What is sublimation and in which substances does it commonly occur?

Sublimation is the process where a substance changes directly from a solid to a gas, bypassing the liquid state.

Sublimation is a type of phase transition, or a change in a substance's state of matter. Unlike other transitions, such as melting or boiling, sublimation does not pass through a liquid phase. This process occurs when the particles in a solid gain enough energy to overcome the forces of attraction between them. The energy required for sublimation is usually higher than for melting or boiling, as it needs to overcome both the forces holding the particles in the solid and those that would hold them in a liquid.

Sublimation is commonly observed in substances like dry ice (solid carbon dioxide), iodine, naphthalene (commonly found in mothballs), and frozen water under certain conditions. When dry ice is exposed to room temperature, it sublimates, turning directly into carbon dioxide gas. This is why you never see liquid dry ice. Similarly, if you've ever noticed that mothballs seem to 'disappear' over time, that's because they're slowly sublimating, turning from a solid directly into a gas.

In the case of water, sublimation can occur under specific conditions. For example, in extremely cold and dry conditions, snow and ice can sublimate into water vapour. This is a common phenomenon in the polar regions and high altitudes where the air is dry, and the temperature is low.

Understanding sublimation is important in various fields, including chemistry, physics, and even in the food industry. For instance, freeze-drying food is a process that relies on sublimation, where water in food is frozen and then sublimated, removing the moisture and preserving the food.