Introduction

Changes around us can be physical or chemical, and separately, reversible or irreversible. An irreversible change proceeds only in one direction until it is complete -- a natural waterfall, smoke spreading from an incense stick, and the diffusion of a flower's fragrance are all irreversible physical changes; they never spontaneously reverse. Chemical reactions can be irreversible too: $C(s)+O_2(g)\xrightarrow{\Delta}CO_2(g)$ and $2KClO_3(s)\xrightarrow{\Delta}2KCl(s)+3O_2(g)$ proceed only from reactants to products, shown by a single arrow, and continue until a reactant is used up. A reversible reaction, by contrast, can proceed in both directions -- the forward reaction (reactants forming products) and the reverse or backward reaction (products re-forming reactants) -- shown with a double arrow, e.g. $H_2(g)+I_2(g)\rightleftharpoons 2HI(g)$ and $CH_3COOH(aq)+H_2O(l)\rightleftharpoons CH_3COO^-(aq)+H_3O^+(aq)$. Whether a reaction behaves reversibly can depend on the system it occurs in. Consider heating calcium carbonate: in an OPEN container, $CaCO_3(s)\xrightarrow{\text{heat}}CaO(s)+CO_2(g)$ looks irreversible, because the CO2 gas escapes and cannot recombine with the CaO. But in a CLOSED container, the same decomposition, $CaCO_3(s)\rightleftharpoons CaO(s)+CO_2(g)$, is genuinely reversible: heated for a long time at a fixed temperature, the amounts of CaCO3, CaO and CO2 eventually stop changing -- the system has reached equilibrium. The reaction has not actually stopped; CaCO3 continues decomposing at exactly the rate that CaO and CO2 recombine to re-form it. Such reactions, which never go to completion and occur in both directions simultaneously, are called reversible reactions, and may be written in general as $A+B\rightleftharpoons C+D$ (reactants on the left, products on the right, double arrow showing reversibility). A closed system exchanges heat but not matter with its surroundings; an open system exchanges both; an isolated system exchanges neither -- this distinction matters because true chemical equilibrium is only observed in a closed (or isolated) system.