And now they will try out some magical chemistry! Where the goal is to see whether you can make Prestidigitation do anything interesting to chemistry, once you know what the chemistry is doing.
In particular, they're going to try making vinegar more and less acidic, wood-ash-in-water more and less alkaline, and they're going to see whether things can be turned into salts that shouldn't be salts and then stop being salts when the Prestidigation wears off an hour later! Or put some actual salt in water, and see if they can change the amount of chlorine that evaporates away, such that the amount of salt recovered via evaporation changes! Or see if you can stop salt crystals from dissolving in water by Prestidigitating them to taste like more stable things than salt!
...and if turns out that, for some of those things, Keltham can do that with Prestidigitation and others can't, he's going to teach them about chemistry layer-by-layer in hopes of figuring out which layer of knowledge is necessary.
By the way, to be clear, this general kind of operation is not guaranteed to revolutionize chemistry within three days. Up to a month can be required! Sometimes it even takes longer than one month! Science Maniac Verrez always says that to everyone, and usually does do it within three days, in dath ilani fiction. Though in one famous reader-trolling incident the next scene in the book took place thirty years later and involved Verrez's grandchildren. Actually Verrez's wife who is constantly trying to prevent him from getting himself killed talks a lot like Ione now that Keltham is thinking about it.