Why can a substance keep the same temperature while melting?

Adding energy does not always raise a substance’s temperature. At a phase transition, a pure substance can absorb energy while its temperature remains constant, provided pressure is fixed and the phases remain in equilibrium. The energy changes the arrangement and interactions of its particles as one phase becomes another. It does not break water molecules into different chemical substances.

The illustration begins with 1 kg of pure ice already at its melting temperature, 0°C at a pressure of 1 atmosphere. The latent heat of fusion is the energy per unit mass needed for melting at that condition. Using 334 kilojoules per kilogram, fully melting this kilogram requires 334 kJ. This is the relation Q = mL: transferred energy equals mass times latent heat.

With 167 kJ added and no energy loss, half the ice has melted. Solid ice and liquid water coexist at 0°C; the added energy changes their proportions rather than raising the equilibrium temperature. The flat section of the energy–temperature graph is the melting plateau. Its width represents an energy requirement, not a fixed amount of elapsed time.

Once all the ice has melted, further energy can warm the liquid. Using an approximate specific heat capacity of 4.2 kJ per kilogram per degree Celsius, another 84 kJ raises this kilogram of water by 20°C. Pressure, purity, energy losses, and whether the sample has reached equilibrium affect real observations. The plateau is therefore a statement about defined conditions, not a promise that every heated material remains at one exact temperature.