Condensation is a simple, everyday science process where water in its gas form changes back into liquid water. This shift from an invisible gas to visible droplets is a very important part of the larger water cycle that moves water all around our planet. It is the opposite of evaporation, which is how liquid water turns into a gas when heated.
Where Does the Water Come From?
Before water can condense, it must exist in the air as a gas called water vapor. Water vapor is water that has been heated so much that its tiny parts, called molecules, are flying around very fast and are spread far apart. Since you cannot see water vapor, it is often described as an invisible gas floating all around us. This invisible water vapor comes from lakes, rivers, and puddles when the sun warms them up. When liquid water gets hot enough, its molecules gain energy and break away from the surface, becoming gaseous water vapor.
What Makes the Water Appear?
Condensation happens when warm, vapor-filled air bumps into something cold. When the speedy water vapor molecules touch a cool surface, they immediately lose energy, transferring heat to the colder object. This loss of energy causes the molecules to slow down quickly.
Imagine the water vapor molecules are like dancers moving quickly and independently. When they slow down, their natural attractions to one another take over, causing them to gather close together. This crowding forces the individual gas molecules to stick together and form tiny groups, which are the very beginnings of liquid water droplets.
When enough of these microscopic droplets collect on a surface, they become big enough for your eyes to see. This change from a fast-moving, separate gas state to a slow-moving, joined liquid state is the complete process of condensation.
Look! Condensation Around Us
One of the easiest places to see condensation is on the outside of a cold glass of lemonade on a warm day. The air right next to the glass is cooled by the icy drink inside. This cooling causes the water vapor in the air to condense directly onto the glass’s surface, creating the little water beads that make the glass look like it is “sweating.”
Condensation is also visible when a bathroom mirror fogs up during a hot shower. The warm, steamy air is full of water vapor that rises to meet the cooler glass of the mirror. When the warm air hits the cold glass, the vapor cools and instantly changes back into a thin layer of liquid water, making the mirror cloudy.
In nature, dew forms when grass and leaves cool down significantly overnight. The water vapor in the warmer night air condenses onto the cool blades of grass, forming tiny morning droplets.
Condensation is also responsible for making clouds. Warm, moist air rises high into the sky where it meets the cold upper atmosphere. The vapor condenses around tiny dust particles to create the massive collections of water droplets we recognize as clouds.