The terms vapor and aerosol are often used interchangeably, especially when discussing topics like humidifiers, vaping devices, or air quality. Understanding the precise scientific distinction between these two states of matter requires examining their fundamental properties and how they are formed. Defining these differences is necessary for accurate communication in science, public health, and engineering.
The Scientific Definition of Vapor
Vapor is defined as a substance in the gas phase at a temperature below its critical temperature. This critical temperature is the highest point at which a substance can exist as a liquid, regardless of the pressure applied. Below this point, the gaseous substance can be condensed back into a liquid simply by increasing the pressure.
A true vapor consists of individual molecules dispersed in the air, making it a single, pure phase of matter, not a mixture. Water vapor is invisible because the molecules are too small to scatter light. The cloudy plume often called “steam” is not pure vapor but is an aerosol—tiny liquid water droplets suspended in the air that have condensed from the gaseous water vapor. Vapor forms through evaporation or boiling, which converts a liquid entirely into a gas.
The Scientific Definition of Aerosol
An aerosol is a colloid, defined as a heterogeneous mixture where tiny solid particles or liquid droplets are suspended in a gas, typically air. Unlike vapor, an aerosol is inherently a two-phase system: particulate matter dispersed within a continuous gaseous medium. The term “aerosol” refers to the entire mixture, including both the particles and the surrounding gas.
The particles are minute, generally having diameters less than 100 micrometers (µm), which allows them to remain suspended for extended periods. Common natural examples include fog, mist, and dust. Human-made aerosols encompass smoke, spray paint, and the output from vaping devices.
How Their Physical Properties Differ
The core difference between vapor and aerosol lies in their fundamental physical nature. Vapor is a single, homogeneous phase—a gas whose individual molecules are uniformly distributed. In contrast, an aerosol is a heterogeneous mixture, or suspension, containing distinct liquid or solid particles dispersed throughout a gas. Vapor is molecular, possessing no measurable particles; the substance exists as individual molecules. Aerosols, however, are defined by the size of their suspended particles, which are measured in micrometers. Particles larger than 10 µm tend to settle out quickly, but smaller particles can remain airborne for hours, creating a persistent suspension.
The stability of the substance also differs significantly. Vapor is thermodynamically stable once it reaches equilibrium with its liquid phase, and it will remain in a gaseous state unless the pressure or temperature changes enough to cause condensation. Aerosols are generally unstable suspensions, with the dispersed particles eventually settling out due to gravity or evaporating into a true vapor over time. Vapor is formed only through a thermal phase change, like boiling or evaporation, while aerosols can be formed by condensation, combustion, or mechanical actions such as grinding or spraying a liquid.