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EQUILIBRIUM 169 this dynamic equilibrium stage that there is characteristic features. We observe that the no change in the concentrations of various mass of ice and water do not change with species in the reaction mixture. Based on the time and the temperature remains constant. extent to which the reactions proceed to reach However, the equilibrium is not static. the state of chemical equilibrium, these may The intense activity can be noticed at the boundary between ice and water. Molecules be classified in three groups. (i) The reactions that proceed nearly from the liquid water collide against ice and to completion and only negligible adhere to it and some molecules of ice escape concentrations of the reactants are into liquid phase. There is no change of mass of ice and water, as the rates of transfer of left. In some cases, it may not be even possible to detect these experimentally. molecules from ice into water and of reverse (ii) The reactions in which only small transfer from water into ice are equal at atmospheric pressure and 273 K. amounts of products are formed and It is obvious that ice and water are in most of the reactants remain unchanged at equilibrium stage. equilibrium only at particular temperature (iii) Thereactionsinwhichtheconcentrations and pressure. For any pure substance at atmospheric pressure, the temperature at of the reactants and products are which the solid and liquid phases are at comparable, when the system is in equilibrium. equilibrium is called the normal melting point or normal freezing point of the substance. The The extent of a reaction in equilibrium system here is in dynamic equilibrium and we varies with the experimental conditions such can infer the following: as concentrations of reactants, temperature, (i) Both the opposing processes occur etc. Optimisation of the operational conditions simultaneously. is very important in industry and laboratory (ii) Both the processes occur at the same so that equilibrium is favorable in the rate so that the amount of ice and water direction of the desired product. Some remains constant. important aspects of equilibrium involving physical and chemical processes are dealt in 6.1.2 L iquid-Vapour Equilibrium this unit along with the equilibrium involving This equilibrium can be better understood if ions in aqueous solutions which is called as we consider the example of a transparent box ionic equilibrium. carrying a U-tube with mercury (manometer). 6.1 EQUIL IBRI UM IN PHYSICAL Drying agent like anhydrous calcium chloride PROCESSES (or phosphorus penta-oxide) is placed for a few hours in the box. After removing the The characteristics of system at equilibrium drying agent by tilting the box on one side, a are better understood if we examine some physicalprocesses.Themostfamiliarexamples watch glass (or petri dish) containing water are phase transformation processes, e.g., is quickly placed inside the box. It will be observed that the mercury level in the right solid liquid limb of the manometer slowly increases and liquid gas finally attains a constant value, that is, the solid gas pressure inside the box increases and reaches 6.1.1 Solid-Liquid Equilibrium a constant value. Also the volume of water in the watch glass decreases (Fig. 6.1). Initially Ice and water kept in a perfectly insulated there was no water vapour (or very less) inside thermos flask (no exchange of heat between its the box. As water evaporated the pressure in contents and the surroundings) at 273K and the atmospheric pressure are in equilibrium the box increased due to addition of water molecules into the gaseous phase inside state and the system shows interesting the box. The rate of evaporation is constant. Reprint 2026-27