How does the boiling point of water decrease with altitude?

Post date: Apr 12, 2015 3:55:32 PM

To support a mini-lesson looking at the boiling point of water at different altitudes, I wanted to know the what the boiling point of water is at different altitudes. My aim was to make graphs showing how the boiling point changes with different altitudes or pressures and to give students some practise at reading graphs and interpreting data.

To do this I needed to generate some data - here is the calculation and code I used for this. The main pieces of physics I was using were:

• • the barometric approximation: this gets me pressure for different heights (assuming a column of air at constant temperature)

  • Water boils when the vapour pressure from the liquid is equal to atmospheric pressure.

  • I used the Antoine equation (a numerical approximation) which relates the vapour pressure of a liquid to it's temperature.

Hit this link to the full article, with comments.

Here are some pretty of the plots I made:

• • As we go higher, the atmospheric pressure decreases.

  • As we go higher, the boiling point of water decreases.

  • At the top of the world's tallest building (Burj Khalifa @828m) the boiling point has already dropped a few degrees.

  • At the top of Mount Everest (@8848m), we'd expect the boiling point to be around 74 C.

This last plot is quite interesting and moves away from the original altitude vs boiling point question and shows us how the boiling point depends on atmospheric pressure. This is actually very useful in everyday life, for example:

• • in pressure cooking. By increasing the pressure inside the cooker, we can increase the boiling point of water which allows food to be boiled at a higher temperature cooking it more quickly and efficiently.

  • for freeze dried foods and vaccines: by reducing the atmospheric pressure we can also reduce the boiling point of water. This physics is used in part for removing the water content without having to heat and therefore damage the substance. This can improve the shelf life of food and can mean some vaccines don't need to be refrigerated (e.g. easier transport to rural communities).

hit this link to the full article and see the pretty graphs