# How could I scientifically explain ice breath?

I'm sure there have been questions on here about fire-breathing dragons, and I recently saw a question about a lightning-breathing dragon, but back in fifth grade I wrote a 150-page story about a dragon with ice breath, and now I'm starting to wonder if it would be possible without large amounts of magic.

My biggest problem is that while humans have had fire for millenia and electricity for over a century, it took a lot longer to come up with a way to generate significantly cold temperatures without using big blocks of ice. In fact, I'm still a bit confused about how refrigerators work.

So, I'm wondering, could an animal evolve to produce temperatures cold enough to freeze water and/or prey? And if so, how?

Any environment where this is possible is acceptable, though an answer that allows for a wide variety of Earth environments is preferable. Also, though I'm thinking dragons, if it's not possible for them but possible for another kind of animal, such information is welcome.

• All it would need to do is spit liquid nitrogen, how it gets and stores it is up for debate Mar 3 '15 at 15:10
• @ratchetfreak yeah, if you know how a dragon can produce, store, and safely expel liquid nitrogen, let me know. Mar 3 '15 at 15:15
• @DaaaahWhoosh Magic, duh. (sees science-based tag) ...uh, I mean... wormholes? Quantum effects? Mar 3 '15 at 15:35
• @DaaaahWhoosh how is a fire breathing dragon any more scientifically plausible? Mar 3 '15 at 16:55
• @JamesRyan You can make fire with a couple of rocks and some fuel, but making something cold is nowhere near as easy (at least as far as I can tell). Mar 3 '15 at 17:05

I wouldn't use liquid nitrogen. I would use liquid carbon dioxide, CO2. There are a few reasons for this.

Physical

CO2 can exist as a liquid at ambient temperatures under sufficient pressure. Nitrogen cannot as its critical temperature is much lower. In layman's terms, at any temperature above 126K (-147C) the density (and other properties) of nitrogen gas and nitrogen liquid become identical, so there is no distinction between gas and liquid and no vaporization cooling occurs when pressure is released. So, if you use nitrogen, your dragon is going to need a way of storing liquid nitrogen, and a means of generating it.

CO2 on the other hand, can exist as a liquid at ambient temperature if sufficient pressure is applied to it (56 atm at 20C). When the pressure is released through a simple valve, molecules break free from the attractive forces in the liquid, which requires energy and therefore causes cooling. This is exactly what happens when a CO2 fire extinguisher is used. (though the idea is to suffocate the fire, a side effect is the production of dry ice.)

Fun with fire extinguishers (don't try this at home!) https://www.youtube.com/watch?v=Z3xyqfCZmSU

When CO2 vaporizes it requires 3 times as much energy per unit mass than liquid nitrogen, therefore its cooling effect at ambient temperature would be greater. Ironically, it is precisely because of this that it has a higher boiling point temperature than liquid nitrogen: -78C at atmospheric pressure instead of -195C. (As an added complication, the freezing point of CO2 is higher than -78C, so it can only exist as a liquid when its boiling point is raised by high pressure.)

http://www.engineeringtoolbox.com/fluids-evaporation-latent-heat-d_147.html

Chemical / biochemical

Where does the dragon get her gas from? If it is nitrogen, she will have to get it from the atmosphere and liquefy it in some way, either via an expander or via a separate refrigeration cycle, both of which seem biologically impossible.

If she uses CO2 she can generate it chemically, and it may then already be under pressure. She may do this in a variety of ways:

1. Use the normal metabolism. It is unlikely that it would be possible to build up high pressures of CO2 in this way, though, without making the blood too acidic.
2. Use CO2 from her fire breath, if any. This would be highly inefficient, and would again have the problem of generating CO2 at ambient
3. Go to a frozen wasteland and eat CO2
4. Eat chalk, and use this to generate CO2, through the reaction CaCO3 + Acid = Ca salt + CO2. The acid could come from normal metabolism.

5. As for 4, except the acid comes from an external source. For example, the dragon may go to a volcano and feast on brimstone (sulphur) which she could then burn (producing fiery breath without the inconvenience of having to generate large amounts of fuel through metabolism.) When she is not breathing fire, the dragon may slowly convert the sulphur to SO3 and H2SO4 in her belly, and react these with chalk to make CO2 under pressure. This is an entirely realistic way for a dragon to produce icy breath without metabolism or mechanical engineering issues. The only remaining issue is materials, which have always been a problem with fire-breathing dragons anyway.

Other gases

Other suitable gases include propane (again requiring more energy per unit mass than nitrogen for vaporization than nitrogen, and I can say from experience that a splash of propane "feels" colder than nitrogen.) The dragon could choose to breathe icy propane, or instead ignite it and breathe fire.

Carbon monoxide, CO is another posibility (highly toxic and moderately flammable as well as being a gas) but its critical point is well below ambient temperature, like nitrogen. Several existing organisms use formic acid as a weapon, and formic acid decomposes catalytically to carbon monoxide and water.

Sulphur dioxide fits in with the brimstone idea. It has a boiling point of -10C, which is perhaps a little high, and is toxic and corrosive. Generating SO2 chemically under pressure would be difficult because of the requirement for atmospheric oxygen, unless the dragon breathed in the air and dived to great depths like a sperm whale in order to compress the oxygen.

• Does that "3× vaporization energy" figure refer to the molar heats of vaporization ($L_v m$)? Mar 3 '15 at 22:27
• @WChargin the table I linked to is specific heat of vaporization (per kg.) Your link is per mol. After conversion there is good agreement between the two tables for nitrogen, but quite a large discrepancy for CO2. The moral is, if you are going to do any serious engineering work, make sure your source is reliable. I don't propose to dig deeper as it doesn't affect the overall answer, but one issue here is to be very clear whether the condensed state of CO2 considered in the table is liquid or solid. Mar 3 '15 at 22:41
• Since liquid CO2 can't exist at normal pressure, the dragon can't spit it. It would expand rapidly in his mouth and either tear it open, or produce an absurd fizzle cloud around his head. Mar 4 '15 at 0:56
• @Oldcat it doesn't seem to be a big problem for plastic fire extinguisher nozzles. You get a big cloud of gaseous CO2 and solid CO2 ice-dust that (due to the pressure) has a lot of kinetic energy and travels a long way. See the video I just added to my answer. I'm not too concerned about the anatomy of dragons' throats. They have to handle fire anyway, so they should be reasonably resilient. Mar 4 '15 at 1:16
• The biggest problem is that the vapor pressure of CO2 at standard temperature is something like 60 atmospheres, and I don't know of any biological structure which could withstand such pressures in order to function as a storage container. Mar 4 '15 at 4:01

Adiabatic cooling might work for this. Among other things, the general principle is used in the creation of liquid nitrogen, so it can certainly get things cold enough.

In this scenario, the dragon breathes an air vortex at an extremely high pressure, not unlike an air vortex tube. As the vortex leaves the dragon's mouth, it expands, and in so doing, cools the space it flows through. Cool that space enough, and the water vapor in in should condense rapidly: cool it further, and the water vapor should flash-freeze. Combined with the wind from the dragon's breath, you have something that looks like breathing ice and projecting it forward, even though the dragon is in fact breathing nothing but air.

This does require the dragon to be able to produce extremely high pressures in its lungs, and to withstand the heat generated by compressing the air to such temperatures, so the dragon (or at least its upper respiratory system) could not be "weak against fire" as seen in some stories and video games. The dragon would also need to be able to release the gas through its mouth at these fantastic pressures without shattering the bones of its face. But it doesn't require the dragon to contain or generate any exotic substances.

You can create a far lesser version of this effect yourself, just by blowing. Try to blow out air at a steady rate, but alter the opening of your mouth. As your mouth gets bigger, the air you're blowing should feel warmer. Shrink your mouth down very small, and the air gets colder: you're not going to make ice this way, but you should be able to feel the difference in temperature. My suggestion is essentially this effect, writ very large.

• I think this is the best answer, it's the first thing I thought of when I saw this question. Because the dragon could be very large and dense I believe a specialized organ system could achieve this effect. Mar 5 '15 at 1:19
• cool blooded and in an already cold environment, the heat could even become beneficial for the creature Jan 26 at 19:27
• This answer is really good. Possibly better than the chosen answer, even, since it does solve the "materials" problem. Jan 27 at 0:47

A specific way this could be achieved by a dragon would be through the endothermic reaction of ammonium chloride, which is found in nature (in volcanic regions) and barium hydroxide, which can be synthesised from nature. Barium hydroxide can be formed by the addition of water to barium oxide, which is formed by heating barium carbonate which is also found in nature (although admittedly not in volcanic regions). Even if the dragon would have difficulty heating the barium carbonate to the appropriate temperature it could force the reaction through the use of catalysts / enzymes, and by constantly removing carbon dioxide from the bladder (a standard biological process) containing the barium carbonate, shifting the equilibrium towards the barium oxide state. The resultant barium oxide could be filtered out by flushing the bladder with something like ethanol which dissolves barium oxide, but not barium carbonate. The dragon may also be able to supply heat to the mix by entering a hot spring or other volcanic nicety.

Ammonium chloride is not particularly poisonous, and is even sometimes used as food flavouring. Barium oxide is more of a problem, being fairly poisonous, however if digested as barium carbonate, and processed in a separate bladder I don't see why it would be a problem.

The dragon could store the barium oxide dissolved in ethanol until it was ready to release its freeze-breath, at which point it would pump water into the barium oxide, which would rapidly precipitate out of the ethanol as it became barium hydroxide, which is mostly insoluble in ethanol. Barium hydroxide octahydrate is a crystal so the remaining ethanol and excess water mix would help to flush the slurry into the ammonium chloride bladder. This mix would then be expelled rapidly into the atmosphere. The mix will cool to ~-20°C (depending on volumes), which is enough to freeze most animals. As a side benefit you also get a strong smell of ammonia, which is very atmospheric (no pun intended).

Liquid nitrogen seems to be the obvious answer. It's inert, raw nitrogen is available in abundance from our atmosphere and producing it is not totally impossible.

All you need is for the dragon to have some sort of nitrogen bladder, where it accumulates nitrogen from the atmosphere (this is pretty doable by biological processes).

When it's sufficiently full, the bladder constricts to significantly increase pressure. This warms the gas up, so you need a cooling system (ie. blood flow) to remove the heat, than you continue the process (optionally adding more nitrogen) until it becomes liquid.

We know that some life forms (like pistol shrimps) can use biological processes to briefly create impressively high pressures; the problematic part would probably be maintaining the pressure (which helpfully also prevents the dragon from freezing over from the inside), but since you apparrently already have flying dragons this only requires little additional handwaving.

When the dragon wants to use his "breath", he will expell the nitrogen as a high-pressure stream. Depending on the dispersal of this stream, it can either have long range and cover a distant target with the supercold liquid, or can be spread over, so that the nitrogen evaporates, cooling a relatively larger volume.

• "keeping the dragon from freezing over from the inside" why would that be a problem? If I understood you correctly, nitrogen would be stored at room temperature, wouldn't it? Mar 3 '15 at 16:49
• @skysurf3000 Right, if you never release the pressure, it should stay at body temperature. I'll edit the answer. Mar 3 '15 at 16:50
• The pistol shrimp actually creates an incredibly low pressure, which upon collapse, creates a loud bang. The high pressure acoustic waves are actually less than one atmosphere in pressure, far below the 200 atmospheres required to make room temperature liquid nitrogen. Mar 3 '15 at 17:03
• @Samuel Well, I was talking about the shockwave that happens when the cavitation collapses, but I guess that's what I get for baiting marine biologists. Do you have a better example of animals abusing mechanical advantage to create high pressures? Mar 3 '15 at 20:57
• @MikeL. I'm no biologist, just an electrical engineer, but I believe a crocodile bite will put you on track. Mar 3 '15 at 21:28

There's really two possibilities I can see offhand:

1. Endothermic Chemical Reactions. This is a chemical reaction where you take two reagents, and when put together they consume heat as part of the reaction. So your dragon could store the two separately, and the "breath" would be where the two intersect and hit, consuming heat and freezing the area.

2. Pressure changes. Think about phase diagrams - basically as you increase pressure, the freezing point of liquids goes down. What this means for you is that an animal with a high pressure storage area could have, say, supercooled water stored in liquid form below what's normally freezing. It could then release the water as a breath weapon, and it would immediately crystallize and turn to ice because of the lower pressure, freezing the enemy.

I think Endothermic chemicals are your best bet, although presumably there's some reason creatures haven't evolved them on Earth.

• Increasing the pressure lowers the melting point of a liquid, true enough, but how do you actually cool it? The tricks use to cool and liquify gas don't work on liquids. Mar 3 '15 at 15:41
• A staple of ice dragons is that they live in cold environments - maybe it flies high in the atmosphere and uses the cold upper air as a coolant? But I think using something like nitrogen, as suggested by another answer, is probably better/more realistic. Mar 3 '15 at 15:43
• @DanSmolinske High enough for the temperature drop to help much with that seems like it'd be high enough that there might not be enough oxygen to breathe, let alone air to create lift...
– user
Mar 3 '15 at 15:46
• @DanSmolinske That is a staple, although if they're living (and presumably hunting) in polar environments, most of their prey is by necessity not going to be too bothered by cold:) Mar 3 '15 at 15:46
• My thought is that it would go into a colder area to build up its breath, then go hunt in a slightly warmer area where the animals will be susceptible. It is kind of funny that "standard" DnD fantasy dragons generally have breath weapons that wouldn't be very effective where they live. Mar 3 '15 at 15:49

1. Endothermic reaction - or at least I hope that's how it is called: As there are some exothermic reactions (which produce heat), there are also endothermic reactions which get all the heat from the reaction, effectively freezing it.

So, moreover:

2. It would not be breath, but spit Somewhere I heard, that poison in snakes evolved from really sour spits. Accepting it as truth, there would be dragon species with very (???) spits which would cause a bit of endothermic reaction on the skin of the prey.

3. It will work only specifically Using my poor basic knowledge of biology, and wisdom from funny internet videos, I would assume, that such dragons could freeze only (furry) animals: Their spit would cause endothermic reaction on the body of their prey, causing it to freeze to paralysis, or death. I would not suggest such a dragon being able to freeze everything.

4. Make far spitting dragons to succeed It makes evolutionary sense. The farther you can spit, the easier you get your prey.

5. Connect spitting with sound So it would look like they actually breathe out the "winter" (blue spits could do the trick).

Here you go. It is not rock solid, but I think it is a plausible theory to work with.

I think liquid nitrogen is a viable choice for all the reasons mentioned by Mike L. This link also goes to some explanation on how living tissue might be able to survive from freezing over: You can Safely stick your hand in liquid nitrogen which all boils down to the Leidenfrost Effect

*I would've just added a comment but I need more rep.

• Have some rep. Good answer to the question, "how could you be frozen alive?" Dec 1 '15 at 18:11

I'm presuming your dragon is of the conventional mould, and can therefore fly? That so, he/she need simply fly to a decent altitude (up amonst the cirrus, perhaps) where the temperature is naturally below freexing - and then he/she will breath ice crystals with no extra tricks needed.

I think the animal would require:

1. The ability to withstand subzero temperatures inside of it's body. Perhaps with a lot of inner insulation and a strong internal heat system, this would be feasible.

2. It would need a gland that produces chemicals that can flash freeze objects. Liquid nitrogen is the go to chemical here. Your animal would have to be able produce and store a lot of it to be able to use it as a main weapon in battle.

• well, in this case, and if you look at answers above, you will see that CO2 is better Jan 27 at 9:19