Changes of State

The diagram below shows the different changes of state that can occur in a substance.

Energy Transfer

A change of state in a substance, such as melting, boiling, evaporating, condensing, or freezing, requires an energy transfer in the form of heat. This heat energy affects the forces of attraction between the particles in the substance.

When heating or boiling a substance, the energy applied breaks some of the forces of attraction between the particles, allowing the substance to change from a solid to a liquid or a liquid to a gas.

The amount of energy required to make this change depends on the strength of the forces of attraction between the particles in the substance. Stronger forces of attraction require more energy to break, which results in a higher melting point and boiling point.

On the other hand, condensing or freezing a substance involves an energy transfer from the substance to the environment. This produces forces of attraction between the particles, causing a change from a gas to a liquid or a liquid to a solid.

The changes of state are physical changes which can be reversed. As the particles themselves do not change, the chemical properties of the substance also remain unchanged.

Different Changes of state


Melting occurs when a solid is heated to its melting point, which causes it to change into a liquid. This happens because the heat energy applied to a solid is transferred to the kinetic energy of its particles. As a result, the particles gain more energy and start to vibrate and move more, which eventually leads to the solid melting.

  • The point at which a solid melts is the melting point, and each pure solid has a unique melting point.


Boiling takes place when a liquid is heated to its boiling point, causing it to change into a gas. As the liquid is heated, bubbles of gas form throughout it, which escape from the surface of the liquid.

  • The point at which a liquid boils is the boiling point, and each pure substance has a unique boiling point.


Evaporation is a process that occurs when a liquid changes into a gas, but it is slower than boiling. Unlike boiling, evaporation only takes place at the surface of the liquid, and it does not produce bubbles.

Evaporation takes place over a range of temperatures, but heating the liquid will speed up the evaporation process. This is because particles move faster at higher temperatures, so they are more likely to break away from the liquid to become a gas.


Sublimation is a process in which a solid changes directly into a gas without passing through the liquid stage. An example of this is the sublimation of iodine, where iodine crystals are heated and change into a purple iodine gas.


Freezing is the opposite of melting, and it occurs when a liquid is cooled. As the temperature decreases, the forces of attraction between particles become stronger, causing the particles to get closer together. The reduced movement of particles and increased forces of attraction cause the liquid to change into a solid.

The temperature at which a liquid changes into a solid is called the freezing point, which is the same as the melting point. For example, water freezes and melts at 0ºC.


Condensation is a process in which a gas changes into a liquid. When a gas is cooled, the particles have less energy, so they get closer together. This causes more forces of attraction to form between particles, which groups them together to form a liquid.


Limitations of the Particle Model

The particle model is useful for explaining the behaviour of matter, but it has its limitations. This model states that matter is made up of tiny particles in constant motion.

Let’s look at some of the theory’s limitations:

  • The particle model is a simplified representation of the complex nature of matter. The model generalises all particles as being the same, regardless of the substance they make up. It fails to recognise that particles can have different sizes, shapes, masses, and different forces of attraction between them.
  • Does not take into account the intermolecular forces that exist between different particles in different substances. As a result, it does not fully explain the differences in boiling and melting points between different substances, which are largely determined by intermolecular forces.