So what you want is a self-sustaining endothermic chain reaction until the reagents run out, basically?

@Shalvenay Something like that, yeah. Should be detonatable.

@Shalvenay Unfortunately, such a reaction is chemically impossible, according to the law of enthalpy and reaction rate.

Won't work. The second law of thermodynamics requires that entropy increases, and implies that to decrease entropy, you have to do work that adds more energy to the system than you took out. So, no.

@Luna -- expand that into an answer and you have my upvote!

@nzaman -- expand that into an answer and you have my upvote too!

Taken from other source: the thermal decomposition of calcium carbonate in a blast furnace.

Does something creating a black hole count?

@Mast is there an economically viable way to release a black hole?

Mei (Overwatch) has a endothermic blaster, look up it's lore (If there is any...)

@Zxyrra Blowing-up a star comes to mind. Not sure how economically viable that is, but it's kind-of doable.

@Zxyrra Technically we can simulate the sun with nuclear fusion. I guess speeding that up would pretty much kill it.

I feel like your radius of effect wouldn't be great, even if you could get this working - exothermic bombs tend to force large amounts of energy outwards, but an endothermic bomb's ability to "suck" energy in would be much less effective.

@Mast both fusion power and blowing up stars tend to release energy, not consume it

@Zxyrra Yes, but if the star is big enough, the final result of the explosion could be a black hole, which is definitely eating more energy than it releases. Think long-term :-)

I'm surprised nobody mentioned Ice-Nine, the substance in Kurt Vonnegut's novel Cat's Cradle, which causes people and water to freeze by raising the freezing temperature significantly.

@Samuel Perhaps no one has mentioned it because the question says no handwavium.

Can't you just lob a big tank of R134a and a hand grenade at them? That'd explode and freeze everything within a few meters (and suffocate everyone else in the room)

@Sam If you think that would work make a case for it as an answer - but I'm guessing the grenade would release more energy than the coolant would take up.

@Zxyrra Well, this particular polymorph for water isn't known to exist (and likely can't). But the concept is not "handwavium". Of course, the way your question is worded, a cool breeze would qualify.

@Samuel For clarity handwavium generally describes substances with desired qualities that cannot be manufactured (such as an impossible polymorph of water); additionally cool breezes cannot be detonated as far as I know.

@Zxyrra For clarity, the concept doesn't require that the substance is a polymorph of water. Cool breezes can be generated in a plethora of ways that begin with a "detonation".

I would use a small Spherical portal pointing to a really cold place, but the reality-check tag is present.. :(

This question appears to be a duplicate of chemistry.stackexchange.com/questions/41979/…

I was thinking about half a glass of water, but that's not really endothermic...

Seriously, you should've checked out the other SEs first ;) chemistry.stackexchange.com/questions/41979/…

You cannot 'force' or attract light into your device, except by black-hole'ish gravity. The energy from electricity cannot be harvested without some sort of circuit in which the device connects both poles of the power source. And as others stated, 0 Kelvin is the minimum. If you consider how much damage you can do with something held at +270°C you have an estimation of the upper limit of what can be done with some device producing constant -270°C (~0K).