What Defines a Gas?
A gas is a state of matter that exists above its critical temperature. This means that a gas cannot be condensed into a liquid by simply increasing pressure, no matter how much pressure is applied, as long as the temperature remains above its critical point. Gases uniformly fill any container they occupy and are highly compressible.
What Defines a Vapor?
A vapor, on the other hand, is a gaseous substance that exists below its critical temperature. This characteristic means that a vapor can be condensed into a liquid by increasing pressure, or by decreasing temperature, or both. Common examples include water vapor, which can condense into liquid water (rain) at everyday temperatures.
Critical Temperature and Pressure Explained
The distinction between a gas and a vapor hinges on the concept of the critical point, which includes critical temperature and critical pressure. The critical temperature is the highest temperature at which a substance can exist as a liquid. Above this temperature, it is always a gas. The critical pressure is the pressure required to liquefy a gas at its critical temperature. If a substance is above its critical temperature, it is a true gas; if it's below and in its gaseous phase, it's a vapor.
Practical Applications of This Distinction
Understanding the difference between a gas and a vapor is crucial in various scientific and industrial applications, such as refrigeration, atmospheric science, and chemical engineering. For instance, refrigerants often operate in their vapor phase because they can be easily compressed and expanded to absorb and release heat. In meteorology, water vapor is distinct from atmospheric gases because it readily undergoes phase changes, forming clouds and precipitation.