In my story there're alchemists but I can't find a way to use alchemy in combat. Please help me!
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I assume you’re talking about this kind of alchemy, which was half chemistry and half people trying to turn lead into gold. Doing battle is not a requirement of having a magic system. You don’t need to kill people with magic to make it a real system.
If you still insist on hurting people, a large part of alchemy was transmutation (turning one item into something else). There are endless possibilities in battle when you can turn your enemy’s sword into, say, a chicken. Also, if your alchemist has succeeded into turning metal into gold, turning someone’s sword or armor into gold would give you a massive advantage. Firstly, gold is really heavy. Secondly, it’s really soft. Thirdly, it’s rare and perhaps an alchemist evades their enemies by turning their weapons into gold, whereupon the enemy is attacked by greedy passersby, and the alchemist can escape in the chaos.
Also, alchemy was the earliest form of chemistry. Alchemists could have discovered the compounds that go boom when you put them together. Perhaps alchemists aren’t the best hand-to-hand fighters, but they are good at throwing bombs.
I am a bit of an alchemy buff so I want to add to this.
There are non-Western types of alchemy. Alchemy did not originate only in Renaissance Europe. There was European alchemy, but there was also Islamic alchemy, Chinese alchemy, Indian alchemy, and Greco-Roman alchemy.
That being said, most types of alchemy had one or both of these goals:
To transform something that wasn’t gold (lead, mercury, etc.) into gold.
To find a means of becoming immortal.
Related to this, Chinese alchemy, for example, was very much medically-based. They actually invented gunpowder while attempting to make a potion for eternal life.
As clevergirlhelps said, you don’t necessarily need to have a combat-based magic system for it to be a real magic system. That being said, if you do want it to be combat based, there are a few directions you can go in. Obviously just using it for combat medicine is one possible choice. Another choice is to go further than just turning lead/mercury/etc. into gold and have a more general transmutation idea (like in FMA).
Because alchemy was never just one thing, you can take the basic idea of alchemy (in whatever form you want) and change it to fit your needs. Depending on how far you change it, you might not necessarily want to call it alchemy, but you can. You can go in the chemistry direction, or the transmutation direction. You can go in the direction of creating potions to do the work for the fighters (healing potions, strength potions, etc.) You can look at different types of alchemic symbols. You could have it be based in the planets, because that was a big thing especially in Western alchemy.
My suggestion would be to wander around the Wikipedia pages on alchemy for a while and see what catches your fancy.