Describe how temperature, surface area and air movement over a surface affect evaporation

2.2.3 Melting, boiling and evaporation

Evaporation is the process where molecules at the surface of a liquid gain enough energy to escape into the gas phase. Unlike boiling, it can happen at any temperature, and its rate depends on several factors.

🌡️ Effect of Temperature

Increasing the temperature raises the average kinetic energy of the liquid molecules (recall \$E_k = \frac{1}{2}mv^2\$). More molecules have sufficient energy to overcome intermolecular forces and escape, so the evaporation rate increases.

• Higher temperature → more energetic molecules → faster evaporation.
• Lower temperature → fewer energetic molecules → slower evaporation.

🌬️ Effect of Surface Area

Evaporation occurs only at the surface. A larger surface area exposes more molecules to the air, allowing more to escape simultaneously.

• Increase surface area (e.g., spreading water thinly) → higher evaporation rate.
• Decrease surface area (e.g., a deep narrow container) → lower evaporation rate.

💨 Effect of Air Movement (Wind)

Moving air removes water vapour molecules that have just escaped from the liquid surface. This lowers the local humidity and maintains a concentration gradient that drives more molecules to leave the liquid.

• Stronger air flow → quicker removal of vapour → higher evaporation rate.
• Still air → vapour builds up → evaporation slows down.

📊 Summary Table

Remember: Evaporation is a surface phenomenon, unlike boiling which occurs throughout the liquid. By controlling temperature, surface area, and airflow, we can speed up or slow down evaporation – useful in drying clothes, cooling sweat, or designing industrial processes.