Explosives as we know them are exothermic: they produce heat and light, or, in other words, energy is expelled.

Let's consider endothermic bombs: weapons that, upon detonation, consume heat and / or light and / or nearby electricity.

Assuming there is sufficient funding to develop and manufacture this technology, and for whatever reason, there is a use for it, is it feasible to construct a detonated weapon with this purpose?

If so, where can energy be drawn from, or what will it actually do? Will nearby electricity be cut out? Will the room be drained of all light? Will it suddenly get colder? etc.

Please, no handwavium. Also note that "economically viable" as part of "feasible" would be nice but isn't necessary for an accepted answer.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Tim B Dec 1 '16 at 16:06

10 Answers 10


Gas expansion

This process is already around us on an everyday basis. It is the thing that makes it rain.

When you have a gas closed in some capsule and around it is lower pressure (and ideally vacuum), when you release the gas it will start expanding rapidly with the loss of internal energy, effectively cooling the gas and its surroundings. It does not work same for every gas; for example, He and H gases have the effect reversed on room temperature (they do achieve cooling on way lower temperatures, however).

In the real world, it works in clouds. When the Earth produces and releases hot air, it rises because it is lighter than cold air. The higher you are, the less pressure is around and raising gas expands, which cools it. At the height of ~2km, it will hit the temperature needed for condensation of water gas into water drops, effectively making "clouds" you can see. When it expands higher and higher (it has to be fed from the bottom because heavier and therefore colder gas tends to drop down as a rain), it possibly can make even storm clouds (Cumulonimbus), which are really tall. At the top parts of that, it effectively expands to nearly vacuum, thus cooling itself A LOT. At that point, a hailstorm may form, which is just extremely cold rain.

The bomb

For a bomb utilising such an expansion, you don't need any reaction, nor ignition, nor any kind of magic. You only need some capsule that will store A LOT of gas that can be released. Upon releasing the gas, it will expand rapidly (thus make the effect of "explosion", pressure wave etc), and it will cool down in the process. If it is released fast enough, it will consume its internal energy for expansion, thus cooling surrounding objects.

The problem to make such a bomb is that you need to start with room temperature, high pressurised gas, which is hard to find. If you compress the gas, it tends to raise its temperature, so you need to do it in steps. Also, the capsule that you use must be strong enough to hold such a pressure. That might also be a problem, nowadays used gas cylinders are not really robust and hold too little of gas for your bomb purposes.

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    $\begingroup$ @Annonymus Preparing the bomb will be really hot job, compressing stuff can heat it a lot. Yes, more heat than it eats later, but you can deal with it in factory. Probably you will end up with gas in fluid form, which probably will be somehow stable, if not exposed to higher temperatures and will have containment strong enough. It will want to explode and expand. However the change from liquid to gas on expanding is another heat-eater. I would look on behaviour of gas cylinders and their safety. It will be really costy ammo compared to conventional explosives (and sensitive to temperatures) $\endgroup$ – Antoine Hejlík Nov 30 '16 at 14:14
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    $\begingroup$ I mean on explosion actually. The expansion of the gas will eat some heat but the friction between the gas and anything around the bomb as well as the compression of the stuff around the bomb (likely air) will also produce heat. I'm not actually sure how much energy is added/subtracted by either process but at a rough estimate it seems to me like more heat would be generated than absorbed $\endgroup$ – Annonymus Nov 30 '16 at 14:18
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    $\begingroup$ @RudolfL.Jelínek No, if you compress the gas so much it is neutrons, the expansion results in unstable free neutrons, which rapidly decay over a few minutes, converting a non-trivial percentage of their mass to energy. I suspect that this will overwealm the decompression energy costs. $\endgroup$ – Yakk Nov 30 '16 at 19:01
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    $\begingroup$ An effect many of us are familiar with when we use products like canned air or compressed. Frost forms on the container during prolonged use. Nice answer. $\endgroup$ – Samuel Nov 30 '16 at 21:22
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    $\begingroup$ @RudolfL.Jelínek well, no. You cannot compress indefinetly, soon or later you will get liquid state, then more compressing will just start to be really hard thing to do (aka "not economically feasible"). But it is for each substance different, according to its en.wikipedia.org/wiki/Phase_diagram . $\endgroup$ – Antoine Hejlík Dec 1 '16 at 7:24

This could be a bit of an underwhelming answer, but it is the best I can do while sticking to realism.

For absorbing heat

One simple solution is to use a device that causes rapid evaporation/sublimation of a liquid or solid. We already have something similar—a bottle full of liquid nitrogen. To make the device better, you can pressurize it to prevent ineffective evaporation, to make the device easier to store. (so that your character can say things like "hey look, this centuries-abandoned arsenal seem to contain some endothermic bombs, we can use that to...")

For absorbing light

The Ozone layer is absorbing ultraviolet light right now—at least on the sunlit side of this planet, hopefully—however, gases like ozone are frequency-specific in their absorption of light, meaning that light of a different colour/frequency cannot be absorbed by the same gas, so a mixture of many gases is needed to absorb a broad range of light (you also have to be aware of re-emission where the energized gas release that energy in a lower frequency).
This will be boring, but if the purpose is to block light rather than remove the photonic energy, just use a smoke bomb.

For electricity

I can only imagine this being useful when the target is some kind of machinery that runs on electricity. Traditional EMPs will probably do the job, but if you want to physically reduce the amount of electricity running in the system, try a graphite bomb, which will cause short-circuiting.

Some physics/chemistry

An endothermic chain reaction is not possible because of this equation: $$ΔG = ΔH - T\times ΔS$$ Where ΔG is the change in Gibbs free energy, which must be a negative number for spontaneous reactions (those that can go on without people helping along)

ΔH is the change in enthalpy, in endothermic reactions, it is positive

T is temperature, it will decrease in the case of endothermic reactions, it is always positive because it is measured in Kelvins

ΔS is change in entropy, cooling generally means that this value is positive, but it can be negative in the case of evaporation

So, these being said, as the endothermic reaction carries on, T will decrease, so no matter what the posivity/negativity of your ΔS is, the reaction will always start becoming non-spontaneous because the effect of ΔS is decreasing while ΔH is positive and constant, causing ΔG to become positive.

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  • $\begingroup$ A weapon that consumes electricity could have a major advantage over an EMP when you want to deny an opponent the use of their electronics, yet leave the device intact. $\endgroup$ – a CVn Nov 30 '16 at 8:48
  • $\begingroup$ Don't know how scientific this is but if the device could absorb all the electricity in an area wouldn't this make a very good clean bomb? It would also disrupt the electrical impulses in the human brain, causing death while not destroying any infrastructure. $\endgroup$ – Snowlockk Nov 30 '16 at 9:54
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    $\begingroup$ @Snowlockk I am not sure what you mean by "absorbing electricity", since electricity is really just moving charged particles. You can slow them down perhaps, if you know how exactly they are moving and use a magnetic field, but for small and complex circuitry, like the brain or a computer, you might as well just look for the plug. $\endgroup$ – Luna Nov 30 '16 at 18:29
  • $\begingroup$ It should be noted that a simple foil chaff bomb will absorb radio waves. Usually used for simply disrupting radar, but an appropriate design could "kill" most radio communications over a limited area. $\endgroup$ – Hot Licks Dec 1 '16 at 22:23

Surprisingly (at least to me, at first) there really are chemical reactions that both go forward usefully quickly and are strongly endothermic. Nor are exotic chemicals necessarily required:

A classic example is mixing ice and salt to get a lower-than-freezing temperature -- as was done to make ice cream, before refrigeration was available.

Another example -- one that works fine starting from strictly room temperature reactants are the "instant ice packs" used in first aid (great for sprains); see https://en.wikipedia.org/wiki/Ice_pack#Instant_ice_packs

The mechanism for what appears to be a violation of the first law of Thermodynamics is that the second Law gets involved, via the Gibbs Free energy: https://en.wikipedia.org/wiki/Gibbs_free_energy

I don't know of any such reaction that would be much use as a weapon, except against a poor goldfish in a bowl :-(

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    $\begingroup$ if you created something like a fuel air bomb with these you might, but the effect would still be localized and minor, once it cools the surroundings enough there is not enough ambient heat to sustain the reaction. they are self limiting. You have made a few square blocks cold and... that's it. $\endgroup$ – John Nov 30 '16 at 14:21
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    $\begingroup$ In the ice cream maker case, the salt just lowers the freezing point of water, making liquid possible at lower temperatures. The ice was already below freezing, and the salt dissolving into the water doesn't take any energy out of the system. $\endgroup$ – mskfisher Nov 30 '16 at 14:40
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    $\begingroup$ @mskfisher: mixing ice and salt actually lowers the temperature, below that of the ice itself! The entropic term (T times delta S) overpowers the enthalpy term. Strange, but it works; try an old-fashoned ice cream freezer and see! (Experiments with tasty results are the best kind ;-) $\endgroup$ – Catalyst Nov 30 '16 at 16:49
  • $\begingroup$ The mixture of salt an ice has a lower melting point, causing part of it to melt, absorbing heat (heat of fusion), until the temperature has dropped to the new melting point. $\endgroup$ – AI0867 Jan 13 at 21:27

The endothermic bomb would be a heat sink. The problem for a point sink is that you can only go to ~0 Kelvin which is not that impressive compared to the exothermic at least ~500 Kelvins. The point sink simply has no "suction" comparable to "expansion" from the heat source. Thus there is no "explosion" from a point sink.

Thus the bomb would need to spread the substance to a big volume. One mechanism could be that the molecules are pressurized before they are released and that in the pressure the molecules will not react, but as released the gas spreads and then does the reaction. For that there can be made a kind of approximation of the power that the sink would "suck". Inversely you could then try to find a reaction that could produce this required effect.

One thing could be that the energy for the reactions is sucked directly from the target. It would not be an explosion, but more like a gas attack.

EDIT: One thing to notice is that unlike in exothermic explosion the frontier pushes itself, in endothermic the frontier would be "sucked" as soon as the reaction starts. Thus the reaction needs to be slower. The effective mechanism could be the expansion of stabilizing pressure after the sink has "sucked" energy. In exothermic the effective mechanism is the expansion at the beginning and there is then some mild suction after for the pressure to stabilize. Exothermic reactions simply are better because they can expand and they have no similar thermic limit as endothermic.

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  • $\begingroup$ "Impressive" may depend on how you phrase it. You can heat the environment to 500 Kelvins and you've only doubled the energy. But reduce the environment to 2 Kelvins, and it's been reduced to less than a hundredth of its original energy. $\endgroup$ – James Beninger Dec 1 '16 at 5:01
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    $\begingroup$ Yes, "impressive" is relative, but it's about the difference. Just lighting a match changes temperature more than what can be achieved only in a cryonic lab. The potential of making a endothermic "explosion" is simply too inferior to exothermic. $\endgroup$ – user3644640 Dec 1 '16 at 7:31
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    $\begingroup$ In purely destructive terms, you're right. But a "freeze bomb" that sucks out every bit of energy from an area impresses me just as much, if not more. From a military perspective, it would be superior in many circumstances. It would be less damaging to infrastructure, and the people (prisoners) may be revivable since they were flash-frozen, giving their blood no chance to crystalize. $\endgroup$ – James Beninger Dec 1 '16 at 14:03

No handwavium but not possible with our current technology: convert energy into matter. This will require considerable amount of energy and will suck the heat around it. To get 1g of matter, you need $8 \times 10^{14}$ joules of energy, which is quite a lot. If I did the math right, it will reduce the temperature of 10 million tons by 190 degrees.

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    $\begingroup$ I'd be interested in seeing the math you did to reach that answer. Also, how would one go about converting energy into matter? $\endgroup$ – automaton Nov 30 '16 at 17:40
  • $\begingroup$ No idea about how. That requires proper understanding of how the universe really works. But the math is simple: E = mc^2 (m is in kg and c is in meters/sec, E will be in joules), convert joules to calories, 1 cal = 1g of water heated by 1 degree. This will give you a rough estimate. $\endgroup$ – Cem Kalyoncu Nov 30 '16 at 21:29

Endothermic Explosive

Entropic explosives are driven by entropy rather than enthalpy.

Many modern explosives are actually endothermic. However the effect is nothing like what you are expecting. In short, for the reaction to be driven by entropy, it would basically need to generate a HUGE amount of gas in a short space of time.

The mechanical, concussive effect would make it almost indistingushable from an Exothermic explosive.

More information can be found on the Chemistry SE https://chemistry.stackexchange.com/questions/41979/are-non-exothermic-explosions-possible

Cold Bomb

However, what you described, is not an endothermic explosive, but rather a cold bomb. A devices that removes Entropy from its surroundings instantly.

This breaks the second law of thermodynamics.

When considering the bomb and its surrounds as a closed system. The bomb is able to reduce entropy from a closed system. This is forbidden by the second law.

Any such device, must sidestep this law. Possibly by "openning up the system", using a wormhole.

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    $\begingroup$ You need to include idea, that the factory making the bomb is also in that closed extended system. In factory you will do reverse process by preparing the bomb. For example for making pressurized gas bomb, you will need a lot of work and a lot of heat will be emitted. Then the whole system will be thermodynamically okay, and locally decreasing entropy is okay. Whole unverse system does matter. $\endgroup$ – Antoine Hejlík Dec 1 '16 at 7:30

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, for instance, if you have two objects at room temperature, you'll need to do work, i.e., transfer energy into the system, to move heat from one to the other. That means your bomb can't be at room temperature, since to remove Q Joules of heat from the room, you'll need to add W Joules of work done, so the total energy in the system is now Q+W Joules, and for any combination of Q and W, the total energy of the system is greater than Q alone.
"Bomb" implies a chemical reaction. To generate a highly endothermic reaction, you run into the same issue as above. The lowest energy state, i.e., what's left after every possible exothermic reaction possible, given the reagents present, is the most stable, just as a body at the bottom of a cliff is gravitationally more stable than at the top. In order to get your chemical mixture to move to a different combination, you'll need to supply energy, as described above. This allows you to move to a different chemical mixture that CAN potentially move to a mixture of lower energy state and WILL, unless there is an energy barrier in the way. Think of it as standing on a cliff with a high wall between you and the edge. The higher the wall, the less risk of falling off. On the flip side, if you're starting from the bottom of the cliff, the higher the wall, the more energy you need to expend initially climbing it, and the more energy that will be given back out when you climb down the other side. enter image description here

In thermodynamic terms, you need a high initial concentration of energy to start the reaction, followed by part of it being given back. Now, bear in mind that the energy source is going to be air at room temperature. As I pointed out in the first paragraph, some 200 words ago, you can't get much energy out of a system at the same temperature, without doing work, so let's assume a fuse to start the process, give it a boost over the cliff.
The fuse pushes the first few molecules into the higher energy state, and they give back some energy, a fraction of what was supplied, which can push a fraction of the number of molecules that initially reacted. So, let's say 40% reacted in the second round. In the third, 40% of the second round number will react, and so on. The reaction cannot sustain itself; more energy is needed.

Bottom line, from above, the only energy absorbed, is that generated by the fuse. On the other hand, part of the energy released as the products roll down the wall, is lost to the surroundings. Net result, energy gain by the environment.
TL:DR, endothermic bombs don't work, unless of course, you're using them to damp a runaway exothermic reaction.

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  • $\begingroup$ Don't paraphrase. The law includes an important stipulation in that it "requires that entropy increases" in a closed system. If entropy in the universe is increased during the creation and detonation of the "bomb" then the law is not violated. $\endgroup$ – Samuel Nov 30 '16 at 21:15
  • $\begingroup$ @Samuel: Unless it's a closed system, we may as well throw away the books and say, "anything goes". $\endgroup$ – nzaman Dec 1 '16 at 13:16
  • $\begingroup$ Also wrong. It's certainly not "anything goes" and it's not necessarily "nothing goes". If it is the latter case, your reasoning is still wrong. $\endgroup$ – Samuel Dec 1 '16 at 15:51
  • $\begingroup$ @Samuel: If the universe isn't a closed loop, then as you've stated, the laws of thermodynamics don't apply. Also, the laws of conservation become false and perpetual motion becomes possible. As far as I'm concerned, that's anything goes, magic, supernatural, whatever else you might decide to call it. And what's "nothing goes" and how does it affect my reasoning? $\endgroup$ – nzaman Dec 2 '16 at 5:56

Short Answer


Long Answer

There are 2 parts to your question.

The bomb part. I am assuming this means that there is a spontaneous reaction with an epicenter and a radially propagating explosion.

The endothermic part. This means that as the reaction progresses, it cools the surroundings as it passes.

Let us assume that such a reaction exists, which was spontaneous (had negative Gibbs energy) and endothermic:

$$ G(p,T) = U + pV - TS \\ G(p,T) = H - TS $$

This would mean that as the reaction proceeds, the reactants and products would expand (given that the assumptions surrounding the explosion). For an endothermic reaction, $\Delta H$ is positive (as the internal energy rises $U$ increases from absorbing heat and the volume, $V$, is increasing for the fluid components of the reaction - from spreading out due to the explosion)

$T$ is decreasing over time. (as its an endothermic reaction)

$S$ is dependent on the nature of the reaction. In order for the reaction to be spontaneous at the start (our assumption for the start of the reaction), as the reaction went along, assuming that there mechanics of the reaction stayed the same (not true - will explain why later), this would still mean that the overall Gibbs energy would tend towards a positive value over time.

When it reaches 0, it stops being spontaneous and the reaction will stop being spontaneous. (it will propagate as long as it is kinetically permissible, i.e. activation constraints are satisfied)

Why I think $S$ will decrease over time

The reaction spreads radially so the reaction components themselves have to diffuse radially from the epicenter.

Considering the Boltzmann (stochastic, state-based) entropy as a measure of the system, in the system of the explosion, the fluid components' internal energy only decreases. This means that the overall disorder of the system decreases - thereby decreasing the entropy. The gaseous components dominate the measurement of entropy therefore, this means that there $S$ will decrease over time.

An answer but not to your question

Drop a balloon filled with liquid nitrogen and it will be close to what I think you're imagining.

This isn't really a 'reaction' but will have the effect of an endothermic bomb (the shockblast will be from the expanding nitrogen) and the surroundings will be cooled due to nitrogen absorbing the latent heat.

How about a hypothetical endothermic self-replicating nanothermitic reaction?

The principle behind a self-replicating nanothermitic reaction is that the the reaction components produce the feed stock required for the reaction to continue indefinitely from the surroundings.

The absorbed energy would provide a access to the high-energy quantum states required to pass the activation energy barrier (of this hypothetical reaction pathway) and the solid products emitted would be left at such a low temperature that they supercool their surroundings as they pass through.

This is just food for thought, but I guess but I don't think that the reaction conditions on Earth can sustain such a reaction.

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A short response not repeating what's been said in other answers, just adding conceptual clarification:

Assuming that you can indeed cause a large endothermic reaction (which wouldn't be an "explosion"), one thing to realize is that an exothermic reaction really just produces heat. Yes, light is a byproduct - when something heats it emits a range of EM radiation, including visible light if hot enough - but in the end it's all just heat. Chemicals endothermically reacting simply get cold, and therefore won't directly "absorb" light, electricity, or any other form of energy. The net result of a large endothermic reaction is that the immediate area gets colder than it was before.

Still, what might rapid cooling do? Rapid contraction of some brittle solids makes them fracture/shatter (try dipping hot glass in ice water). Unprotected electronic systems will collect frost from water vapor in the air, potentially shorting them out when they thaw. Living creatures get frostbite. All in all, the effect is pretty mild if the temperature isn't sustained, and weaponizing it would be difficult without alien tech or other handwaving.

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If your 'civilization' was in the ocean, or other body of water, there are several reactions that would quickly pull heat from the water, ammonium chloride, for example when mixed with water reduces the temperature of the water around it. YouTube

Encase the ammonium chloride in a 'bomb' and detonate it in the water.

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