Why do some substances skip the liquid state when heated?

Some substances skip the liquid state when heated due to a process called sublimation, where they transition directly from solid to gas.

Sublimation is a fascinating phenomenon in which a substance transitions from a solid state directly to a gas state, bypassing the liquid state entirely. This occurs when the atmospheric pressure is too low for the substance to exist in a liquid state. The particles in the solid gain enough energy to overcome not just the forces holding them together in the solid but also the forces that would normally hold them together in the liquid state.

A common example of a substance that sublimates is dry ice, or solid carbon dioxide. When dry ice is exposed to room temperature, it doesn't melt into a liquid. Instead, it turns directly into carbon dioxide gas. This is because the pressure at which carbon dioxide turns into a liquid (5.1 atmospheres) is much higher than normal atmospheric pressure. Therefore, at normal atmospheric pressure, it skips the liquid state and sublimates.

Another example is iodine. When iodine crystals are heated gently, they change directly into a purple vapour without passing through a liquid state. This is due to the relatively weak forces of attraction between its molecules, which are easily overcome when energy is supplied in the form of heat.

In contrast, substances like water do not sublimate under normal conditions because the atmospheric pressure is high enough for the substance to exist in all three states: solid, liquid, and gas. However, under certain conditions, such as extremely low pressure or temperature, even water can sublimate.

Understanding sublimation is important in chemistry as it helps us understand the unique properties of different substances and how they behave under different conditions. It also has practical applications in various fields, such as in freeze-drying food for preservation.