Question

During a very cold winter, the temperature may remain below freezing for extended periods. However, fallen snow can still disappear, even though it cannot melt. This is possible because a solid can vaporize directly, without passing through the liquid state. Is this process (sublimation) a physical or a chemical change?

Short answer

Sublimation is a process where a substance transitions directly from the solid phase to the gas phase without passing through the liquid phase. Since sublimation involves changes in the state of matter without altering the chemical composition of the substance and is reversible, it is considered a physical change.

Step-by-step solution

Definition of Sublimation

Sublimation is the process through which a substance directly transitions from the solid phase to the gas phase without passing through the intermediate liquid phase. In other words, it is when a solid turns into a gas without first becoming a liquid. In this case, snow disappears even though it doesn't melt due to the cold temperatures, which is an example of sublimation.

Physical Change

A physical change is a transformation that occurs without altering the chemical composition of a substance. It usually involves changes in the state of matter, such as melting, freezing, evaporation, or condensation. Physical changes are typically reversible, and the original substance can be recovered.

Chemical Change

A chemical change, on the other hand, involves the formation of new substances with different properties than the original substance. Chemical changes happen when chemical bonds are formed or broken, and they are typically not reversible. Examples of chemical changes include combustion, decomposition, and oxidation.

Identifying Sublimation as a Physical or Chemical Change

In sublimation, the substance, like snow, directly vaporizes from the solid state to the gas state without changing its chemical composition. The process is reversible, as the substance can be recovered by condensing the gas back to the solid form (deposition). Therefore, sublimation is a physical change as it does not involve the formation or breaking of chemical bonds, and the original substance can be recovered.