flask.

By dousing the flask with cold water, the water and the water vapor inside cool down. In particular, the volume of the steam decreases and as a result the pressure above the water decreases. As a result, however, the boiling temperature of the water drops and it now boils again (despite cooling from the outside).

The pressure-dependent boiling temperature is used in the pressure cooker, where cooking takes place under slightly increased pressure.

During the boiling time, water vapor forms, which displaces the air from the inside of the pot. Once the air has been completely displaced from the pot, steam flows out of the valve and overpressure builds up inside the pot. The pressure increase is regulated by a pressure relief valve that can be set to two cooking levels. As a result of higher pressure, the boiling point of the water increases. The liquid only boils at 105 ° C (level I) or at 120 ° C (level II). The higher temperature in the pot reduces the cooking time and results in lower energy consumption.

The advertisement promises a 40% energy saving and a 30% time saving.