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Fire arrows are a pretty common trope, however, according to Lindybeige, they're not really that great. In contrast to fire, what if you make arrows that create an endothermic reaction upon impact, freezing its surroundings.

I have a rough design idea for these: The head of the arrow carries a container with two separated compounds, which mix together upon impact when the container breaks, and start an endothermic reaction. Now onto the question.

There are a few questions I have; what materials should be used in the making of the arrows, including the container and the compounds inside (the compounds should be as reactive as possible)?; and what could they freeze, flesh, blood, armor (what armor materials?)?

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    $\begingroup$ Endothermic reactions are not powerful at all, in sense of energy amount they are able to absorb. You are much better off packing your projectiles with dry ice or liquid nitrogen than any of endothermic ingredients. $\endgroup$ – Alexander Jun 15 '17 at 17:42
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    $\begingroup$ Sorry. You need magic for this one. $\endgroup$ – M i ech Jun 15 '17 at 19:00
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    $\begingroup$ You could create arrows that cause fire that don't actually burn in flight; e.g. with a glass bulb in the head containing a binary ignition agent (e.g. sodium and moist chlorine in separate compartments in the glass bulb). The bulb breaks on impact and the ignition agent starts the fire. $\endgroup$ – Pak Jun 16 '17 at 19:02
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You're talking about shooting someone with an instant cold pack. WebMD recommends keeping a cloth between the ice pack and the skin and limiting exposure to 15-20 minutes. As a weapon it's not going to be very effective.

I'm not sure why you'd want to in the first place. Even if you shot someone with a vial of liquid nitrogen you'd be looking at some added frostbite. This will be unpleasant to whomever has been shot. But the arrow sticking out of them is of much greater concern.

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    $\begingroup$ Like with fire arrows, your best target isn't a person, but a thing. Encasing an object in ice due to an endothermic reaction could significantly hamper the use of an instrument of war. Freezing the engine of a plane would cause it to fall from the sky, for instance. $\endgroup$ – Isaac Kotlicky Jun 15 '17 at 17:30
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    $\begingroup$ A couple grams of liquid nitrogen isn't going to do much. $\endgroup$ – sphennings Jun 15 '17 at 17:35
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    $\begingroup$ You're not going to do that with a couple of grams of liquid nitrogen. $\endgroup$ – sphennings Jun 15 '17 at 17:38
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    $\begingroup$ The Asker is asking what they could freeze. The answer is not much. $\endgroup$ – sphennings Jun 15 '17 at 17:47
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    $\begingroup$ This answer addresses the real issue here, you could handwave something to have a ridiculously high heat capacity and also to have a really low temperature, however those are pointless unless the arrow has good contact, essentially unless it penetrates the armor or splatters onto it in a way that increases the contact area dramatically it's going to be hard to freeze anything at all just because there's not enough surface for heat transfer. $\endgroup$ – Taha Attari Jun 17 '17 at 1:54
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If you're ok with a little magic or handwavium, you could have the arrow equipped with a device that attempts to create matter by comverting energy. In doing so, it would draw in tons of heat from the surrounding area.

The matter created would be nanoscopically small, but all the heat in a radius around the impact point would be gone, thus you'd have a sphere of solidified air surrounding the target, which would also be frozen. The air would thaw quickly enough, varying slightly with the surrounding temperature. It'll give off a really cool steam as well, as the air goes back to a gaseous state. In contrast, a human target would remain frozen longer than the air, since the freezing point is so much higher.

This may not be what you're looking for, as it's not exactly an endothermic reaction, but it is a potential soft-science way to make a freezer arrow.

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    $\begingroup$ It wouldn't be "solid" air, as it isn't in any sort of connected structure. The most that would happen is all the air freezes in place and then falls to the ground, forming a thin layer of very cold air particles. There it forms the solid form. Also, something tells me you'd also get interesting effects from absorbing the energy of electrons and nuclear energy states and other kinds of energy. $\endgroup$ – ltmauve Jun 15 '17 at 20:45
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    $\begingroup$ @Itmauve, true. I guess it'd just look like a bunch of snow had hit the ground. That'd be a cool side effect to a freezer arrow. But thanks for the correction. $\endgroup$ – Iter Jun 15 '17 at 20:48
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One way a frozen arrow could disable a target is to deploy a gas that would freeze the lungs and incapacitate a living being. If an arrow depending on size could carry enough helium. Helium can be cooled to below -400 Fahrenheit in gas form.

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    $\begingroup$ Sadly, although crazy cold, it's ability to freeze things comes in at around a 1:1 weight ratio, so you need a kilo of the stuff to freeze a kilo of water (not to mention that it takes up far more space). I'd recommend liquid ammonia, much easier to get, much denser (so the capsules are smaller), it won't boil away when exposed to light (unlike the helium) and as an added bonus... it's highly poisonous. $\endgroup$ – Samwise Jun 16 '17 at 0:28
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The coldest you could probably get with that starategy is maybe 400 degrees F. This will hurt, and cause lots of damage while stuck in the target, but it most likely won’t freeze anything. It would bounce off armor. Cloth, maybe. They(the arrows) will freeze flesh, tightening the skin and destroying cells. It’ll take a lot of freeze stuff though. But there’s the danger that the arrow will explode before firing, hurting its user.

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  • $\begingroup$ 400 F is far above boiling. You must mean some other temperature. $\endgroup$ – Vincent Oct 21 '17 at 4:22

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