Evaporation, Condensation, and Boiling
Evaporation is the change of a liquid into a gas at its surface, boiling is the rapid change into gas throughout the liquid at the boiling point, and condensation is the change of a gas back into a liquid. In brief: a puddle dries by evaporation at any temperature, water in a pot boils at 100 °C at sea level, and water droplets on a cold glass form by condensation of water vapor from the air.
These three processes drive the weather, cooking, cooling of the human body and many industrial methods. This article compares evaporation with boiling, explains condensation, shows which factors speed evaporation up, gives a worked heat calculation and clears up common confusion about steam and clouds.
What is evaporation?
The particles in a liquid move at different speeds. Particles at the surface that move fast enough can escape into the air as gas. This happens at any temperature below the boiling point, though it is faster when the liquid is warmer. Evaporation takes place only at the surface and is a slow process compared with boiling.
Because the fastest particles leave, the average energy of the remaining liquid drops. This is why evaporation causes cooling. Sweat evaporating from your skin cools you down.
What is boiling?
Boiling happens when a liquid is heated until its vapor pressure equals the pressure of the surroundings. Bubbles of vapor form inside the liquid, rise and burst at the surface. For water at standard atmospheric pressure (101.3 kPa) this occurs at 100 °C.
During boiling, the temperature of the liquid stays constant at the boiling point, even though heat is still being supplied. The energy is used to separate particles from each other, not to raise the temperature.
What is condensation?
Condensation is the reverse of evaporation. When a gas is cooled, its particles slow down and attract each other, forming a liquid. It releases energy to the surroundings. Dew on grass in the morning, fog, clouds and the mist on a bathroom mirror are all examples of condensation.
Condensation also explains why the outside of a cold bottle gets wet. Water vapor in the air touches the cold surface, loses energy and turns to liquid. The water did not leak through the glass.
Evaporation and boiling compared
| Feature | Evaporation | Boiling |
|---|---|---|
| Where it occurs | Surface only | Throughout the liquid |
| Temperature | Any temperature below the boiling point | Only at the boiling point |
| Speed | Slow | Fast |
| Bubbles | No | Yes |
| Effect on liquid temperature | Cools the liquid | Temperature stays constant |
What affects the rate of evaporation?
- Temperature: warmer liquids have more fast-moving particles, so they evaporate faster.
- Surface area: a wide, shallow dish dries faster than a narrow glass with the same amount of water.
- Air movement: wind or a fan carries vapor away, which lets more liquid escape.
- Humidity: when the air already holds much water vapor, evaporation slows down.
- Type of liquid: volatile liquids such as acetone and ethanol evaporate faster than water.
Boiling points of common liquids
| Liquid | Boiling point (°C) |
|---|---|
| Liquid nitrogen | −196 |
| Acetone | 56 |
| Ethanol | 78 |
| Water | 100 |
| Mercury | 357 |
The boiling point also depends on pressure. At the top of a high mountain the air pressure is lower, so water boils below 100 °C, and a pressure cooker raises the boiling point by increasing the pressure.
Worked example: heat needed to boil water
The specific latent heat of vaporization of water is about 2,260 J/g, and its specific heat capacity is 4.18 J/(g·°C).
- Heating 200 g of water from 20 °C to 100 °C. Q = 200 × 4.18 × 80 = 66,880 J.
- Boiling away 200 g of water at 100 °C. Q = 200 × 2,260 = 452,000 J.
- Total. 66,880 + 452,000 = 518,880 J, or about 519 kJ.
Turning water to steam takes about seven times more energy than heating the same water from 20 °C to the boiling point. This large energy requirement explains why steam burns are so severe.
Where do these changes matter?
- The water cycle: evaporation from oceans and condensation in clouds move water around the planet.
- Cooling: sweat, evaporative coolers and refrigerators all use the cooling effect of evaporation.
- Laundry: clothes dry as the water evaporates, which is faster on a warm, windy day.
- Power: power stations boil water to make steam that drives turbines.
How did people explain these changes?
For a long time, evaporating water was thought to turn into air. In the 1700s, Joseph Black’s work on latent heat explained why boiling needs a steady supply of heat without any rise in temperature. In the 1800s, John Dalton’s experiments on water vapor and the kinetic theory of gases showed that evaporation can be explained through the motion of particles.
Which mistakes are common?
Mistake 1: Thinking that boiling and evaporation are the same
Both produce vapor, but boiling happens throughout the liquid at a specific temperature, and evaporation happens only at the surface.
Mistake 2: Saying that boiling water gets hotter if you turn up the heat
At a given pressure, boiling water stays at the boiling point. Higher heat only produces vapor faster.
Mistake 3: Believing that the white cloud over a kettle is steam
Steam is invisible water vapor. The visible cloud is made of tiny liquid droplets formed by condensation.
Mistake 4: Assuming that condensation comes from inside a cold glass
The droplets come from water vapor in the surrounding air.