Vapors represent a state of matter that embodies the transition between the familiar liquid or solid form and the expansive gaseous state. A vapor is the gaseous form of a substance that exists as a liquid or solid under normal conditions. Its existence is a direct consequence of the substance’s temperature relative to a specific thermal boundary.
The Scientific Definition of a Vapor
A vapor is defined as a substance in its gaseous phase at a temperature below its critical temperature. This classification separates a vapor from a true gas. Water vapor, for instance, is the gaseous phase of water, but it can easily revert to liquid water.
The critical temperature is central to defining a vapor. It represents the highest temperature at which a substance can exist as a liquid, regardless of the pressure applied. Above this temperature, the substance cannot be condensed into a liquid simply by increasing pressure. For water, this point is 374 degrees Celsius (647 Kelvin), meaning water in the atmosphere is always a vapor, not a true gas, under normal conditions.
Because a vapor is below its critical temperature, it can be liquefied solely by applying sufficient pressure, even without lowering the temperature. This allows the vapor state to coexist in equilibrium with its liquid or solid phase. Vapor molecules possess enough energy to escape the liquid’s intermolecular forces, yet they remain susceptible to being forced back into the condensed state.
Distinguishing Vapors from Gases and Aerosols
The terms vapor, gas, and aerosol are often used interchangeably, but they describe distinct physical states. A true gas exists at a temperature above its critical temperature, meaning its molecules have so much kinetic energy that pressure alone cannot force them back into the liquid state. Nitrogen and oxygen in the air are examples of true gases at room temperature.
A vapor, in contrast, is the gaseous state of a substance that is below its critical temperature. This distinction is based entirely on the thermal condition. If the substance can be compressed back into a liquid at its current temperature, it is a vapor; if not, it is a gas.
An aerosol is physically different from both a vapor and a gas because it is not a homogenous gaseous phase. An aerosol is a suspension of tiny solid particles or liquid droplets dispersed within a gas. Mist, fog, and smoke are common examples. They are visible because the suspended particles are large enough to scatter light, whereas a true vapor, such as pure steam, is invisible because its individual molecules are too small to scatter light.
How Vapors Form: Evaporation and Boiling
Vapors are created when a liquid undergoes a phase change through either evaporation or boiling. Evaporation is a slow, surface-level phenomenon that occurs below the boiling point. Molecules with enough kinetic energy near the liquid’s surface overcome intermolecular forces and escape into the gaseous phase.
The rate of evaporation increases with temperature, but the bulk of the liquid remains in the liquid state. This process is responsible for puddles drying up or wet laundry becoming dry, and it does not require the entire liquid to reach a specific high temperature.
Boiling is a more vigorous process that occurs when the liquid reaches a temperature where its internal vapor pressure equals the surrounding atmospheric pressure. At this point, the liquid vaporizes throughout its entire volume, forming bubbles of vapor that rise and escape. The boiling temperature is a fixed point for a given substance at a specific pressure, unlike evaporation, which occurs across a range of temperatures.