How does sublimation occur and why does it not pass through the liquid phase?
The correct answer and explanation is:
Correct Answer:
Sublimation occurs when a substance changes directly from a solid to a gas without passing through the liquid phase. This happens under specific conditions of temperature and pressure, typically when the substance’s vapor pressure exceeds atmospheric pressure at a temperature below its melting point.
Explanation:
Sublimation is a physical process in which a solid turns directly into a gas without first becoming a liquid. This transformation is possible because particles in some solids gain enough energy to overcome their intermolecular forces and escape into the gas phase without the intermediate liquid state. This typically occurs under low pressure and when the temperature is below the substance’s melting point.
The key to understanding sublimation lies in the phase diagram of a substance. A phase diagram shows the conditions of temperature and pressure at which the three states of matter — solid, liquid, and gas — coexist. In some cases, there is a region on this diagram where the solid and gas phases are directly connected. This region allows the solid to transition to gas without entering the liquid phase. The specific point where this occurs is known as the triple point. If the ambient pressure is below this point, a substance cannot exist as a liquid, so it moves directly from solid to gas when heated.
Substances like dry ice (solid carbon dioxide), iodine, and naphthalene demonstrate sublimation visibly. For example, dry ice, when exposed to room temperature, skips the liquid phase entirely and turns into carbon dioxide gas. This is because at atmospheric pressure, carbon dioxide’s triple point is at a much lower pressure than that found at Earth’s surface, preventing the liquid phase from forming.
Sublimation is also used in practical applications such as freeze-drying food, purifying substances, and in air fresheners. This process offers an efficient method to change states without involving the liquid form, especially in environments where liquid states are undesirable or unstable.