What Is It Called When a Gas Turns to Liquid? Condensation Explained

When a gas changes into a liquid, the process is called condensation. It is a physical change of state: the substance remains the same, while its molecules move into a different arrangement. Water vapor, for example, becomes liquid water during condensation. The process is the reverse of evaporation, which changes a liquid into a gas.

Condensation often happens when a gas cools. It can also happen when pressure increases. For water vapor in air, condensation occurs when the vapor reaches saturation under the conditions around it. The Encyclopaedia Britannica describes condensation as a change from gas to liquid that can result from cooling, compression, or both.

One example: droplets on a cold glass

Set a cold glass of iced tea on a table. After a while, small droplets may appear on its outside. The liquid has not leaked through the glass. The droplets form from water vapor in the surrounding air.

Air contains water vapor, which is water in its gaseous state. When air near the glass touches its cold surface, that air cools. If it cools to the dew point, some of its water vapor condenses into liquid droplets on the glass. The water has come from the air, not from inside the drink.

The dew point is the temperature at which air, with its current water vapor content, becomes saturated. If the air cools further, some of its vapor can condense. The dew point depends on how much water vapor is in the air: air with more vapor generally has a higher dew point than drier air. This helps explain why a cold glass may collect droplets in humid conditions but stay mostly dry in dry air.

The glass does not need to be colder than every surrounding object. It needs to cool the thin layer of nearby air enough for that air to reach its dew point. The droplets appear where water vapor meets the cold surface and changes state.

What changes during condensation

Gas molecules move freely and are relatively far apart compared with molecules in a liquid. As a gas cools, its molecules lose kinetic energy and move more slowly. When conditions allow, attractions between molecules bring them closer together, forming a liquid. The molecules themselves remain the same substance. Water vapor and liquid water are both H₂O, so condensation is a physical phase change, not a chemical reaction.

Cooling is a common way to cause condensation, but it is not the only one. Raising pressure can also encourage a gas to become liquid. The precise conditions depend on the substance and its temperature. For water vapor in air, the familiar case is cooling moist air to its dew point. A rise in the amount of water vapor in a given volume of air can also bring it to saturation, after which some vapor may condense if suitable conditions are present.

Condensation releases heat to the surroundings. This is the reverse of vaporization, which takes in heat as a liquid becomes a gas. The heat transfer can matter in physical systems, but it does not change the basic definition: condensation is gas becoming liquid.

Condensation and related terms

Condensation is often discussed alongside evaporation because the two processes run in opposite directions. Evaporation changes liquid into gas, while condensation changes gas into liquid. At a liquid’s surface, molecules can evaporate into the air. At the same time, vapor molecules can return to the liquid by condensing. In a closed system, the rates of evaporation and condensation can become equal, leaving the amounts of liquid and vapor steady even though molecules continue to move between the two states.

Boiling is also a change from liquid to gas, but it is different from evaporation at the surface. During boiling, vapor bubbles form throughout the liquid when it reaches the boiling conditions for the surrounding pressure. Neither boiling nor evaporation is another name for condensation; both describe the opposite direction of change.

The word “condensation” can also refer to liquid water forming from water vapor in the atmosphere. The USGS Water Science School describes condensation as water vapor in the air becoming liquid water. This meaning connects the basic phase change to processes in the water cycle, where vapor can cool and form liquid droplets. The term still refers to the same change of state.

Key Takeaways

• Condensation is the process in which a gas changes into a liquid.
• Cooling can cause condensation; increasing pressure can also do so under suitable conditions.
• The dew point is the temperature at which air, given its water vapor content, becomes saturated.
• Droplets on a cold glass form when water vapor in the surrounding air cools and condenses on the glass.
• During condensation, the substance stays the same. Its state changes from gas to liquid.