Evaporation is when a liquid slowly turns into a gas without boiling. It happens at the surface of a liquid, at any temperature, all the time. When your wet hair dries, when a puddle disappears on a sunny day, or when laundry dries on a clothesline, you are watching evaporation. Heat, wind, and the amount of surface area all affect how fast evaporation happens. Evaporation is different from boiling because it happens at temperatures below the boiling point and only at the surface.
Place equal amounts of water in a shallow plate and a tall narrow glass. Set both in the same spot and check them each day — the plate dries up faster because it has more surface area. Also compare wet paper towels placed in a sunny spot, a shady spot, and in front of a fan to see how heat and wind affect drying speed.
You already learned about boiling — the dramatic, bubbly process where liquid water turns into gas at 100 degrees Celsius. But there is a quieter version of the same change happening all around you, all the time, at much lower temperatures. It is called evaporation, and it is the reason puddles dry up, wet hair dries, and spilled juice vanishes from the counter.
Evaporation happens at the surface of a liquid. Even when water is not boiling, some of the particles at the very top are moving fast enough to break free and fly off into the air as gas. Not all of them — just the fastest ones at the surface. This is why evaporation is slow compared to boiling. Boiling turns liquid to gas throughout the whole body of water; evaporation only works at the top.
Three things speed up evaporation. First, heat: a puddle in the sun dries faster than one in the shade because the sun's warmth gives more particles the energy they need to escape. Second, wind: moving air carries away the vapor that forms above the surface, making room for more particles to escape. That is why a fan helps dry things faster. Third, surface area: water spread out in a thin layer dries faster than the same amount of water sitting in a tall cup, because more of the liquid is exposed to the air.
The water that evaporates does not disappear — it is still out there, just invisible. Water vapor is a gas mixed into the air around you. On a humid summer day, the air holds a lot of water vapor, which is why your skin feels sticky and wet clothes take forever to dry. On a dry winter day, there is less vapor in the air, and things dry out fast — including your skin and lips.
Evaporation is the key to many everyday processes. Your body cools itself by sweating — the sweat evaporates off your skin, and the escaping liquid carries heat away with it. Farmers dry crops in the sun. Clothes dryers use both heat and moving air to speed up evaporation. Once you understand evaporation, you start seeing it everywhere.