isochoric process
/ eye-so-KOR-ik /
An isochoric process is a change that happens at constant volume — the container is rigid and cannot expand or shrink. The root choro- refers to space or room, so isochoric means 'same volume'. Picture gas sealed in a strong steel bottle with fixed walls: you can heat it or cool it, and its pressure and temperature will climb or fall, but its volume is locked. It is also called an isovolumetric or constant-volume process.
If the volume never changes, then ΔV = 0, so the pressure-volume work W = P ΔV is zero: a gas in a rigid box does no work on its surroundings because nothing moves. The first law ΔU = Q - W therefore simplifies to ΔU = Q — every joule of heat you add goes entirely into internal energy, raising the temperature and pressure, with none of it siphoned off as work. This makes constant-volume heating the cleanest way to relate heat directly to a temperature change.
A sealed can heated in a fire (which can eventually burst it) and the combustion inside a bomb calorimeter are isochoric. On a PV diagram an isochoric process is a vertical line, and since its enclosed area is zero it confirms visually that no work is done. Gay-Lussac's law — pressure proportional to absolute temperature at fixed volume — is the ideal-gas rule for this path.
Heat a gas sealed in a rigid steel tank, adding 500 J of heat. Since the walls cannot move, W = 0, so all of it becomes internal energy: ΔU = Q = 500 J. The gas gets hotter and its pressure rises, but it does no work at all.
In a rigid container nothing moves, so all added heat becomes internal energy.
No volume change means no PV work, but it does not mean 'no energy change' — the gas can still gain internal energy and reach a much higher temperature and pressure, which is exactly why sealed containers can explode when heated.