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EQUILIBRIUM 171 6.1.4 Equilibrium Involving Dissolution pressure of the gas above the solvent. of Solid or Gases in Liquids This amount decreases with increase of Solids in liquids temperature. The soda water bottle is sealed under pressure of gas when its solubility in We know from our experience that we can water is high. As soon as the bottle is opened, dissolve only a limited amount of salt or some of the dissolved carbon dioxide gas sugar in a given amount of water at room temperature. If we make a thick sugar syrup escapes to reach a new equilibrium condition required for the lower pressure, namely its solution by dissolving sugar at a higher partial pressure in the atmosphere. This is temperature, sugar crystals separate out if we coolthesyruptotheroomtemperature.Wecall how the soda water in bottle when left open it a saturated solution when no more of solute to the air for some time, turns ‘flat’. It can be generalised that: can be dissolved in it at a given temperature. The concentration of the solute in a saturated (i) For solid liquid equilibrium, there is solution depends upon the temperature. In only one temperature (melting point) at 1 atm (1.013 bar) at which the two a saturated solution, a dynamic equilibrium phases can coexist. If there is no exits between the solute molecules in the solid state and in the solution: exchange of heat with the surroundings, the mass of the two phases remains Sugar (solution) Sugar (solid), and constant. the rate of dissolution of sugar = rate of crystallisation of sugar. (ii) For liquid vapour equilibrium, the vapour pressure is constant at a given Equality of the two rates and dynamic temperature. nature of equilibrium has been confirmed with the help of radioactive sugar. If we drop some (iii)For dissolution of solids in liquids, the solubility is constant at a given radioactive sugar into saturated solution of non-radioactive sugar, then after some time temperature. radioactivity is observed both in the solution (iv) For dissolution of gases in liquids, and in the solid sugar. Initially there were no the concentration of a gas in liquid radioactive sugar molecules in the solution is proportional to the pressure but due to dynamic nature of equilibrium, (concentration) of the gas over the liquid. there is exchange between the radioactive These observations are summarised in and non-radioactive sugar molecules between Table 6.1 the two phases. The ratio of the radioactive Table 6.1 SomeFeaturesofPhysicalEquilibria to non-radioactive molecules in the solution increases till it attains a constant value. Process Conclusion Liquid Vapour pH2Oconstant at given Gases in liquids H O (l) H O (g) temperature When a soda water bottle is opened, some of 2 2 the carbon dioxide gas dissolved in it fizzes Solid Liquid Melting point is fixed at H O (s) H O (l) constant pressure out rapidly. The phenomenon arises due 2 2 to difference in solubility of carbon dioxide Solute(s) Solute Concentration of solute at different pressures. There is equilibrium (solution) in solution is constant between the molecules in the gaseous state Sugar(s) Sugar at a given temperature and the molecules dissolved in the liquid (solution) under pressure i.e., Gas(g) Gas (aq) [gas(aq)]/[gas(g)] is CO (gas) CO (in solution) 2 2 constant at a given This equilibrium is governed by Henry’s temperature law, which states that the mass of a gas CO 2g) CO 2aq) [CO2(aq)]/[CO2(g)] is constant at a given dissolved in a given mass of a solvent at any temperature is proportional to the temperature Reprint 2026-27