Equilibrium in Physical Processes

Physical processes can reach equilibrium too, whenever they are carried out in a closed system. (a) Liquid-vapour equilibrium: when liquid water evaporates inside a sealed flask, water molecules escape the liquid surface into the vapour phase, building up vapour pressure; they also condense back into the liquid. Initially evaporation dominates (little vapour is present), but as vapour builds up, condensation speeds up while evaporation slows, until the two rates become exactly equal -- an equilibrium state, $H_2O(l)\rightleftharpoons H_2O(\text{vapour})$, at which the vapour pressure stays constant. This constant pressure is called the equilibrium (or saturated, or aqueous-tension) vapour pressure at that temperature, and it increases with temperature; water's saturated vapour pressure is 1.013 bar (1 atm) at 100 degC, which is why water boils at 100 degC under 1 atm -- the normal boiling point of any pure liquid is the temperature at which its saturated vapour pressure equals the surrounding (atmospheric) pressure. (b) Solid-liquid equilibrium: a mixture of ice and water in a perfectly insulated (isolated) flask at 273 K stays at a constant temperature indefinitely, with ice and water present together in solid-liquid equilibrium. (c) Solid-vapour equilibrium: solid iodine placed in a closed vessel sublimes to give violet iodine vapour; the vapour's colour intensity increases at first, then stabilises once the rate of sublimation equals the rate of condensation back to solid, $I_2(s)\rightleftharpoons I_2(g)$. Camphor and ammonium chloride show the same kind of solid-vapour equilibrium on sublimation. A related equilibrium occurs in a saturated solution: if excess sugar is dissolved in warm water to make a syrup and the syrup is then cooled, sugar crystals separate out, because at the lower …