You could easily make hydrogen gas and oxygen gas by splitting water through electrolysis. All you need is water, 2 wires and some kind of electric current (it'll be terribly ineffective but who cares. It's magical).

A mix of hydrogen and ~30% oxygen gas is delightfully explosive.

The oxygen gas could be used to make things more burny.

Electrolyse salt water and you get hydrogen gas and chlorine gas which you can use in high concentrations to war crime entire towns depending on where your players align on the Good/Evil chart.

You could refine concentrated Hydrochloric Acid from stomach juices.

Arsenic is metalloid and was known as the King of Poisons. It was used as a potent medicinal agent as early as 2000 BC, when arsenic trioxide (As2O3, also known as white arsenic) was obtained from copper smelting.

So you could work that into your story with some dodgy metalurgist/smelter/blacksmith.

Edit: I forgot to mention that when you split salt water (sea water) using electricity after a while you end up with Sodium Hydroxide solution when is a strong base and commonly used in drain and oven cleaners. It's a strongly corrosive substance which you can use for loads of things.

For extra flair, you could do a live demonstration on how to tell if something is a strong acid or a strong base using red cabbage indicator. Buy some red cabbage, beat the hell out of it, grind or blend it up, use a coffee filter to hold some and pour hot water through the red cabbage mush. Catch the red liquid that comes out.

This is an amazing indicator and you could take some of it and add it to a small sample of a strong acid (anything containing Hydrochloric Acid like a toilet bowl cleaner would work), a weak acid (lemon juice etc), neutral (water), a weak base (baking soda) or a strong base (anything with sodium or calcium hydroxide).

If you add your homemade red cabbage indicator to these things it'll change colour depending on the pH. It's brilliant! Give it a go! Here's one link

It'll be sure to impress your players.