Today’s lesson Describe evaporation in terms of the escape of more energetic molecules from the surface of a liquid. Relate evaporation to the consequent cooling. Demonstrate understanding of how temperature, surface area and air flow over a surface influence evaporation.

Evaporation

Consider a beaker of water at room temperature

Evaporation The molecules of water are moving around at different speeds, some fast, some slow. speed of molecule (m/s) # of molecules at a particular speed Average speed

Evaporation If a molecule is at the surface, and moving fast enough, it may escape the liquid. This is called evaporation. Freedom!

Evaporation Since the average speed of the remaining molecules must now be lower, the temperature of the liquid drops (since temperature is a measure of the kinetic energy of the molecules). Freedom!

Evaporation Evaporation can thus take place at any temperature.

Increasing the rate of evaporation Increasing the temperature.

Increasing the rate of evaporation Increasing the temperature means that more molecules are moving fast enough to escape.

Increasing the rate of evaporation Increasing the surface area

Increasing the rate of evaporation Increasing the surface area means that more molecules are at the surface.

Increasing the rate of evaporation Increasing the air flow over the surface

Increasing the rate of evaporation Increasing the air flow over the surface so that molecules are carried away before they can fall back into the liquid

Increasing the rate of evaporation Decreasing the humidity of the surrounding atmosphere

Increasing the rate of evaporation Decreasing the humidity of the surrounding atmosphere to stop water molecules from the atmosphere entering the liquid.

OK, now try to answer some questions! Finish for HOMEWORK – Due next Thursday