Evaporation

Evaporation is when particles at the surface of a liquid escape into the air and become a gas. You see it when a puddle slowly disappears or when sweat dries from your skin.

Particle view

In a liquid, particles move randomly and have different energies. Some particles at the surface have enough energy to break away from their neighbours and leave as gas. This is evaporation. It can happen at any temperature below the boiling point and only from the surface, so no bubbles form inside the liquid.

Why evaporation causes cooling

The fastest (most energetic) particles leave first. The particles left behind have a lower average kinetic energy, so the liquid’s temperature falls. Temperature is linked to average kinetic energy, so when energy is removed, temperature drops.

An object touching the liquid also loses thermal energy as evaporation happens. This is why alcohol gel feels cold on your skin and why sweating cools the body.

Factors that increase evaporation

• Higher temperature: particles have more energy, so more can escape.
• Larger surface area: more particles are at the surface and can leave.
• Moving air (wind): carries away escaping particles, so more can keep leaving.

Evaporation versus boiling

• Where it happens: Evaporation occurs at the surface only; boiling happens throughout the liquid.
• Temperature: Evaporation at any temperature; boiling at a fixed boiling point (for water, 100 °C at standard pressure).
• Bubbles: No bubbles in evaporation; boiling forms bubbles of gas.
• Speed: Evaporation is usually slow; boiling is rapid.
• Temperature change: During boiling, temperature stays constant while energy is supplied; during evaporation, the liquid often cools unless energy is supplied.

Real-world links

• Clothes dry faster on a warm, windy day.
• Sweating removes thermal energy from the skin to keep body temperature steady.
• A wet thermometer bulb reads lower as the liquid evaporates and cools it.

Common misconceptions

• Evaporation does not need boiling; it happens at any temperature.
• Evaporation happens at the surface, not inside the liquid.
• Covering a liquid or still air slows evaporation; blowing air speeds it up.