When energy is continuously supplied to a block of ice, the temperature remains fixed at 0 degrees Celsius until the last shard disappears. This observation seems to defy the expectation that adding energy to a substance should always make it warmer. The explanation lies in the fundamental distinction between the two concepts we use to measure a substance’s thermal state and the specific way water molecules are structured in their solid form. Understanding this process requires looking closely at how energy is absorbed and what it is used for at the molecular level.
Temperature Versus Heat
Heat is the transfer of thermal energy between substances due to a temperature difference, and it is measured in units like Joules. Temperature is a physical property that measures the average kinetic energy of the particles within a substance. Kinetic energy is the energy of motion, meaning that temperature indicates how fast the molecules are moving or vibrating. When an object’s temperature rises, the speed of its constituent molecules increases. This connection between added energy and increased molecular speed changes during a phase transition like melting.
Breaking Molecular Bonds
The energy supplied to the ice is repurposed away from increasing molecular speed. When water freezes, the molecules arrange themselves into a rigid, crystalline lattice structure held together by attractive forces known as hydrogen bonds. These bonds must be overcome for the solid structure to break apart and form a liquid. The incoming thermal energy is used to pull the water molecules apart, disrupting the organized structure of the ice. This energy is absorbed and stored as potential energy, not kinetic energy. This explains why the average kinetic energy—the temperature—does not change while the energy is diverted into structural reorganization.
The Coexistence of Ice and Water
The temperature plateau persists because the system reaches a state of equilibrium at the melting point. As long as ice remains mixed with water, the system is actively undergoing a phase change from solid to liquid. At 0 degrees Celsius, the ice and the liquid water coexist, and all incoming heat is prioritized for melting the remaining solid. On a molecular scale, energy input is immediately absorbed to break hydrogen bonds and transition molecules from the solid lattice into the liquid state. This continuous process ensures the energy converts ice into water, rather than increasing the kinetic energy of the molecules.
The temperature of the mixture is held constant until the transformation is 100% complete. A significant amount of energy is required for this conversion, known as the heat of fusion, which is approximately 334 Joules for every gram of ice melted. This large energy requirement means the temperature of the ice-water mixture is effectively stabilized at 0 degrees Celsius until all the ice is gone.
Resuming Temperature Increase
The temperature only begins to rise once the last piece of ice has converted into liquid water. With the phase transition complete, the liquid water can no longer absorb the incoming heat as potential energy for structural change. Any further thermal energy supplied now directly contributes to increasing the average kinetic energy of the water molecules. Since the energy is no longer breaking bonds, molecular motion increases, and the temperature of the liquid water climbs above 0 degrees Celsius.