IGCSE Chemistry 0620: Evaporation, Boiling and Condensation Compared

A meteorology student records three everyday observations of water changing state during a warm, humid day.

Observation 1: A shallow puddle of rainwater on a pavement slowly shrinks and disappears over several hours in the sun, even though the air temperature never rises above about $28\,^{\circ}\text{C}$ .

Observation 2: In the school kitchen, a kettle of water is left switched on until the water bubbles vigorously throughout the whole kettle, with the water's temperature holding steady at $100\,^{\circ}\text{C}$ .

Observation 3: A tall glass of iced lemonade, taken outside on the same humid day, quickly becomes covered in small droplets of liquid water on its cold outer surface.

(a) Name the change of state occurring in Observation 1, and state whether this change happens only at the surface of the liquid or throughout the whole liquid. [2]

(b) Name the change of state occurring in Observation 2, and state one key difference between this change of state and the one in Observation 1, in terms of the temperature at which each occurs. [2]

(c) Name the change of state occurring on the outside of the glass in Observation 3, and explain, in terms of particle energy, why this change occurs on the cold glass surface rather than in the surrounding warm air. [3]

Part (a): The shrinking puddle

The puddle changes from liquid water to water vapour (gas), escaping into the air, without ever reaching $100\,^{\circ}\text{C}$ . This gradual change of state, occurring below the boiling point, is called evaporation.

Evaporation happens only at the surface of the liquid. It is the particles right at the exposed top layer of the puddle, with enough kinetic energy to escape into the air, that leave as vapour. The bulk of the liquid beneath the surface is not directly involved.

Part (b): The boiling kettle

The kettle’s water changes from liquid to gas (steam) throughout the whole container, with bubbles of vapour forming inside the liquid itself, not just at its surface. This change of state is called boiling.

The key difference from Observation 1 is temperature: boiling occurs at one single, fixed temperature (the boiling point. $100\,^{\circ}\text{C}$ for water at normal atmospheric pressure), whereas evaporation can occur at a range of temperatures, including well below the boiling point, as seen with the puddle at around $28\,^{\circ}\text{C}$ .

Part (c): Droplets on the cold glass

The droplets forming on the outside of the glass are liquid water appearing from the surrounding air. This is condensation, the change of state from gas to liquid.

Water vapour particles in the warm, humid air are constantly moving and colliding with everything around them, including the cold glass surface. When these fast-moving gas particles collide with the cold glass, they transfer some of their kinetic (thermal) energy to the glass and slow down. Once enough particles have slowed down sufficiently, the forces of attraction between them become strong enough, relative to their reduced kinetic energy, to pull the particles together into a liquid.

This does not happen out in the surrounding warm air, because the air particles there retain enough kinetic energy to keep overcoming the forces of attraction between them and stay as a gas.

Final answers

(a) Evaporation; occurs only at the liquid’s surface.
(b) Boiling; boiling occurs only at one fixed temperature, while evaporation can occur at any temperature.
(c) Condensation; water vapour particles lose kinetic energy on contact with the cold glass, slowing enough for attractive forces to pull them together into a liquid.