When matter changes physical states, such as during freezing or melting, a fundamental transfer of energy is always involved. This change in state, known as a phase transition, requires energy to be either added to or removed from the system. All substances exist in different states depending on the thermal energy they contain. The nature of this energy transfer determines whether the process is exothermic or endothermic.
The Definition of Exothermic Energy Transfer
The process of liquid water turning into solid ice is classified as an exothermic change. Exothermic processes release thermal energy, or heat, into the immediate environment. Although freezing is a cooling process, the classification refers to the energy leaving the water itself. Water molecules transition from a higher-energy liquid state to a lower-energy solid state. They must expel this excess energy into the surroundings for the phase change to occur, making the freezing process exothermic.
The cooling of the water is the necessary condition to bring the liquid to the freezing temperature. Once at this temperature, the physical act of the liquid becoming solid expels the stored thermal energy. This principle applies to other transitions where a substance becomes more ordered, such as when water vapor condenses into liquid droplets.
Energy Release at the Molecular Level
The energy release occurs because water molecules (H2O) transition from random, high-speed movement to a highly ordered, fixed structure. In liquid water, molecules have enough kinetic energy to constantly break and reform temporary hydrogen bonds, allowing them to slide past one another. As the liquid cools, the molecules slow down, allowing attractive forces to dominate. They then lock into a precise, crystalline lattice structure.
This rigid arrangement of ice is a lower-energy configuration than the dynamic liquid state. When molecules settle into this stable pattern, the excess energy held in the liquid phase must be shed into the environment. This specific amount of heat released during the liquid-to-solid transition is called the latent heat of fusion. For water, this value is approximately 334 Joules of energy released for every gram that freezes. This continuous expulsion of energy keeps the temperature constant at 0°C until all the water has solidified.
The Opposite Process: When Water Absorbs Energy
The opposite process of freezing, known as melting, is classified as an endothermic change. An endothermic process is defined by the absorption of thermal energy from the environment. To change solid ice back into liquid water, energy must be continuously supplied.
This energy input is required to break the strong, fixed hydrogen bonds holding the molecules in the rigid ice lattice structure. As heat is absorbed, it is used solely to overcome these intermolecular forces, not to raise the ice’s temperature. The molecules gain enough kinetic energy to move freely, returning the water to its liquid state. Only after all the ice has converted will continued heat application begin to raise the liquid’s temperature.