Cooling down water by BOILING it

Let’s cool down some water by boiling it. The water in this beaker is hot, but it’s not boiling because the molecules in the beaker don’t have enough kinetic energy right now to rapidly fight against the air pressure from the outside that’s squeezing them into a liquid state.

But let’s put them under less pressure. I’m going to suck some of the water up into this syringe, and then I’m going to seal the syringe with some transparent tape right over the hole. Perfect, nice and airtight.

Now when I pull down the syringe, the air pressure inside will drop, and many of the liquid molecules in here will be able to fight against that lower pressure and become a gas. Oh yeah, look at that, the water is boiling! This is literally boiling water, but we have not warmed it up at all.

In fact, now that it’s boiling, it’s actually colder. That’s because the temperature of this water is just the average kinetic energy of all the molecules in the sample. But by boiling it like this, we’ve allowed those molecules with the highest kinetic energy to escape as a gas. So what’s left in liquid form are actually colder molecules on average.