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Physical Science: Session 3

A Closer Look: Evaporation Versus Boiling

Are evaporation and boiling the same processes?

At first glance, it would seem that evaporation and boiling might be the same process, because both start with a liquid and end up with a gas. However, there are some important macroscopic and microscopic differences that help make the distinction.

What are the “macroscopic” differences?

boiling water
Boiling water.

Evaporation happens only at the surface of a liquid and occurs at any temperature (so long as the substance is a liquid at that temperature). However, as most people are aware, liquids evaporates faster at a higher temperature.

Boiling, on the other hand, happens throughout the bulk of a liquid, usually starting from some site on the inside of the container and rising in a bubble to the surface. It only happens when the temperature is above the boiling point of that substance.

What are the microscopic differences?

Boiling occurs when the average motion of particles is fast enough to overcome the forces holding them close together. This happens evenly throughout a boiling liquid because the temperature is uniform throughout.

Evaporation happens for the following two reasons:

  1. Not all particles in the liquid are moving at the same speed and, as a result, the faster particles are more likely to overcome the forces they feel from their neighbors.
  2. The particles at the surface of the liquid are only held in place by forces from the neighboring particles beneath them, whereas particles in the middle of the liquid have forces holding them on all sides. Thus, particles at the surface find it easier to break away from the liquid.

In the cases of both boiling and evaporation, the force between two particles is always present. The greater the space between the particles becomes, however, the weaker the force is between them. To break the bond between two particles, one particle has to be moving fast enough to overcome the pull of the other, until it gets so far away that pull is diminished. An analogy would be if you tried to jump off the Earth: we cannot jump fast enough to get to where Earth's gravity has less pull, but a rocket can.

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