Lesson 1 of 12 · 9 min

Equilibrium in physical processes

NCERT § "Equilibrium in Physical Processes"

A sealed bottle of soda can sit on a dhaba counter for a week and stay fizzy. Open it and it hisses within seconds. Nothing was added, so what changed?

The story this chapter follows: Soda bottle, ammonia flask, vinegar

A sealed bottle of soda on a dhaba counter, a 1 L steel flask where nitrogen and hydrogen make ammonia at 500 K, and a 0.1 M solution of acetic acid, the acid in vinegar. The flask is a teaching model whose numbers (1.0 mol N₂ and 3.0 mol H₂ settling to 0.50, 1.50 and 1.00 mol) run through Kc, Kp, Q, ΔG and Le Chatelier; the acetic acid carries every ionic lesson.

The lesson in notes

In short

At equilibrium the forward and reverse processes continue at equal rates, so measurable properties stay constant; the equilibrium is dynamic, not static.

Solid-liquid equilibrium: ice and water coexist at 273 K at atmospheric pressure, and the rate of melting equals the rate of freezing; the normal melting point is the temperature at which this happens at 1 atm.

Liquid-vapour equilibrium: in a closed vessel the vapour pressure becomes constant when evaporation and condensation rates match; the normal boiling point is the temperature at which vapour pressure equals 1.013 bar (1 atm).

Solid-vapour equilibrium is seen with substances that sublime, such as iodine, camphor and ammonium chloride.

A saturated solution is in equilibrium with undissolved solute: dissolution and crystallisation occur at the same rate.

Henry's law links gas solubility to pressure: at fixed temperature, the amount of a gas that dissolves in a fixed amount of solvent rises in step with the gas pressure over it, and it falls as temperature rises. Opening a soda bottle drops the CO₂ pressure, so dissolved CO₂ escapes until a new equilibrium is reached.

What all these cases share: a closed system held at one temperature, two opposite processes running at matching rates, and observable properties such as pressure or concentration that stop changing.