Le Chatelier's principle (pronounced or ), also called Chatelier's principle (or the Equilibrium Law), is a principle of chemistry used to predict the effect of a change in conditions on chemical equilibria. The principle is named after French chemist Henry Louis Le Chatelier, and sometimes also credited to Karl Ferdinand Braun, who discovered it independently. It can be stated as: Phenomena in apparent contradiction to Le Chatelier's principle can also arise in systems of simultaneous equilibrium (see response reactions). Le Chatelier's principle is sometimes alluded to in discussions of topics other than thermodynamics. Thermodynamic statement Introduction The following thermodynamic statement is abstract, in general terms, but, for introduction, it may help the reader to bear in mind a simple example: A body of gas in a cylinder with a piston has its pressure externally controlled through pressure on the piston. The body of gas starts in a state of internal thermodynamic equilibrium by setting its own volume. The externally controlled pressure is then 'perturbed' by a controlled finite amount, say and the body of gas sets its own volume again through a change in volume If the piston and cylinder are insulated so that the gas cannot gain or lose energy as heat, then the Le Chatelier–Braun principle has nothing to say, because, though the temperature of the gas may change, there is available no possibility of 'moderation' or 'feedback'. But if the piston and cylinder walls conduct heat between the body of gas and a closed external 'heat reservoir' of externally controlled temperature and internal energy, then 'moderation' or 'feedback' can occur, or be investigated, through temperature change in, or heat transfer to or from, the body of gas; and the Le Chatelier–Braun principle tells about it. General scenario The Le Chatelier–Braun principle analyzes the qualitative behaviour of a thermodynamic system when a designated one of its externally controlled state variables, say changes by an amount the 'driving change', causing a change the 'response of prime interest', in its conjugate state variable all other externally controlled state variables remaining constant. The response illustrates 'negative feedback' and so is 'moderated' in ways evident in two related thermodynamic equilibria. Obviously, one of has to be intensive, the other extensive. Also as a necessary part of the scenario, there is a designated auxiliary 'moderating' or 'feedback' state variable , with its conjugate state variable For this to be of interest, the 'moderating'