Matter constantly undergoes transformations, which scientists classify as either physical or chemical changes. Sublimation is the process where a solid turns directly into a gas, completely bypassing the liquid state. Understanding which category sublimation falls into requires a clear framework for how matter changes at the molecular level.
Distinguishing Physical and Chemical Changes
The fundamental difference between the two types of changes lies in whether a new chemical substance is created. A physical change alters a substance’s form or state, but its underlying molecular structure remains unchanged. For example, when an ice cube melts into liquid water, the substance is still water (H2O). These changes are often reversible, such as refreezing the liquid water back into a solid.
A chemical change always results in the formation of entirely new substances with different chemical properties. This transformation involves the breaking and reforming of chemical bonds between atoms, fundamentally altering the starting material’s composition. Burning a log is a classic example; the wood reacts with oxygen to form new compounds like ash, carbon dioxide, and water vapor. These reactions are generally difficult or impossible to reverse using simple physical means.
The Mechanism of Sublimation
Sublimation is a phase transition defined by a direct jump from the solid phase to the gaseous phase, bypassing the liquid state. For sublimation to take place, the solid must absorb energy from the surroundings, making it an endothermic process. This absorbed heat provides enough energy for the individual molecules to overcome the forces holding them in the rigid solid structure.
The phenomenon is most famously observed with dry ice, the solid form of carbon dioxide (CO2). When dry ice is exposed to standard atmospheric conditions, it immediately converts into gaseous carbon dioxide, producing a dramatic fog effect. Other substances, such as iodine crystals and naphthalene, also readily sublime at room temperature. The ability of a solid to sublime depends on its vapor pressure relative to the surrounding pressure and temperature.
Why Sublimation is a Physical Change
Sublimation is classified as a physical change because the chemical identity of the substance is preserved throughout the transition. When solid carbon dioxide sublimates, the resulting gas is still composed of CO2 molecules. No new chemical compounds are created, and no chemical bonds within the molecules are broken. The transformation is merely a change in the state of matter.
The energy absorbed during sublimation is used only to weaken and overcome the intermolecular forces, which are the relatively weak attractive forces between separate molecules. These forces, such as London dispersion forces or dipole-dipole interactions, hold the solid structure together. They are significantly weaker than the strong covalent or ionic bonds within the molecules. Because the stronger internal chemical bonds remain intact, the molecular formula and chemical properties of the substance remain the same in both the solid and gaseous forms. Sublimation is a reversible process that only affects the physical spacing and movement of the molecules, not their fundamental composition.