Why Do Substances Sublime?

Almost everyone of us must have experienced the phenomenon of sublimation, be it the disappearing case of naphthalene balls without a trace or the magical effects of dry ice in a theatrical performance. Sublimation is defined as the conversion of a substance from its solid state to its gaseous state without passing through the liquid phase. But, why do substances sublime?

Before I answer this, let me give you a brief background. To convert any substance from one state to another, two factors play a pivotal role: temperature and pressure. If temperature is increased, state change happens from solid to liquid to gas but if pressure is increased, state change happens from gas to liquid to solid.

For this discussion, let us consider dry ice (solid CO₂ ). Carbon dioxide exists in its solid state at -78.5°C, which is way below that of water (0°C). At STP (Standard Temperature and Pressure), carbon dioxide exists as a gas.

I would like to clear one thing here- Liquid CO₂ DOES exist. The only factor that makes it impossible to naturally obtain liquid CO₂ is pressure. CO₂ exists as a liquid at a pressure condition of 5.1 atm (atmospheric pressure) and temperature between –56.6 and +31.1°C. Though the temperature can be naturally obtained on Earth, the pressure of 5.1 atm is impossible to obtain naturally. The Earth’s atmospheric pressure is 1 atm. So, to obtain liquid carbon dioxide, we need to quintuple the atmospheric pressure! That wouldn’t be the best thing to do as it would affect all organisms adversely.

Similarly, any sublimable substance (ammonium chloride, naphthalene, camphor, iodine) would not be able to exist in liquid form due to unfavourable pressure conditions (naturally, that is).

So, how was it? Feel free to comment in the Comments section.

Author: Venkata Bhamidipati

Source Of Picture: Wikipedia