Fun one from today: student was measuring copper content in a penny by dissolving it in conc nitric acid and doing science titrations on it. I'm left with remainder of the copper/acid to dispose of (fair enough, my job and all). So I neutralise it with bicarb, filter precipitate off for the bin, filtrate can go down the sink.
But what are these tiny blue/green crystals forming overnight in the (water based) filtrate?
(Awaits obligatory AS reference ;)