1968

BZ Reaction

Boris Belousov (1893–1970), Alan Mathison Turing (1912–1954), Anatol Zhabotinsky (1938–2008)

Most chemical reactions have a time course that’s fairly easy to follow. In general, the starting material gradually gets consumed, the products gradually accumulate, and the reactions can be sped up or slowed down by changing the conditions—especially temperature. Not so with the BZ (Belousov-Zhabotinsky) reaction, the most famous of the class known as oscillating reactions, which swing out further than usual from chemical equilibrium and then swing back the other way, overshooting back and forth until they eventually run down. If you run the BZ reaction in a standard flask with stirring, the solution’s color keeps changing periodically. If it is run in a shallow dish with no stirring, expanding waves and spirals of color move through the solution in patterns reminiscent of mineral cross-sections, bacterial colonies, and animal camouflage.

Soviet chemist Boris Belousov discovered the basic mixtures during the 1950s but had trouble publishing his results because they seemed hard to believe and were difficult to explain. Soviet chemist Anatol Zhabotinsky, a graduate student, rediscovered the reaction in 1961, but it was still unknown outside a few groups of Russian chemists until he published a short description of it at a 1968 conference. We know the basic ideas behind the reaction—bromine goes up and down through various oxidation states and species, with reactions running in either direction depending on local concentrations. There’s always another reactant that gets gradually consumed, since the reaction certainly doesn’t violate the laws of thermodynamics—it just takes a bumpier ride downhill than most. The detailed mechanism involves at least eighteen steps and twenty-one separate chemical species, so getting a complete picture of the reaction process is no small matter.

The resemblance to the patterns found in living creatures is no coincidence. In 1952, the brilliant British mathematician and computer scientist Alan Turing, well known for his instrumental role in breaking the Germans’ Enigma code during World War II, worked out a theory of pattern formations in biology that also predicted the existence of oscillating reactions like the Belousov-Zhabotinsky. He would surely have been happy to see a real one in action.

The classic BZ reaction in a shallow dish. The bright yellow bands move slowly through the solution, driven by a complex series of reaction steps.