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The heating and AC system in the building where I occasionally work, works like this (basically a thermostat):

The administrators set some temperature that is maintained automatically. Say, in the winter, the outside temperature is 0 °C. The admins will set the desired inside temperature to 20 °C. There is some thermometer behind a little steel panel. It measures it to be cold in the room; therefore the heat comes on. I can trick the heat into coming on even more, by rubbing some alcohol or acetone on the steel panel. The acetone evaporates endothermically, making the steel panel REALLY cold; then the heater thinks that it's REALLY cold in the room and pumps the heat in.

The same system is in place in the summer, but in reverse. However, I can't trick the panel in the same way. If I put acetone on the panel, it'll cool off, and then the A/C unit thinks the room is just fine, because the panel is cool. I'm wondering if there is some liquid that will evaporate exothermically, and heat up the panel, in order to make the system think I need more A/C?

I don't believe that exothermic evaporate exists; here is a list of heat of vaporization for various substances; all positive (endothermic). http://www.engineeringtoolbox.com/fluids-evaporation-latent-heat-d_147.html But maybe I'll be surprised and it does exist!

I have been holding my laptop's hot backside up to the panel instead, but that's a bit too manual for my taste.

Edit: To be a bit clearer, let me specify that this is just a thought experiment. I'm not actually expecting to find something which doesn't make sense like an exothermically evaporating liquid.

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    $\begingroup$ Why can't you just use the heatpack for summer? en.wikipedia.org/wiki/Heating_pad#Chemical It should be safe enough. $\endgroup$
    – Mark
    Commented Jul 12, 2017 at 11:47
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    $\begingroup$ Remember those USB coffee cup heaters? I would've never guessed I will ever see use case for these, but apparently here's one. $\endgroup$
    – Dmitry Grigoryev
    Commented Jul 12, 2017 at 13:49
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    $\begingroup$ Last I checked, your body puts out a lot of heat. Just lean on it and check your cell phone. $\endgroup$
    – rtaft
    Commented Jul 12, 2017 at 17:54
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    $\begingroup$ You have a textbook XY problem here... What you want is to turn the air conditioning on, but instead you are asking about exothermically evaporating liquids. (In fact this is more classic than most classic XY problems...) $\endgroup$
    – Stack Exchange Supports Israel
    Commented Jul 13, 2017 at 0:30
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    $\begingroup$ If your room is too warm in the summer then the first thing you should try is turn the thermostat down, or since it's locked, ask whoever has the key to turn the thermostat down. $\endgroup$
    – Stack Exchange Supports Israel
    Commented Jul 13, 2017 at 0:31

6 Answers 6

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No such liquid, safe or otherwise, can exist. Evaporation is a strictly endothermic process in all cases.

The change in state from liquid to gas is marked by the individual particles gaining enough translational kinetic energy to overcome the mutual attractions present in the liquid phase to "fly free" in the gas phase. It is logically inconsistent for a substance to increase its internal energy and release energy to the surroundings as heat in the same process.

In order to achieve both evaporation and a release of energy, one would have to find a liquid that reacts to (a) release heat and (b) form gaseous products. The energy required to move from the liquid to the gas phase is substantial; more than likely the only reactions exothermic enough to provide a net release of heat are combustion reactions.

I somehow doubt dumping a flammable liquid into your thermostat, inserting a wick, and lighting it on fire is a satisfactory solution for you.

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    $\begingroup$ I somehow doubt dumping a flammable liquid into your thermostat, inserting a wick, and lighting it on fire is a satisfactory solution for you. That would be a great solution, although the OP wanted safe. $\endgroup$
    – Pritt says Reinstate Monica
    Commented Jul 12, 2017 at 10:01
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    $\begingroup$ Of course, you could do the opposite - let some liquid condense on the thermostat. Like, spraying it with super-heated steam. Don't call me when the building burns down or something :P But it certainly isn't as convenient as evaporating a liquid. $\endgroup$
    – Luaan
    Commented Jul 12, 2017 at 13:58
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    $\begingroup$ One could also argue from thermodynamics: The transition from liquid to gas increases the internal entropy of the substance. If the reaction was exothermic, it would increase the entropy of its surroundings as well (or leave the surroundings alone and increase internal temperature and entropy even more), and it would therefore be spontaneous at all temperatures. It is precicely because it is endothermic, and thus lowers the entropy of the suroundings, that the reaction becomes spontaneous only above a certain temperature. $\endgroup$
    – Arthur
    Commented Jul 12, 2017 at 14:14
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    $\begingroup$ Good point, @Arthur! $\endgroup$
    – hBy2Py
    Commented Jul 12, 2017 at 14:15
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    $\begingroup$ There's always the option of using some alcohol-based hand sanitizers. I've seen many that are viscous enough to put on a vertical panel and they burn pretty low (small flame, not jumpy). Not really safe but safer than using gasoline. $\endgroup$
    – Delioth
    Commented Jul 12, 2017 at 15:17
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Silly question, but why can't you get the thermostat adjusted to the temperature you want? You've got a nice control mechanism built in to the room and you want to tinker with it... to make it hotter in winter and colder in summer.

But if there is some reason you can't get the thermostat adjusted, instead of using an exothermic evaporating liquid (as hBy2Py says, because evaporation is increasing the kinetic energy in each molecule compared to the bulk liquid it's got to be endothermic) why not get a peltier cooler such as at http://tetech.com/peltier-thermoelectric-cooler-modules/?gclid=Cj0KEQjwkZfLBRCzg-69tJy84N8BEiQAffAwqlE6p-prIsm3udii0_qoEVpGT2gSRqXm4AgVWUNjos8aAolJ8P8HAQ and magnet it onto the panel in the right place. Feed it the appropriate (low) voltage one way it heats up the panel, reverse the voltage it cools it down.

That's far safer than evaporating acetone or ether!

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    $\begingroup$ To be honest, I was more interested in seeing people's answers/ideas about heating/cooling the panel thing than actually changing the temperature. But with regards to adjusting the thermostat, the thermometer inside the panel is not very good because it's right where the vent is. Even when I'm not tricking it, when the A/C is on the panel thinks it's colder than it really is because it's near where the cold air blows out, and that makes the A/C stop too early. Same problem when I want heat. So adjusting the thermostat doesn't really help much. $\endgroup$
    – iammax
    Commented Jul 12, 2017 at 14:47
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    $\begingroup$ Ahh, a really well designed HVAC install then! A peltier cooler controlled by a remote thermostat might be the way forward. $\endgroup$
    – matt
    Commented Jul 12, 2017 at 14:51
  • $\begingroup$ I'm pretty sure they did it on purpose to save money. It's at a big city university so comfort is not a priority... $\endgroup$
    – iammax
    Commented Jul 12, 2017 at 14:53
  • $\begingroup$ Or, wrap the thermostat with insulation, maybe? $\endgroup$
    – hBy2Py
    Commented Jul 12, 2017 at 15:55
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    $\begingroup$ @iammax : I've seen students install thin cardboard baffles to redirect the outflow away from the thermostat. This sounds like a job for duct tape. (... or Duck tape, if you get condensation.) $\endgroup$
    – Eric Towers
    Commented Jul 12, 2017 at 23:28
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Can't you use something like a hair drier? It's a bit noisy but you don't need much time to trigger it.
Else, you can use the coffee machine warm water, you just need it to touch the panel, not pour it onto it...
I second the heating pad idea.
If I would follow your principle I would find a way to have some evaporated liquid condensate at thermostat plate level, in a reverse way to what you have (inside a bottle, glass is a good thermic conductor). You can evaporate a few water inside a bottle then let it cool by transfering its energy to its environment. Problem being that it will also actually warm the room.

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    $\begingroup$ A heating blanket would be even better. $\endgroup$
    – Agent_L
    Commented Jul 12, 2017 at 13:48
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Forgive me for saying, but this feels like a problem statement in which the setup of the question inadvertently obscures the actual solution space. You state a number of constraining requirements, that other answers have dutifully adhered to by telling you there is no solution, but the actual problem does not seem to have those constraints.

For example is the phase of matter truly a requirement? Must it be a liquid? Must the mechanism of action be limited to evaporation? Are those really requirements?

It seems to me there is nothing about the explanation that requires those constraints. It seems to me that the constraints are actually: something that is clever, not unsightly or likely to lead to damage, and of course effective at making the thermostat turn on the air conditioning. In other words something as clever as your acetone solution, which is in fact quite clever.

However, as stated the question reads like "I'm hot, how do I build a fire that's cold?" And so, of course, hBy2Py is absolutely correct to tell you that there is no such fire.

Could the answer not be in the form of a solid? Maybe a mixture of particulate solids? Perhaps fine particulates that could even be aerosolized?

Because if that's the case I'd draw your attention to rust, iron oxide. You know the kind of thing you might find in disposable hand warmers. A fine particulate mixture of iron, activated charcoal, salt, vermiculite, and water. Cheap, safe, exothermic, and clever.

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Not at all sure about the practicality, but rather than trying to come up with a liquid that exothermically goes to a gas, could you take e.g. gallium (which is supposed to liquify in your hand), put that on and when it "freezes", that should give off heat (although I have no idea what the specific heat of fusion of gallium is)

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To avoid having to stand there or wire up the panel, I would aim some low power laser to the thermostat cover. This can be started and stopped remotely. A dark sticker would enhance energy absorption.

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