Many chemical reactions are known in which the concentrations of two different species increase and decrease alternately in a periodic manner before reaching equilibrium. These reactions are called "oscillating reactions". The Belousov- Zhabotinsky reaction and the Briggs-Rauscher reaction are examples. A dramatic visual effect can be created by making the changes in concentrations visible using indicators.When the reaction mixture is continuously stirred, the concentrations and therefore the color of the solution changes periodically with time. This is called "oscillations in time". But if the reaction mixture is left undisturbed, different regions within the solution may acquire different concentrations of the species, giving rise to beautiful ripple-like patterns. This is called "oscillations in space".
Watch this interesting video showing the Belousov- Zhabotinsky reaction in a stirred solution. The actual oscillations usually are very slow with a period of about two to four minutes, but the video is made to run a bit faster, so you don't get bored watching the reaction.
Watch the same Belousov- Zhabotinsky reaction taking place in a petri dish undisturbed, giving rise to colorful visual patterns. This video runs at eight times the normal speed of the reaction, so that you do not become bored watching it.