Vapor

A vapor is a substance that exists in a gas phase at a specific temperature. This temperature must be lower than the substance's critical temperature. The critical temperature is the highest temperature where a liquid can exist at any pressure. For example, water has a critical temperature of 374 °C (647 K). Because the temperature is below this point, the vapor can be condensed into a liquid. This happens by increasing the pressure without reducing the temperature.

Vapors are different from fixed gases. A fixed gas is a gas where no liquid or solid can form at the current temperature. Air at typical ambient temperatures is a fixed gas. Vapors are also distinct from aerosols. An aerosol is a suspension of tiny liquid or solid particles within a gas. While vapors are pure gas phases, aerosols contain these small suspended particles.

When a vapor is in contact with a liquid or a solid, they can coexist. This relationship is called equilibrium. In this state, the two phases are in a steady balance. The gas has a specific pressure called the equilibrium vapor pressure. This pressure is determined by the temperature of the substance. Interestingly, the equilibrium vapor pressure is not affected by how much liquid or solid is present.

Scientists study the movement of molecules within a vapor using the kinetic theory of gases. They look at three types of motion: vibrational, rotational, and translational. If the vapor pressure rises above the equilibrium value, the vapor becomes supersaturated. This means there is more vapor than the equilibrium allows. The vapor will then condense on nucleation sites, such as tiny particles of dust.

This process of condensation is very useful in science. Cloud chambers use this principle to visualize particles of radiation. These particles cause water droplets to nucleate and form. Vapors are also used in laboratory processes like distillation. This method helps separate substances. Vapors are also used for headspace extraction from liquid samples. This is a step taken before using gas chromatography.

There are specific mathematical ways to measure and predict vapor behavior. The amount of vapor is quantified by its partial pressure. In a gravitational field, vapors obey the barometric formula. This is the same way conventional atmospheric gases behave. We also look at the normal boiling point of a liquid. This is the temperature where the vapor pressure equals normal atmospheric pressure. For mixtures, scientists use Raoult's law. This law states that the partial pressure of a component is the product of its vapor pressure and its mole fraction. The total vapor pressure is the sum of all component partial pressures.

You can see the effects of vapor in many common places. Water vapor near the Earth's surface can condense into fog, mist, or haar.

Perfumes also rely on vapor. Chemicals in perfume vaporize at different temperatures and rates. These are known as scent accords or notes. Even lighting uses this science. Mercury-vapor lamps and sodium vapor lamps create light from atoms in excited states.

Finally, it is important to understand how vapors interact with fire. Flammable liquids do not actually burn when they are ignited. Instead, it is the vapor cloud above the liquid that burns. This only happens if the concentration of the vapor is between the lower flammable limit (LFL) and the upper flammable limit (UFL). Understanding these limits is a key part of science and safety.