What is the coldest substance?

Answer

A particular substance isn't itself the "coldest" or "hottest." Substances can exist at many temperatures, and every single substance can be as cold as you want it to be! If you had a very special refrigerator, you could bring any substance down to absolute zero (which is 0 K or -273.15 °C or -459.4 °F)! Absolute zero is as cold as you can go, but there is no reason why any substance could not be that cold.

We often think of things like liquid nitrogen as being very cold, and that's true. But there is nothing special about the nitrogen... it is a substance like anything else, it has just cooled to very low temperature. Think about hot water from the faucet and an ice cube -- they are both just water. There is nothing special about the ice cube except that it was in the freezer, whereas the hot water was heated.

Liquid gases are very cold (because if they warm up, they don't remain liquids, but turn back into gases). So liquid nitrogen is very cold (liquid nitrogen boils at boils at 77.35 K, which is -195.8 °C or -320.44 °F. However, liquid hydrogen would be much colder because it boils at 20.268 K, which is -252.88 °C or -423.184 °F). But frozen or solid hydrogen would be colder yet! The freezing point of hydrogen gas is 14.03 K, which is -259.13 °C or -434.425 °F. Now that's cold! Liquid helium has the lowest boiling point of all the elements, with a boiling point of 4.125 K and a freezing point of 0.95 K!

First answer by JEK. Last edit by JEK. Contributor trust: 2047 [recommend contributor]. Question popularity: 19 [recommend question]