Physics

# The Triple Point Is When a Liquid Can Boil and Freeze at Once

When water is a solid, it's ice. When it's a gas, it's steam. When it's a liquid, it's, well, water. What if we told you that a substance could be all three of those states of matter at once? Yup. This freaky intersection is called the triple point.

## Betcha Didn't Know This Was Possible

The triple point is a truly mind-mangling concept. A substance at the triple point is simultaneously a liquid, solid, and gas. How does this happen? It's all about temperature and pressure. The triple point can be described as the temperature and pressure that puts the three aforementioned states of matter into thermodynamic equilibrium — that is, a condition where no one state of matter is trying to change into any other state.

The boiling liquid causes high-energy molecules to rise as a gas, which in turn lowers the temperature of the boiling liquid and makes it freeze. That cycle continues as long as the substance stays at that triple-point temperature and pressure. For water, that triple point is at 32.02 degrees Fahrenheit (0.01 degree Celsius) and .006 atm. For reference, the Earth's average atmospheric pressure is 1 atm.

## It's Freaky and Useful

Seeing a substance flirt with the triple point is a little mind-blowing, and would be a really cool party trick. (Oh, you don't go to parties with vacuum chambers?) But the triple point is more than just a freaky science experiment. According to the National Physics Laboratory, "triple points make ideal reference points for the calibration of thermometers." Additionally, as reported by the NPL, "Ethylene carbonate has a triple point temperature of 36.315°C which, being close to body temperature, makes it a highly useful reference point for the calibration of clinical thermometers, while benzoic acid has a triple point temperature of 122.33°C, close to the sterilising temperature of medical drip solutions."