The transformation of materials between solid, liquid, and gas states represents a fundamental aspect of chemistry and physics, and each change involves a transfer of energy. When considering dry ice, which is the solid form of carbon dioxide (\(\text{CO}_2\)), its unique behavior of bypassing the liquid phase and turning directly into a gas often prompts questions about how energy is exchanged with its surroundings. This process, known as sublimation, is a clear physical change that allows scientists and the public to observe energy principles in action. Understanding whether this process requires an input of energy or releases energy is key to grasping the thermal dynamics at play.
Understanding Energy Transfer: Endothermic vs. Exothermic
Energy transfer in physical and chemical changes is classified into two primary categories based on the direction of heat flow relative to the system. An endothermic process is one where the system absorbs energy, typically in the form of heat, from its environment. This absorption of energy causes the surrounding area to feel cooler because heat is moving “in” to the system.
The opposite action is an exothermic process, where the system releases or gives off energy to the surroundings. This release of heat causes the temperature of the immediate environment to rise, making the surroundings feel noticeably warmer. It is the direction of this heat flow that ultimately determines the thermal sensation experienced by an observer.
The Process of Dry Ice Sublimation
Dry ice is the common name for solid carbon dioxide (\(\text{CO}_2\)). Unlike regular water ice, dry ice does not melt into a liquid when it warms up at standard atmospheric pressure. Instead, it undergoes a phase transition called sublimation, which is the direct conversion from a solid state to a gaseous state.
This bypass of the liquid phase occurs because carbon dioxide has an extremely low triple point, the temperature and pressure at which all three phases—solid, liquid, and gas—can coexist. At the normal pressure found on Earth’s surface, the temperature of dry ice, which is approximately \(-78.5^\circ \text{C}\) (\(-109.3^\circ \text{F}\)), is far below the point required for it to turn into a liquid. Therefore, as soon as the solid absorbs any thermal energy, the molecules gain enough kinetic energy to leap straight into the gas phase.
Why Sublimation is an Endothermic Process
The process of dry ice sublimation is classified as endothermic because it requires a continuous input of energy to occur. For the solid \(\text{CO}_2\) to convert into a gas, energy must be supplied to overcome the weak intermolecular forces holding the molecules rigidly in their solid structure.
The required energy is pulled directly from the dry ice’s immediate surroundings, such as the air, a container, or a surface it is resting on. By absorbing heat from its environment, the dry ice causes a dramatic cooling effect on anything it touches. The energy absorbed, known as the latent heat of sublimation, is used to break the intermolecular bonds, allowing the \(\text{CO}_2\) molecules to escape as a gas. This mechanism of heat absorption from the environment is the defining characteristic that makes dry ice sublimation an endothermic process. The enthalpy of sublimation for dry ice is approximately 571 kilojoules per kilogram, demonstrating the significant energy requirement for the phase change.
Everyday Examples of Energy Transfer
Phase changes and chemical reactions that absorb or release heat are common occurrences in daily life, providing clear demonstrations of energy transfer principles. The evaporation of rubbing alcohol from the skin is an example of an endothermic process, where the liquid absorbs heat from the body to become a gas, resulting in a cooling sensation. Similarly, the melting of ice in a drink absorbs heat from the surrounding liquid, which is why the beverage becomes colder.
In contrast, the burning of a candle or a piece of wood provides a familiar example of an exothermic process. Combustion is a chemical reaction that releases a large amount of stored chemical energy in the form of heat and light into the environment. Another common exothermic example is the chemical reaction inside a commercial hand warmer, which involves the rapid oxidation of iron powder that releases warmth to the hands.