2.14    describe experiments to demonstrate that a more reactive halogen will displace a less reactive halogen from a solution of one of its salts

If a halogen is added to a salt that contains a less reactive halogen, displacement will occur. e.g.

Cl₂(aq) + 2KI(aq) ® 2KCl(aq) + I₂(aq)

The original salt solution (potassium iodide here) is colourless, but the halogen elements are coloured.  We see the colourless solution turn to a brown colour here, due to the iodine formed.

If potassium bromide solution had been used, the colourless salt solution would turn to a yellow or orange colour, due to the bromine that was formed.

Note     I2 + NaCl → no reaction, because iodine is LESS reactive than chlorine and can’t displace it.