When matter changes from one physical state to another, such as from liquid to solid, energy is either absorbed or released. These transformations, known as phase changes, are fundamental to many natural phenomena. The specific nature of this energy transfer defines the process.
Understanding Energy Changes: Exothermic vs. Endothermic
Exothermic processes release energy into their surroundings, typically as heat. The term “exothermic” means heat exits the system. Common examples include burning wood or natural gas, which generate warmth and light, and the rusting of iron.
Conversely, endothermic processes absorb energy from their surroundings, usually as heat. This absorption causes the surroundings to cool. Melting ice, which draws heat from its environment to transform into liquid water, and the evaporation of water, which cools the skin, are classic examples.
Freezing: An Exothermic Process
Freezing is an exothermic process, meaning a substance releases heat as it transitions from liquid to solid. At a molecular level, liquid molecules possess more kinetic energy and are less rigidly arranged than in a solid. As a liquid cools, its molecules lose kinetic energy and slow down.
During the freezing process, these liquid molecules begin to arrange themselves into a more ordered and stable crystalline structure. The formation of these more stable bonds and the transition to a lower energy, more organized state result in the release of energy to the surroundings.
This released energy is known as the latent heat of fusion (or solidification). It represents the specific amount of energy that must be removed for a substance to solidify at a constant temperature. The substance’s temperature remains constant at its freezing point until all liquid has solidified, with latent heat given off to the environment.
Everyday Examples of Freezing
The exothermic nature of freezing is evident in many common situations.
When water is placed in a freezer to make ice cubes, the water releases heat to the colder air inside the appliance. Freezers operate by actively removing this heat from their interior, transferring it outside the unit, which allows the water to solidify into ice.
Another familiar example is the formation of frost on cold surfaces. Frost occurs when water vapor directly transforms into ice crystals, a process called deposition, which also releases latent heat. This release of energy contributes to the growth of the ice layer.
Reusable heat packs provide a practical demonstration of freezing as an exothermic process. Many of these packs contain a supersaturated solution, often sodium acetate, which can be triggered to crystallize. As the dissolved substance rapidly solidifies, it releases a significant amount of latent heat, warming the pack.