In everyday conversation, the terms “gas” and “vapor” are often used without distinction, leading to confusion about their scientific meaning. Both involve substances in the gaseous state, but they differ fundamentally based on their physical properties and temperature. While all vapors are technically a type of gas, the classification is based on a precise thermodynamic boundary that dictates how the substance behaves under pressure. Understanding this difference is necessary for accurately describing physical and chemical processes.
Defining the Gaseous State
A substance in the gaseous state is characterized by particles that move randomly and rapidly with minimal attraction. A substance is classified as a true gas when its temperature is above its critical temperature. In this high-energy state, the particles possess too much kinetic energy to be easily forced closer together.
Because of this high energy, a true gas cannot be returned to its liquid state by increasing pressure alone. If the temperature remains above the critical temperature, the gas molecules will not condense back into a liquid, regardless of the external force applied.
Defining Vapor
In contrast to a true gas, a vapor is the gaseous phase of a substance existing at a temperature below its critical temperature. This lower temperature allows a vapor to easily co-exist in equilibrium with its liquid or solid form, such as steam with liquid water.
The key characteristic of a vapor is its susceptibility to condensation through compression. If the pressure on a vapor is increased while the temperature is held constant, the substance will revert to its liquid state without further cooling. This capability to condense easily is what separates a vapor from a true gas.
Fumes from rubbing alcohol or the “cloud” that forms when hot water boils are correctly identified as vapors. These substances are cool enough to be liquefied by a change in pressure.
The Critical Distinction
The critical temperature is the line that separates a gas from a vapor, representing the highest temperature at which a substance can exist as a liquid. Every chemical substance has a unique critical temperature and corresponding critical pressure, which define the substance’s critical point. This point marks the end of the phase boundary between the liquid and gaseous states.
For water, the critical temperature is approximately 373.9 degrees Celsius. Gaseous water below this temperature is called water vapor, such as the steam that rises from a kettle. Above 373.9 degrees Celsius, gaseous water becomes a true gas and cannot be liquefied, regardless of the pressure applied.
The distinction is based on the substance’s potential to return to its liquid phase. A vapor is below its critical temperature, retaining the potential to be easily condensed back into a liquid state through simple compression. A true gas, existing above this thermal threshold, has lost that potential.