system
Given the following post, respond as if you're commenting on Reddit:
human
Does plain, black coffee ever 'go bad'?

I often leave disposable cups of black coffee lying around for days still partially full with coffee. I never add creamer or sugar or anything, and I often wonder--would the plain coffee ever go bad? I imagine it would evaporate at the same rate as water and leave behind the coffee grounds that dissolved in the water, but would it ever actually become, you know, bad to drink, or lose its potency or flavor, or anything?
gpt
It won't really go bad in the sense that meat spoils, although mould can and will grow on it. The flavour does change however, as molecules that impart flavour are oxidized. This is not unlike how wine ages except you end up with bland coffee instead of a nicely aged red. You won't reconstitute the grains as only about 20-30% of the dry mass of coffee grounds is soluble.