why some chemical reactions change color back and forth

The Mechanism

In 1950–51, Soviet chemist and biophysicist Boris Pavlovich Belousov (1893–1970) was trying to build a laboratory analogue of the Krebs cycle. Mixing citric acid, bromate and cerium ions in water, he saw the solution oscillate in color — periodic changes that continued for nearly an hour in a closed, unstirred vessel. He submitted a short paper to the Soviet *Journal of General Chemistry* in 1951. It was rejected almost immediately: the editor's objection was that a closed chemical system oscillating instead of settling smoothly into equilibrium contradicted the Second Law of Thermodynamics. Chemists at the time held that homogeneous closed-system oscillation was simply impossible. Belousov revised, resubmitted, and was rejected by every chemical journal he tried; he eventually placed a report of the discovery in 1959, in an obscure conference booklet, and then largely withdrew from the field. The objection was wrong — not because the Second Law fails, but because it constrains the *overall* direction of a reaction, not the path: a system far from equilibrium can oscillate on the way down, as long as total entropy still increases. Anatol Zhabotinsky took the work up and published a mechanistic analysis in 1964; the system is now called the Belousov–Zhabotinsky reaction and is a textbook case of non-equilibrium thermodynamics and self-organization, producing expanding target rings and rotating spiral waves in a petri dish. Belousov died in 1970. He was awarded the Lenin Prize in 1980 — ten years after his death.