Testing for cations Lesson 11.5

How can we identify a substance in solution? Aqueous sodium hydroxide or aqueous ammonia can be used to identify positive ions in compounds. Positive ions are often called cations because they move to the cathode when a solution is electrolysed. A solid that we want to identify is the best to dissolve in a little water first and use this aqueous solution for the test. The procedure for identifying cation is: Put a small amount of solution into test tube. Add a few drops of aqueous sodium hydroxide. Observe the colour of any precipitate. Add excess aqueous sodium hydroxide and shake the test tube. Record whether precipitate dissolves or not and any colour changes.

The table shows the results is particular cations are present: This procedure can be repeated using aqueous ammonia instead of sodium hydroxide. The table shows the results is particular cations are present: metal cation result with aqueous sodium hydroxide result with aqueous ammonia aluminium,Al3+ white precipitate soluble in excess (colourless solution) insoluble in excess calcium,Ca2+ no precipitate or very slight white precipitate copper(II),Cu2+ light blue precipitate soluble in excess (dark blue solution) iron(II),Fe2+ grey-green precipitate iron(III),Fe3+ reddish-brown precipitate zinc,Zn2+

Sodium hydroxide and ammonia react in a similar way with some of ions. We can use this two alkalis to distinguish the colourless solutions containing aluminium and zinc ions. If the alkalis are not in excess, the formed precipitates are metal hydroxides. Equation for copper, for example, will be: Ca2+(aq) + 2OH-(aq) → Cu(OH)2(s) copper ions + hydroxide ions → copper hydroxide The aluminium and zinc ions dissolve in excess sodium hydroxide because they form soluble aluminates and zincates. When a solution containing ammonium ions is heated with sodium hydroxide, ammonia gas is given off (turns red litmus blue): NH4+(aq) + OH-(aq) → NH3(g) + H2O(l)

We can use a flow chart to identify an unknown cation in an order way.