The terms “gas” and “vapor” are often used interchangeably, leading to confusion about their scientific meaning. Both describe a substance in its gaseous form, a state of matter where particles move freely and randomly to fill the entire volume of a container. However, in the precise language of thermodynamics, the distinction is technical, based entirely on temperature and pressure conditions. This classification determines whether the substance can be converted back into a liquid merely by compression or if it requires cooling first. The key to understanding this difference lies in a specific thermodynamic boundary known as the critical temperature.
The Foundational States of Matter
A substance exists in one of three primary phases: solid, liquid, or gas. These states are defined by the kinetic energy and arrangement of their constituent particles. In a solid, particles are tightly bound and vibrate in fixed positions, giving the substance a definite shape and volume.
Particles in a liquid have enough energy to overcome some attractive forces, allowing them to flow and take the shape of their container while maintaining a fixed volume. The gaseous state, which includes both gases and vapors, is characterized by particles possessing high kinetic energy and weak intermolecular forces. These particles are widely separated and move independently, allowing the substance to occupy the entire volume and shape of its container. A vapor is fundamentally the gaseous phase of a substance, and the classification as “gas” or “vapor” is a technical refinement of this foundational state.
Defining a True Gas
In scientific terms, a substance is classified as a “true gas” when it exists at a temperature above its critical temperature. This classification signifies that the particles have such high kinetic energy that no amount of pressure, however great, can force them close enough to transition into the liquid phase. The extremely high kinetic energy prevents the attractive forces between molecules from dominating, which is necessary for condensation to occur.
For a true gas, the only way to induce a phase change into a liquid is to first lower its temperature below the critical point. Substances like oxygen and nitrogen are often referred to as gases at standard atmospheric conditions. However, in a strict thermodynamic sense, they are only “true gases” when they are heated to temperatures far exceeding their very low critical temperatures.
Defining Vapor and the Gaseous Phase
A substance is correctly termed a “vapor” when it is in its gaseous phase but exists at a temperature below its critical temperature. This characteristic separates a vapor from a true gas, as it means the substance retains the potential to be converted back into a liquid without any cooling. The particles in a vapor still move randomly and fill their container, but their kinetic energy is low enough for intermolecular forces to become significant under certain conditions.
A vapor can be liquefied solely by increasing the pressure upon it, a process known as isothermal compression. Water vapor, often called steam, is the most common example; its critical temperature is \(374\,^{\circ}\text{C}\) (\(705\,^{\circ}\text{F}\)), which is far above everyday temperatures. Gaseous water at room temperature is technically a vapor because it can be condensed into liquid water by increasing the pressure alone. This ability to easily transition to a liquid makes the term “vapor” applicable to the gaseous phase of any substance that is typically a liquid or solid under normal conditions.
The Critical Temperature Differentiator
The critical temperature is the definitive thermodynamic boundary that separates a true gas from a vapor. It is the maximum temperature at which a distinct liquid phase of a substance can exist. Above this temperature, the physical properties of the liquid and gaseous phases become identical, and the substance enters a state known as a supercritical fluid.
For example, the critical temperature of carbon dioxide is approximately \(31.2\,^{\circ}\text{C}\) (\(88.2\,^{\circ}\text{F}\)). Below this temperature, gaseous carbon dioxide is a vapor and can be liquefied by pressure, which is how liquid \(\text{CO}_2\) is stored in fire extinguishers. If the carbon dioxide is heated above \(31.2\,^{\circ}\text{C}\), it becomes a true gas and cannot be liquefied, regardless of how much pressure is applied. The critical temperature serves as the precise benchmark for classification.