Why are Zn and PB compounds not distinguishable upon addition of NaOH solution? (1 mark).

Certain metals like Zinc, Aluminium and Lead react with only hot caustic NaOH/KOH solution to give the corresponding soluble salt with the liberation of hydrogen gas.

Metal         +     Alkali                    →        Salt                         +            Hydrogen

1.Zinc          Zn         +   2NaOH                    →         Na2ZnO2                  +            H2                                                                                          Sodium zincate                                                                    (Colourless)

2.Aluminium   Al            + 2NaOH   + 2H2O      →         2NaAlO2                  +            H2                                                             Sodium meta aluminate                                                                  (Colourless)

3. Lead Pb         +   2NaOH                    →         Na2PbO2                  +            H2                                                                                     Sodium plumbate                                                                (Colourless)

 

So upon additon of excess NaOH/KOH these compounds are not distinguishable.