The concept of a reversible reaction was introduced by Claude Louis Berthollet in 1803, after he had observed the formation of sodium carbonate crystals at the edge of a salt lake (one of the natron lakes in Egypt, in limestone):
:2NaCl + CaCO → NaCO + CaCl
He recognized this as the reverse of the familiar reaction
: NaCO + CaCl→ 2NaCl + CaCO
Until then, chemical reactions were thought to always proceed in one direction. Berthollet reasoned that the excess of salt in the lake helped push the "reverse" reaction towards the formation of sodium carbonate.
In 1864, Peter Waage and Cato Maximilian Guldberg formulated their law of mass action which quantified Berthollets observation. Between 1884 and 1888, Le Chatelier and Braun formulated Le Chateliers principle, which extended the same idea to a more general statement on the effects of factors other than concentration on the position of the equilibrium.