In science, “gas” and “vapor” refer to distinct states of matter, though often used interchangeably in everyday language. This can cause confusion about their physical properties and behaviors. The distinction hinges on specific temperature and pressure conditions that define how a substance behaves and whether it can be easily condensed.
Understanding Gas
A gas is a state of matter characterized by widely separated particles moving randomly with high kinetic energy. Gases do not possess a definite shape or fixed volume, expanding to fill any container. Intermolecular forces between gas particles are significantly weaker than in liquids or solids. Gases are also highly compressible, as their volume can be substantially reduced by increasing pressure due to the large empty space between particles.
Understanding Vapor
A vapor is the gaseous phase of a substance that is typically a liquid or solid at standard room temperature and pressure. Unlike a true gas, a vapor exists below its critical temperature. This means it can be condensed back into its liquid (or solid) form by increasing pressure, even without lowering its temperature. Vapors can also coexist in equilibrium with their liquid or solid phases, such as water vapor above liquid water.
The Critical Distinction
The fundamental difference between a gas and a vapor lies in the critical temperature. Each substance has a unique critical temperature, which is the temperature above which it cannot be liquefied by pressure alone. Above this temperature, particle kinetic energy is too high for intermolecular forces to overcome, preventing liquefaction. A substance existing above its critical temperature is classified as a gas.
Conversely, a substance in its gaseous state below its critical temperature is a vapor. Applying sufficient pressure can force molecules closer, allowing attractive forces to cause condensation into a liquid. For example, water has a critical temperature of 374 °C (705 °F); gaseous water below this temperature is water vapor, while above it, it is a true gas. The critical pressure is the minimum pressure needed to liquefy a substance at its critical temperature. The critical point, defined by both critical temperature and critical pressure, marks where the distinction between liquid and gas phases disappears.
Common Examples and Misconceptions
Everyday language often blurs the lines between gases and vapors, leading to misconceptions. For instance, “natural gas” (predominantly methane) remains a gas at typical room temperatures because its critical temperature is well below these conditions. Oxygen, nitrogen, and helium are also considered gases at ambient conditions due to their very low critical temperatures.
A common misconception involves “steam.” What appears as steam, like the white mist from a boiling kettle, is actually tiny liquid water droplets suspended in the air. Pure water vapor is transparent and invisible. Since water’s critical temperature is 374 °C, water below this temperature is water vapor. It can condense into liquid water droplets through cooling. While all vapors are gases, not all gases are vapors; critical temperature is the defining factor.