When testing for cations, the common reagents used are aqueous NaOH and aqueous NH3.
It is important to be careful when using these reagents, because different products are got / observe when we put these reagents in drops from when we apply these reagents in excess.
For example when NaOH reacts with zinc ions it forms insoluble zinc hydroxide which is usually observed as a white gelatinous precipitate and on further reaction with excess NaOH a soluble complex compound of zinc is formed which causes the white gelatinous precipitate to dissolve
The following video shows the action of NaOH and NH3 solutions on Al3+, Zn2+, and Pb2+ ions
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