Chemistry · Ch 6 — Equilibrium
Solid-Liquid Equilibrium
Solid-Liquid Equilibrium
The Concept of Dynamic Equilibrium in a Solid-Liquid System
The simplest way to understand equilibrium in a chemical system is to look at a physical change: the coexistence of ice and water. Imagine a perfectly insulated thermos flask — one that allows no heat exchange with the surroundings — containing ice and water at 273 K and normal atmospheric pressure. If you observe this flask over time, you notice something striking: the masses of ice and water do not change, and the temperature stays fixed at 273 K.
This might look like a static, dead system. But it is anything but static. At the boundary between the ice and the water, there is intense molecular activity. Water molecules from the liquid phase constantly collide with the ice surface and freeze onto it. Simultaneously, molecules from the solid ice escape into the liquid phase. The reason the masses remain constant is that these two opposing processes occur at exactly the same rate. The number of molecules leaving the ice per second equals the number of molecules joining the ice per second.
This is the essence of dynamic equilibrium: the system appears unchanged at the macroscopic level because two microscopic processes are happening at equal speeds in opposite directions.
For any pure substance at a given pressure, the solid and liquid phases can coexist in equilibrium only at one specific temperature. At atmospheric pressure (1.013 bar), this temperature is called the normal melting point or normal freezing point of the substance.
Properties of Solid-Liquid Equilibrium
From the ice-water system, we can extract two fundamental properties that apply to all systems in dynamic equilibrium.
Property (i): Both opposing processes occur simultaneously.
In the ice-water system, freezing (liquid → solid) and melting (solid → liquid) happen at the same time. Neither process stops. The system is not frozen in time; it is a busy, two-way street where molecules travel in both directions constantly.
Property (ii): Both processes occur at the same rate.
The rate of melting equals the rate of freezing. Because these rates are equal, the net amount of ice and the net amount of water remain constant. There is no net change in the system's composition, even though individual molecules are continuously exchanging phases.