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I would like to create a bomb where a dangerous material is inside of an open bottle (using a mesh screen to keep the material from falling out), then throw the bottle at a creature, and use create or destroy water to create water inside of the bottle and make it explode.
There are many substances that react violently with water, either exploding or releasing toxins. However, I would like to know if this is even possible.
Can I make a bomb with the create or destroy water spell in this way?
tl;dr As written, don't get your hopes up. Best bet is to re-skin an existing spell by keeping its mechanics and changing the narration.
This is clearly in the realm of DM fiat and homebrew. If the table lets this happen, it will be bending some rules for the sake of what the people at the table want. Don't expect this to fly at every or most tables.
Trying to hit a creature with a thrown object is a ranged attack. That uses an action. The spell create or destroy water also requires an action. You usually get one action per round.
Create or destroy water doesn't say it breaks the container. The spell states up to 10 gallons, and that could be limited by the size of the open container. Any excess could simply pour out of the container.
You need to find a material that does what you want in contact with water. It has to exist in the world, and you have to be able to get and handle it.
For an existing spell the character has on their class list, it's easy to change the description of what happens while leaving the mechanics of the spell unmodified.
For example, guiding bolt is also a level 1 spell, it requires a ranged spell attack roll, and it does a considerable amount of damage if it hits. Ask the DM if this character's guiding bolt can be described as a specialized version of create water with a flask or vial the character makes when they prepare their spells.
With this approach, your prepared spell isn't create or destroy water; instead it's Character's molotov (guiding bolt). so you don't get combat use of a non-combat spell. That will avoid balance issues. You still get the desired narrative, and the character has an interesting aspect.
Another spell that might fit the mechanics and be amenable to easily re-skinning is ice knife if the area of effect is important to the narrative or style.
First of all, I am assuming the GM has already let you find some explosive. If this is not the case, you will first need to discuss this with your GM. Such explosives exist in the real world - alkali metals are one example - but they need not do so in your fictional world. It's fantasy, the GM decides.
Even if such materials exist, knowledge of their existence may be limited and only available in the form "don't go to the abandoned mine, the ground explodes when it rains"
Explosives are great in combat, especially gunpowder, but they also present a large threat to whoever carries them. You wouldn't want to be carying gunpowder when the building is on fire.
Likewise, you wouldn't want to be carying alkali metals when water has any chance of getting near it. And water does. Unlike fire, water can appear in many different shapes and sizes, a sudden thunderstorm, a river to wade across or a waterfal behind which lies the BBEG's lair.
Combine that with the constant threat of an enemy or clumsy party member slamming into your backpack turning those safe, sealed glass containers into disasters waiting to happen, there's a reason safety guidelines around alkali metals heavier than Lithium are so strict.
Furthermore, enemies could very well catch on to what you are doing, so the first bottle you throw would better take out any enemy caster capable of casting create water or you'll be getting a swift payback.
In the end this all comes down to 'does your GM think this fits within the rule of cool', so talk to them. There's a pretty good reason real world warfare has never made use of these kinds of substances: on a rainy day the army camp could explode. But there's also a good reason why your GM might allow it: it's epic and this is fantasy.
Furthermore, look at the answer by @GcL: they do a much better job at disecting how this might work within the rules of D&D specifically and how you could keep things balanced.
It would be easier and more action efficient, of course, to use Shape Water's animated water bits to do the same thing. But Create or Destroy Water works as well.
The only pitfalls lie in not having the stuff you presuppose in the question. For example, there's no material in the published game that reacts explosively with water, but you have assured us that in the campaign you are playing in "[t]here are many substances that react violently with water, either exploding or releasing toxins".
So long a that's true, this won't fail for lack of suitable material, like it would in the base game. Similarly, a bottle that can hold your mystery homebrew material with a mesh over the opening and which counts as an open container yet also isn't actually open may be hard to come by. Nevertheless, should you have such a container, you will want for neither bottle nor mesh.
I wouldn't permit it.
There are substances that react violently with water but it's not that violent nor that fast--I wouldn't expect even a first level spell's worth of damage.
Furthermore, there's water in the air. Without an understanding of modern chemistry preparing such materials is going to be impossible short of magic--and without modern chemistry you don't know what you want anyway.
(And if you allow modern knowledge there are far more abusable things. True Polymorph that horse into a ball of U-235.)
The water shape / creation spells etc. look good on the paper, but their intent is to not be abused for whatever physical experiment you try to perform.
In theory, you COULD make a bomb with freezing water in a room, like you COULD cast wish for two neutron stars to rotate around each other and collide just in the second the bbeg is standing in front of you and grill him with a gamma blitz ray.
We jokingly called the spell shape water the "poor man's wish" because one player tried to cheese around with it all time, creating walls of ice so the enemy would not see him and thus he could not be targeted with spells. Well, I always love it when a player uses their imagination about stuff, but there are dedicated spells for this, and shape water in this case was a cantrip that should not substitute for a level 3 spell....
And in your case, just reskin acid splash and have your character do the somatic component like "I mix 2 liquids in a bottle, shake it, and throw it onto the enemy, chanting a chemical formula [as the verbal component]". Perfectly fine, and not game breaking.
First, let's look at the create water spell itself:
You create up to 10 gallons of clean water within range [30 feet] in an open container.
Since the spell is ranged and allows you to cast it within a container, that fits your planned use. It does specify an open container. You might need to work with your GM on this. What does an "open" container mean? Is "a bottle (using a mesh screen to keep the material from falling out)" open? Or since the mesh prevents the material from leaving, is it actually still closed? If the bottle is open, why doesn't environmental water get in and cause an explosion, or just humidity cause the slow degradation of you reactive material? Does your bottle have both a removable stopper and non-removable mesh?
Let's say the GM agrees that something of your design fits your concept of an "open" container. Now let's look at what other mechanisms are already in play that are similar to what you want to do:
Oil usually comes in a clay flask that holds 1 pint. As an action, you can...throw it up to 20 feet, shattering it on impact. Make a ranged Attack against a target creature or object, treating the oil as an Improvised Weapon. On a hit, the target is covered in oil. If the target takes any fire damage before the oil dries (after 1 minute), the target takes an additional 5 fire damage from the burning oil.
From this we know that it is reasonable for you to throw a bottle and have it hit the creature or floor nearby as an Improvised Ranged Weapon Attack. We know that this takes an attack action, and that a subsequent ignition trigger would be a separate action.
Alchemist's Fire (emphasis mine)
This sticky, adhesive fluid ignites when exposed to air. As an action, you can throw this flask up to 20 feet, shattering it on impact. Make a ranged Attack against a creature or object, treating the alchemist's fire as an Improvised Weapon. On a hit, the target takes 1d4 fire damage at the start of each of its turns. A creature can end this damage by using its action to make a DC 10 Dexterity check to extinguish the flames.
This reinforces the conclusions from oil. It also tells us that in-game, alchemical procedures exist to create substances that will react with air and that these can be weaponized routinely enough that there is a standardized price for them in the PHB. Thus, I don't agree with the answers that say that chemicals that react with water would be unlikely to exist in-game, or that they would not be widely known about, or that they could not be safely packaged in containers. If Alchemist's fire exists as canon, what you are trying to make seems very likely at the same level of alchemical technology. In fact, it seems more likely to exist than the technology for drawing metal into wire fine enough for your mesh, since AFAIK there is no PHB support for anything finer than chain mail links.
In terms of support within existing mechanics, then, what you are trying to do is perfectly reasonable.
To extend the comparison with Alchemist's fire, though, look at the cost: 50gp for a flask that does 1d4 damage per round to a single target and can easily be put out as an action.
Are you expecting your bomb to do more damage? Are you expecting it to damage more than one target (as an area of effect)? Are you expecting it to be more difficult to counter (Higher DC save for reducing damage)? If you are expecting it to be in any way better than alchemist's fire, you should expect it to cost more than 50gp, which is already quite costly.
So now think about what you are spending: at least two actions (throwing Attack and spell-casting), a first level spell slot, and more than 50gp in resources. As a GM, if a player was willing to spend all of that I would absolutely allow their bomb to work. As a player, though, I would wonder if your goal of doing damage to an opponent at just 30 feet away couldn't be more efficiently achieved by an existing first level spell - others have mentioned guiding bolt, which gets you 4d6 damage, far better range, a solid secondary effect, and the ability to roll your attack with your spell attack bonus rather than as an Improvised Weapon.
