We talked about how there is a lowest possible temperature, and that it’s absolute zero: -273 °C. You may wonder: how did they discover that? They must have had a bunch of fancy equipment and huge rooms full of machines to achieve low temperatures, right?
No. They couldn’t get anywhere near absolute zero when they discovered it. (Even though now we can, and yes you have to have lots of fancy equipment.) What they did was realized that PV=nRT was a good relation between properties of a gas. Think about this: what if P became 0 in that equation? Then T would also have to be zero. So a gas in a pure vacuum will have zero temperature.
Well, a pure vacuum doesn’t really exist, because if the gas is there, then there can’t be a pure vacuum. But this is still useful, because what they did is measured the P of several gases at 2 T values. That’s easy enough. Then you notice that they all extrapolate back to the same “zero” temperature. This is absolute zero.
Lots of people in the history of science contributed to this, but Joseph Louis Gay-Lussac was the first person (in 1802) to use the number -273.
