Why doesn’t boiling water instantly evaporate at 100 °C? | Maxwell-Boltzmann Distributions

Michelle Ncube
Sep 5, 2025
2 min read

At school we were always taught that water’s boiling point is 100 °C. This idea is engraved in our minds. However, if we were to put a pot of water onto a stove and heat it up until it reaches 100 °C. Why doesn’t the water instantly evaporate into a gas?

What is a liquid?

A liquid is a state of matter in which its particles are free to move past each other, rather than being in fixed positions. In GCSE chemistry we learn that liquids have a definite volume and the atoms/ molecules are held together by intermolecular forces.

These forces allow the liquid to flow and take the shape of its container. They also contribute to why water does not evaporate instantaneously at 100 °C as the individual particles must overcome these forces to escape.

What needs to happen for a liquid to turn into a gas?

For a liquid to turn into a gas, it’s particles would need to gain enough energy to overcome the intermolecular forces that hold them together. In addition, they would also need to overcome the latent heat of vaporisation which is “the amount of energy required to change a substance from a liquid to a as without a change in temperature”.

This would allow for the individual particles to escape, causing the liquid to change state into a gas.

So, why is the evaporation of water gradual?

This gradual nature can be explained by the Maxwell - Boltzmann distribution which explains the range of kinetic energies that particles have at a given temperature.

While this is centred around gases, this can also be applied to liquids as before water even reaches its boiling point, some of its molecules already have enough energy to vaporise which fits the curves shape.

Y axis : Number of molecules at a given speed/velocity

X axis : Particle speed/velocity

According to this distribution, particles in liquids (and gases) do not all move at the same speed. The curve shows how most particles have a moderate amount of kinetic energy, but a few have higher amounts of kinetic energy meaning they are capable of escaping into the gas phase first.

As the water is heated the entire distribution shifts towards higher energies and there’s a gradual change in the amount of particles which exceed the energy threshold needed to overcome the intermolecular forces and latent heat of vaporisation , causing more evaporation.

This makes it impossible for all particles to evaporate instantaneously at 100 °C!

Further reading

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