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?
Best Answer
Possible, yes. Likely, no.
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.
There are difficulties with the setup you describe.
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.
Consider narrative re-skinning a different spell.
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.