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What happens to the gunpowder and other explosives when they get wet? I know that they stop "working" (won't explode anymore) but for what reason? Is it anyhow similar to the fact, that wet wood is burning worse than the dry one?

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In general, putting explosives in water will not prevent them from exploding, because most explosives already contain all of the necessary reactants to produce an explosive chemical reaction. They do not need to draw oxygen from the air, and the reaction happens much too fast for the water to act as a heat sink.

Traditional "black powder" is an exception; it doesn't explode when wet. I don't know exactly why this is, but two plausible reasons are:

  • Unlike most other explosives, black powder is shock-insensitive; it has to be initiated with a flame or spark. Water can easily soak up all the heat energy from a small flame, like the one you get from a percussion cap. This is the same as how it's harder to set fire to a living tree than a dry log, but if the fire gets hot enough it will still catch.

  • Black powder is a "low" explosive, meaning that the chemical reaction happens relatively slowly (for an explosion), which gives the water more time to absorb heat energy and quench the reaction.

Worse, black powder can be permanently ruined by water. This is because it's a mixture of saltpeter (potassium nitrate, $\ce{KNO3}$), sulfur, and charcoal, and saltpeter is water-soluble. If you get it wet enough, the saltpeter can be completely washed out of the mixture. This doesn't always happen; the 19th century industrial manufacturing process for black powder involved deliberately dampening it, both to make it less dangerous during manufacture, and to make saltpeter crystals form inside each particle of charcoal. But they were careful to use only just enough water.

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    $\begingroup$ I suspect an important difference between the water-insensitive explosives and black powder is that it's a low explosive. I.e. the water has time to cool portions of the mixture, before they are themselves ignited by a supersonic shock front. In a high explosive, I'd figure this isn't possible, because in order for the water to take away latent heat it needs to vaporize, thereby expanding. But while the detonation shock front is passing through, the high pressure inhibits this. $\endgroup$ – leftaroundabout May 13 '17 at 22:47
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    $\begingroup$ @leftaroundabout It may also be relevant that black powder is shock-insensitive, needing a flame to initiate an explosion; water can plausibly absorb the heat energy of a small flame (e.g. from a percussion cap) before the carbon-KNO3 reaction has a chance to get started. $\endgroup$ – zwol May 13 '17 at 22:54
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There's some problems with this question.

gunpowder and other explosives when they get wet? ... (won't explode anymore)

"Gunpowder and other explosives" encompasses thousands of substances, all with different properties. And: how wet? And what exactly is meant by "won't explode" -- there's several different types of explosions.

Moreover, OP's premise is wrong. It is not true that explosives, as a general rule become non-explosive when wetted. Most military explosives and many construction/mining explosives are completely waterproof (although their initiation systems may not be.) The product family of commercial emulsion explosives actually need water to be explosive. When completely dried they burn instead of exploding.

The most common explosive in the world -- ANFO -- is rendered (practically) non-explosive by dampening it, but not for the reason you suspect. In this material, the oxidizer (ammonium nitrate) is easily wetted, but the fuel (oil) is repelled by water. Hence when it is dampened the components separate. It is still theoretically possible to detonate it in this state, but it requires an abnormally powerful initiating charge (booster), explodes with greatly reduced power, and produces a lot of toxic fumes. Since the shot-firer probably didn't expect to need an abnormally powerful booster, it will most likely fail to explode.

Note that this occurs strictly because the fuel is repelled by water. It is possible to make a very similar explosive using a hydrophilic fuel. This composition will be able to tolerate much higher levels of dampness. However if enough water is present, the ammonium nitrate will dissolve completely and wash away. To prevent this, a gelling agent may also be added.

The most interesting case is perhaps smokeless powder (gunpowder for modern firearms.) This is a precision application of explosives: the powder needs to burn at a very precisely controlled rate, both to control the pressure that develops inside the firearm, and to give consistent muzzle velocity so you can shoot accurately and hit your target. Allowing small amounts of moisture to contact the powder will alter the burn rate by an unpredictable amount, rendering the powder unfit to use in a firearm, but by no means non-explosive.

This is widely assumed to be due to the cooling effect. That is, each burning granule rises in temperature slower because of its water content, which causes its neighbor to heat slower, and take longer to ignite. However precise measurements show the burning rate change to be larger than can be explained this way, so there must be more to it. However the exact mechanism doesn't seem to be known.

Unlike in ANFO, or even black powder, the water doesn't do any harm to smokeless powder. In powder factories, it is routine to process the powder wet, for safety reasons, and dry it only as one of the last steps before packaging. For several reasons it is infeasible to do this anywhere but in a powder factory.

Although burn rate is affected by even fractions of a percent of water, smokeless powder doesn't become non-explosive until a lot of water is added. Under transport safety regulations, NC based explosives change from "1.3 explosive" to "4.1 flammable solid" at 25% or greater moisture. However, even at 25% water, it is far from safe. It will still burn fiercely, but the burn rate is controlled by the rate of evaporation of water from the surface of the mass. Under some circumstances, this can still be ferocious. In magazines where I have worked, the point at which scrap powder no longer counts against the NEQ limit (Licensed Nett Explosive Quantity) is when it is Under Water: that is, totally soaked in water, and submerged, with at least two inches of standing water above the surface and some sort of lid to prevent evaporation. Even in this state, it wasn't considered to be unable to explode: just unable to be ignited.

Which is why I earlier asked what exactly is meant by "won't explode"? Nitrocellulose explosives are nearly always used as low explosives (propellants), which react by extremely fast burning. However, if suitably compacted (to give good propagation to the shock wave) and given a powerful booster to start it, high-nitrogen nitrocellulose can also be detonated as a high explosive. This reaction can occur even if soaking wet and fully submerged.

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For combustion to take place, there needs to be an oxidizer, fuel, and heat. When a substance gets wet, it is surrounded with water and is not in contact with atmospheric oxygen, arguably the most common oxidizer. In the absence of any other oxidizers, combustion stops. Underwater fuses incorporate strong oxidizers like perchlorate salts so the reactions can continue in the absence of atmospheric oxygen. Water is also an incredible heat sink and will steal heat from the reaction and dampen it in that way.

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    $\begingroup$ The question was specifically about explosives (which always come with an anærobic oxidiser), wasn't it? $\endgroup$ – leftaroundabout May 13 '17 at 22:37

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