Here is a very nice puzzle - But unfortunately you have to ressurect a little chemistry knowledge to fully appreciate it...

You have two identical test tubes. Both tubes look identical - they both contain colourless, odoreless, aqueous solutions. But someone told you that one of them contains a solution of sodium hydroxide (caustic soda), and the other one a solution of potassium aluminum sulfate (potash alum).

How do you quickly tell them apart?