States of matter

In everyday life, there are three states of matter - solids, liquids and gases. The differences between the three states are due to the arrangement and spacing of the particles, and their motion.

Solids, liquids and gases

The particles in a solid:

  • are in a regular arrangement
  • vibrate about a fixed position
  • sit very closely together
Particles in a solid, tightly packed together in order.

The particles in a liquid:

  • are randomly arranged
  • move around each other
  • sit close together
Particles in a liquid, packed together, very fluid.

The particles in a gas:

  • are randomly arranged
  • move quickly in all directions
  • are far apart
Particles in a gas, random order, not together.

Changing state

Transferring energy to or from a substance can change its state. Heating a substance in the solid state will cause it to melt, which changes it to the liquid state. Continued heating will cause the substance to evaporate or boil, which changes it to the gas state.

Some substances in the solid state can change straight to the gas state. This process is called sublimation.

A substance in the gas state condenses when it is cooled, which changes it to the liquid state. Continued cooling causes it to freeze, which changes it to the solid state.

Flow chart showing processes between solid, liquid and gas, using water, ice and steam from a kettle as an example. Labels show all the processes in how one can change to another.Water changing state

Evaporation and boiling

During evaporation, particles with enough energy can escape the liquid as a gas. Unless energy is transferred from the surroundings by heating, the liquid gradually becomes colder. Boiling happens when enough energy is transferred that the substance evaporates as fast as it can.

Conservation of mass

The number of particles does not change during a change of state, only their spacing and arrangement. As a result, the total mass has not changed. It does not matter if a substance melts, freezes, boils, evaporates, condenses or sublimates - the mass does not change.

These changes in state are called physical changes because the process can be reversed. For example, ice is water in the solid state:

  • ice melts to form water in the liquid state when it is heated
  • water in the liquid state becomes ice when it is cooled down

This is different from the changes seen in a chemical reaction, when the changes cannot be reversed so easily.