General Characteristics of Equilibria Involving Physical Processes

Equilibrium in Physical Processes: A Concept-First View

Before diving into the list of characteristics, it helps to understand what a physical process at equilibrium actually looks like. Think of a sealed bottle of water left on a table. Water molecules at the surface are constantly escaping into the air above (evaporation), while water vapour molecules in the air are constantly crashing back into the liquid (condensation). At equilibrium, these two opposing processes happen at exactly the same rate. The bottle is closed — no water vapour can escape, and no new water can enter. The temperature is fixed. Under these conditions, the amount of liquid water and the amount of water vapour in the bottle stop changing. The system has reached a dynamic balance.

This is the core idea behind every equilibrium involving a physical change. The five characteristics listed below are the general truths that hold for all such equilibria — whether you are melting ice, dissolving sugar, or evaporating bromine.

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Characteristic (i): Equilibrium Requires a Closed System at a Fixed Temperature

Equilibrium is possible only in a closed system at a given temperature.

A closed system is one that can exchange energy (heat) with its surroundings but cannot exchange matter. If the system were open — say, an uncovered glass of water — water vapour would continuously escape into the room. The forward process (evaporation) would keep happening, but the reverse process (condensation) would never catch up because the vapour molecules are lost. The system would never reach a stable state; it would simply keep losing water until the glass is dry.

Temperature must also be fixed. The rates of the forward and reverse processes depend on temperature. If the temperature changes, the balance shifts, and the system moves to a new equilibrium position. A constant temperature ensures that the rates remain equal over time.

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Characteristic (ii): Dynamic but Stable Condition — Equal Rates of Opposing Processes

Both the opposing processes occur at the same rate, and there is a dynamic but stable condition.

This is the most fundamental characteristic. At equilibrium, the forward and reverse processes do not stop. They continue to occur, but at identical rates. This is why we call it dynamic equilibrium — the system is active at the microscopic level, even though it appears static at the macroscopic level.

Consider the example of ice melting in a closed container at $0\,^{\circ}\mathrm{C}$. Ice molecules break free from the solid lattice and enter the liquid phase (melting). Simultaneously, liquid water molecules strike the ice surface and rejoin the solid lattice (freezing). At equilibrium, the number of molecules leaving the ice per second equals the number of molecules joining the ice per second. The mass of ice and the mass of liquid water remain constant, but individual molecules are constantly switching phases.

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Characteristic (iii): All Measurable Properties of the System Remain Constant

All measurable properties of the system remain constant.

Because the forward and reverse rates are equal, there is no net change in any macroscopic property. These properties include:

• Pressure (for a gas-liquid or gas-solid equilibrium)
• Temperature (must be constant for equilibrium to exist)
• Concentration (of dissolved substances in a solution)
• Density (of the phases present)
• Colour (if any of the substances are coloured)
• Mass (of each phase)

The constancy of these properties is the experimental evidence that equilibrium has been reached. If you measure the vapour pressure above a liquid in a closed container and it stops changing, you know the system is at equilibrium.

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Characteristic (iv): Equilibrium Is Characterised by a Constant Value of a Parameter at a Given Temperature

When equilibrium is attained for a physical process, it is characterised by a constant value of one of its parameters at a given temperature.

For each type of physical equilibrium, there is a specific parameter that becomes constant at a fixed temperature. This parameter is a quantitative measure of the extent of the process. The textbook provides Table 6.1, which lists these parameters for the common physical processes.

Table 6.1: Constant Parameters at Equilibrium for Physical Processes

For example, at $25\,^{\circ}\mathrm{C}$, the equilibrium vapour pressure of water is about $23.8\ \mathrm{mm\ Hg}$. This value is fixed — it does not depend on how much liquid water or vapour is present, as long as both phases coexist and the temperature is constant. Similarly, the solubility of sodium chloride in water at $25\,^{\circ}\mathrm{C}$ is $36.0\ \mathrm{g}$ per $100\ \mathrm{mL}$ of water. This is a constant at that temperature.

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Characteristic (v): The Magnitude of the Constant Parameter Indicates the Extent of the Process

The magnitude of such quantities at any stage indicates the extent to which the physical process has proceeded before reaching equilibrium.

This characteristic connects the constant parameter to the position of equilibrium. A larger value of the constant parameter means the forward process has proceeded to a greater extent before equilibrium is reached. …