When substances change state, energy is either absorbed or released. These transformations, known as phase changes, involve energy transfers. Understanding these dynamics helps explain many physical phenomena.
The Nature of Freezing
Water freezing is an exothermic process. This means that as liquid water transforms into solid ice, it releases energy into its surroundings. The prefix “exo-” signifies “outside,” indicating that heat is emitted from the system to the environment. This release of energy is a defining characteristic of exothermic processes, distinguishing them from endothermic processes which absorb energy.
Energy Transfer During Freezing
The exothermic nature of freezing stems from the molecular changes occurring during the phase transition. In liquid water, molecules possess kinetic energy, allowing them to move freely and randomly. As water cools, its molecules lose kinetic energy and slow down. This reduction in kinetic energy is observed as a decrease in temperature.
When water reaches its freezing point, 0°C (32°F) at standard atmospheric pressure, molecules begin to arrange into a more ordered, crystalline structure—ice. This arrangement involves the formation of new hydrogen bonds between water molecules. The formation of chemical bonds, including hydrogen bonds, releases energy. This released energy is known as the latent heat of fusion.
For water, the latent heat of fusion is 334 joules per gram (or about 80 calories per gram). This means that for every gram of water that freezes, 334 joules of energy are liberated to the surroundings. The process of its molecules locking into a solid structure generates and releases heat. This heat transfer occurs without a change in the water’s temperature until all the liquid has solidified.
Common Misconceptions About Freezing
A common misconception is that freezing must be an endothermic process because the surroundings feel colder. People often associate “cold” with the absorption of heat. However, the sensation of cold when touching ice is due to heat transferring from your warmer hand to the colder ice. The ice is absorbing heat from your hand to melt, which is an endothermic process.
Conversely, when water freezes, water molecules release energy as they transition to a lower energy state within the solid structure. This energy is released into the surrounding environment. The “cold” feeling often associated with freezing is the result of the environment losing heat to the cooling system that facilitates the freezing, not the freezing process itself absorbing heat.
Observing Freezing in Everyday Life
The exothermic nature of water freezing has practical applications and can be observed in various everyday phenomena. For instance, fruit growers often spray their crops with water when temperatures are expected to drop below freezing. As this water freezes on the plants, it releases latent heat of fusion. This released heat helps to keep the plant tissues at or around 0°C, protecting them from frost damage, even if the ambient air temperature drops lower.
Similarly, the formation of ice on lakes illustrates this principle. As the surface water of a lake cools and begins to freeze, it releases heat into the atmosphere. This release of energy slows down the cooling of the water beneath the forming ice layer. The insulating properties of the newly formed ice layer also play a role in regulating the temperature of the water below, allowing aquatic life to survive the winter.