Observing matter changing from one form to another is called a phase transition, or a change of state. Matter commonly exists as a solid, liquid, or gas. The conversion between these forms is driven by changes in energy. These transitions are constant occurrences, shaping everything from weather patterns to the moisture that collects on a cold beverage.
The Term for Gas Turning Liquid
The physical process in which a substance transitions from its gaseous state to its liquid state is called condensation. This process is the reverse of vaporization, where a liquid turns into a gas. Condensation requires the gaseous matter to lose energy, typically heat, for the change to occur. When gas molecules slow down, attractive forces pull them closer together, forming a liquid. This conversion releases latent heat into the surroundings, which was absorbed during vaporization. The temperature at which this phase change occurs is the condensation point, which is the same as the boiling point for that substance at a given pressure.
The Molecular Process Behind the Change
The shift from gas to liquid is fundamentally about the kinetic energy of the molecules involved. In the gaseous state, molecules possess high kinetic energy, moving rapidly and randomly with large distances between them. For condensation to begin, this molecular motion must decrease, which happens when the gas is cooled and loses thermal energy. As the gas cools, molecules slow down and their kinetic energy drops. This reduction in speed allows intermolecular forces of attraction to become effective, pulling the molecules into contact. They then cluster together, forming the denser liquid state. This clustering is often initiated on a cooler surface or around a microscopic particle in the air, known as a condensation nucleus.
Saturation and Dew Point
Condensation is closely linked to the saturation point. Air can only hold a certain amount of water vapor. When the air temperature drops to the dew point, the air becomes saturated, meaning its relative humidity reaches 100%. Further cooling or addition of water vapor past this point causes the excess vapor to condense into liquid droplets. The dew point is the temperature at which the rate of condensation exceeds the rate of evaporation, leading to the visible formation of liquid water.
Observing the Transition in Everyday Life
Condensation is a constantly occurring natural phenomenon easily observable in daily life. A common example is the formation of moisture on the outside of a cold glass of water or soda. The air immediately surrounding the cold surface cools down, causing the water vapor to reach its dew point and turn into liquid droplets on the glass. In the atmosphere, condensation creates clouds and fog. As warm, moist air rises, it cools, and water vapor condenses around tiny airborne particles to form the small liquid water droplets that make up a cloud. Near the ground, dew forms when the air cools overnight, condensing onto surfaces like grass and car windshields that have cooled below the dew point. Other instances include the moisture seen when breathing outside on a cold day or the fogging of a bathroom mirror after a hot shower.
Related Changes of State
Condensation is one of several phase transitions that matter undergoes. The inverse of condensation is vaporization, the change from a liquid to a gas. Vaporization includes both evaporation (occurring below the boiling point) and boiling (occurring at a specific temperature and pressure). Matter also transitions between the liquid and solid states. The change from a liquid to a solid is called freezing or solidification, which involves removing energy. Conversely, the transition from a solid to a liquid is known as melting, which requires the addition of heat energy. Finally, some substances bypass the liquid phase entirely: sublimation is the direct change from a solid to a gas, and deposition is the reverse, the change from a gas to a solid.