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Comment by prerok | Hacker News Reader

What I don't get is why there is no mention of evaporating effect on the energy level of the system.

Evaporating water takes quite a lot of energy so the rest of the water is cooled:

> During evaporation, energetic molecules leave the liquid phase, which lowers the average energy of the remaining liquid molecules. The remaining liquid molecules can then absorb energy from their surroundings. This process can take place at any temperature because some of the molecules in a liquid will always have enough energy to enter the gas phase.

Source for the quote: https://highschoolenergy.acs.org/content/hsef/en/how-can-ene...

I always thought this was the reason when observing the effect when trying to pour hot instead of cold water to defrost your windscreen during cold winter months :)

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> pour hot instead of cold water to defrost your windscreen

This sounds like a risky experiment, the windshield is likely to shatter with such thermal differences and the associated tensions.

If I am not mistaken, a windshield is created already with internal pressure to ensure it will shatter to pieces when the local pressure changes abruptly (via punching e windshield with a hard pointy object for instance)

When I was a kid we used hot water to defrost the windscreen regularly. A couple of hundred times over multiple winters. Temperatures around -20. No cracks or anything like that ever occurred.

When was that, out of curiosity? I seem to remember that the structure of the windshields (at least in Europe) change in the 70's from ones that would break with a crack, to the ones that would shatter in plenty of small pieces (due to the internal tension, and at least two layers)

Maybe they meant hot from the tap and not boiling, which will certainly crack the windshield.

I saw that in the 90s but people were definitely doing it in the 70s as well. I am not aware of any stories of cracks or shattered windscreens because of it, though I am sure that if you do that with boiling water and not-so-frozen windscreen often enough, you would get that result :)

It was definitely not boiling but hot from tap, so could be about 60°C, and, yeah, somehow the windshield did survive it. It's probably not that bad because you actually pour the water on the frosted windscreen so most of the heat would dissipate already before actually reaching the material.

This makes sense to me. Surface layer partly evaporates, it immediately cools down remaining surface layer and then convection spreads it around. It would enable quick "forced" cooling similar to AC and due to convection, it can continue for a long time as warm water and cold water rotate.

Quite counter intuitive. You would expect to have water cooling slow down when it reaches temperature of colder water.

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