Puzzle: do you know chemistry?


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?

Comments (10)

  1. X.Static says:

    Shake the test tubes? A bit rusty on my chemistry though…

  2. CJ says:

    Read the labels? 🙂

    My chemistry is very rusty.

  3. Anil says:

    Taste them ?

    potassium aluminum sulfate should be salty

    NaOH should be…caustic soda-ish ?

  4. hDrummer says:

    drops of water is the solvation 🙂

  5. Jeff Parker says:

    you could put them on ice as well.

    Causetic soda will quickly melt the ice and dry eveything up.

    However not sure how the aluminum sulfate will react, it might not be very pleasant.

  6. AdiOltean says:

    Hint: if you mix the two solutions then some chemical reaction will happen…

  7. me says:

    Well… if you put a little bit of one in the other you get a precipitations if you are putting a little bit of the NaOH solution in the KAlSO4 solution (it would be Al(OH)2) but no precipitation if you are putting only a little bit of KAlSO4 in the NaOH solution becouse then you make Al(OH)6 wich is soluble in water…

    I think

  8. AdiOltean says:

    >> Well… if you put a little bit of one in the other you get a precipitations if you are putting a little bit of the NaOH solution in the KAlSO4 solution (it would be Al(OH)2) but no precipitation if you are putting only a little bit of KAlSO4 in the NaOH solution becouse then you make Al(OH)6 wich is soluble in water… I think

    Almost correct. The formulas are a little bit incorrect but you are definitely on the right track.

    Al(OH)3 (which orms by combining a soluble salt of aluminum with a strong base such as NaOH) is visible as a white gluish gelly which will get dissolved in an excess of NaOH.

    So, if you just put a little bit from one test tube on another, you will see a little bit of Al(OH)3 forming. Now, if you shake the tube, the precipitate will dissapear if this is the NaOH tube. The reason is that, in an excess of a strong basis, Al(OH)3 combines with the [OH]- to its soluble form [Al(OH)4]- .