In everyday conversation, the terms gas and vapor are often used interchangeably to describe any substance floating in the air. However, this common usage obscures a fundamental and precise distinction in physics and chemistry. Scientifically, the difference between a gas and a vapor is a strict classification based on the substance’s physical state relative to its critical temperature threshold. This threshold determines how the substance behaves when subjected to changes in pressure, which is key to understanding why these terms are not interchangeable.
What Defines a True Gas
A substance is classified as a “true gas” when its temperature is maintained above its characteristic critical temperature. The critical temperature is a unique physical property for every substance that dictates molecular behavior. For a true gas, molecules possess high kinetic energy, meaning they are moving too rapidly to be forced together into a liquid state.
The defining characteristic of a true gas is that it cannot be liquefied by pressure alone. Increasing the pressure only results in a more compressed gas. To change a true gas into a liquid, the temperature must first be lowered below its critical temperature to reduce molecular motion. Oxygen and nitrogen are true gases at room temperature because their critical temperatures are far below this point (around -119°C and -147°C, respectively).
The gaseous state of these substances is considered permanent under ordinary conditions. The forces of attraction between the molecules are overpowered by the energy of motion. Even at immense pressures, the true gas remains a single, highly compressed fluid, never exhibiting the distinct boundary that separates a liquid from its gaseous phase.
Vapor: The State of Equilibrium
A vapor is the gaseous phase of a substance that exists at a temperature below its critical temperature. This temperature difference allows a vapor to behave fundamentally differently from a true gas. Because the substance is below its critical temperature, the attractive forces between its molecules are strong enough to allow for a phase transition.
The essential distinction for a vapor is its ability to coexist in equilibrium with its liquid or solid phase. For instance, the gaseous water above liquid water in a closed container is correctly referred to as water vapor. The vapor can be condensed back into a liquid simply by increasing the pressure, a process known as isothermal compression.
Water vapor, for example, has a critical temperature of approximately 374°C, meaning that at the typical atmospheric temperature of 20°C, it is well below this threshold. This low temperature relative to the critical point allows the vapor to readily condense onto a cool surface, which is a common observation in daily life. This ease of conversion back to the liquid phase under pressure is the hallmark of a vapor.
The Critical Boundary: Why the Terms Are Not Interchangeable
The scientific distinction between a gas and a vapor rests on the concept of the critical point, which is the precise combination of a substance’s critical temperature and critical pressure. The critical temperature is the highest temperature at which a substance can still form a distinct liquid surface when pressure is applied. Above this temperature, a separate liquid phase cannot exist, regardless of the pressure applied.
This boundary serves as the physical divider between the two terms. Any substance in its gaseous state above its critical temperature is classified as a true gas, which resists liquefaction by pressure alone. Conversely, any substance in its gaseous state below its critical temperature is classified as a vapor, which can be liquefied by increasing the pressure.
At the critical point, the densities of the liquid and gaseous phases become equal, and the visible boundary vanishes, forming a single, homogeneous fluid known as a supercritical fluid. This point highlights why the terms are not interchangeable: they describe two different thermodynamic states relative to this fixed point. The temperature determines which method of liquefaction is possible—cooling for a true gas, or compression for a vapor—making the terms fundamentally distinct identifiers in physical science.