Freezing Hot // Boiling Cold

Fact: Water can freeze and boil at the same time.

Get the temperature and pressure absolutely spot on and you can watch a cup of water boil while ice forms on the top. It might sound like magic but once again it’s due to the power of physics.

This precise balance of temperature and pressure at which water can exist as a solid (ice), a liquid (water) and a gas (water vapour) all at the same time is called the ‘triple point’. It occurs at 0.01 C and 611.657 Pascal’s of pressure: at this point, any small change in either temperature or pressure can cause all the water to boil off into water vapour, to freeze into ice or to turn back into liquid water.

A phase diagram of water showing where the triple point is and why its possible to have ice, steam and water all at the same time. Image by Cmglee, from Wikipedia.

This phenomenon occurs because the triple point represents the lowest temperature and pressure at which liquid water can possibly exist.  At this precise point we are at the boundary of all three states, and so changing from one to either of the others is incredibly easy. The conditions required to achieve this are so precise that the triple point of water is commonly used to calibrate the most accurate laboratory thermometers.

The triple point isn’t unique to water: most elements and compounds have a precise ‘triple point’ at which they exist in all three states of matter simultaneously. Water is the coolest though, because who doesn’t want to see boiling water with icebergs in it.

‘Another one of Mr Shaunak’s Little Bites of Science’
Image under Creative Commons License


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