On heating particles gain kinetic energy and move faster. In evaporation and boiling the highest kinetic energy molecules can ‘escape’ from the attractive forces of the other liquid particles. The particles lose any order and become completely free to form a gas or vapour. Energy is needed to overcome the attractive forces in the liquid and is taken in from the surroundings.


This means heat is taken in, so evaporation or boiling are endothermic (require heat to be added) processes. If the temperature is high enough boiling takes place. Boiling is rapid evaporation anywhere in the bulk liquid and at a fixed temperature called the boiling point and requires continuous addition of heat.


The rate of boiling is limited by the rate of heat transfer into the liquid. Evaporation takes place more slowly at any temperature between the melting point and boiling point, and only from the surface, and results in the liquid becoming cooler due to loss of higher kinetic energy particles.


Condensing (gas to liquid)

  • On cooling, gas particles lose kinetic energy and eventually become attracted together to form a liquid.
  • There is an increase in order as the particles are much closer together and can form clumps of molecules.
  • The process requires heat to be lost to the surroundings i.e. heat given out, so condensation is exothermic.

This is why steam has such a scalding effect, it’s not just hot, but you get extra heat transfer to your skin due to the exothermic condensation on your surface!


Melting (solid to liquid)

When a solid is heated the particles vibrate more strongly as they gain kinetic energy and the particle attractive forces are weakened.

  • Eventually, at the melting point, the attractive forces are too weak to hold the particles in the structure together in an ordered way and so the solid melts.
  • The particles become free to move around and lose their ordered arrangement. Energy is needed to overcome the attractive forces and give the particles increased kinetic energy of vibration.
  • So heat is taken in from the surroundings and melting is an endothermic process.


Freezing (liquid to solid)

On cooling, liquid particles lose kinetic energy and so can become more strongly attracted to each other.

Eventually at the freezing point the forces of attraction are sufficient to remove any remaining freedom and the particles come together to form the ordered solid arrangement.

Since heat must be removed to the surroundings freezing is an exothermic process.



This is when a solid, on heating, directly changes into a gas, and the gas on cooling re-forms a solid directly.

Theory in terms of particles:

When the solid is heated the particles vibrate with increasing force from the added thermal energy.

If the particles have enough kinetic energy of vibration to partially overcome the particle-particle attractive forces you would expect the solid to melt. However, if the particles have enough energy at this point that would have led to boiling, the liquid will not form and the solid turns directly into a gas.


Overall, this is an endothermic change as energy absorbed and ‘taken in’ to the system. On cooling, the particles move slower and have less kinetic energy. Eventually, when the particle kinetic energy is low enough, it will allow the particle-particle attractive forces to produce a liquid.

But the energy may be low enough to permit direct formation of the solid, i.e. the particles do not have enough kinetic energy to maintain a liquid state! Overall this is an exothermic change, energy released and ‘given out’ to the surroundings.



  • Anything that has mass and occupies space (has volume)
  • Matter is composed of particles (molecules, ions, atoms)

Spaced apart and seen with scanning electron microscope

  • Are in constant motion attracting one another with inter-particle forces (or cohesive)
  • Strength of interparticle force and space between particles determines the state.


See also






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