When you place a tray of water in the freezer, you initiate a change of state involving a significant energy transfer. Despite the resulting ice being cold to the touch, the change from liquid to solid water is classified as an exothermic process. This means that during freezing, the water actively releases energy into its immediate environment.
Defining Endothermic and Exothermic Processes
Processes involving energy transfer are categorized based on the direction of heat flow relative to the system undergoing the change. An exothermic process releases thermal energy, or heat, from the system into the surroundings. A common example is the burning of wood, which releases heat and light.
Conversely, an endothermic process absorbs thermal energy from the surroundings into the system. A chemical cold pack is a practical example, absorbing heat from the body or surrounding air to achieve a cooling effect. The prefixes “exo-” (outside) and “endo-” (inside) refer to whether heat flows out of or into the defined system.
The Science of Freezing: Is Making Ice Exothermic?
The formation of ice is exothermic because water molecules must shed energy to transition from a liquid’s chaotic motion to a solid’s highly organized structure. In the liquid state, water molecules move quickly and randomly, possessing high kinetic energy. To solidify, these molecules must slow down and lock into the rigid, crystalline hexagonal lattice that defines ice. This transition represents a shift to a more stable, lower-energy state.
The energy removed to achieve this structural change is known as the latent heat of fusion. This heat is released during the phase change at a constant temperature, which for pure water is 0°C (32°F). For every gram of water converting into ice, approximately 334 Joules (or 80 calories) of heat energy must be extracted. This energy relates solely to the rearrangement of molecules into a solid form, not the cooling of the water’s temperature.
The freezer provides a mechanism to continually remove this latent heat that the water is releasing. The refrigeration system pumps the released thermal energy out of the ice tray and into the surrounding environment, which is why the back of a freezer often feels warm. If this heat were not continuously removed, the water would cease to freeze.
Comparing Phase Changes: Melting and Freezing
The exothermic nature of freezing is best understood when contrasted with the opposite phase change: melting. Melting is an endothermic process, requiring an input of energy to occur. The ice must absorb heat from the surroundings to break the strong hydrogen bonds holding the crystalline lattice structure together.
This absorbed heat provides the energy for water molecules to overcome their fixed positions and return to the higher-energy, freely moving liquid state. Melting requires the absorption of the same amount of latent heat of fusion that freezing releases. These opposing processes demonstrate a balanced energy exchange: bond formation releases energy, while bond breakage requires energy input.