This is an interesting variation of an oscillating chemical reaction.
The reaction is carried out in a petri dish. Initially the solution is red in colour. After a while, tiny blue spots will start to appear. Each spot will slowly expand, eventually producing a series of concentric rings. The reaction may continue for half an hour, or even longer. The system can oscillate both in time and in space, generating a pattern of ever widening concentric circles. The reaction is extremely sensitive to chloride ions, so all chloride must be excluded.