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

Particles in a solid, tightly packed together in order.

The particles in a solid:

  • are in a regular arrangement
  • vibrate about a fixed position
  • sit very closely together
Particles in a liquid, packed together, very fluid.

The particles in a liquid:

  • are randomly arranged
  • move around each other
  • sit close together
Particles in a gas, random order, not together.

The particles in a gas:

  • are randomly arranged
  • move very quickly in all directions
  • are far apart

Changing state

Adding or removing energy from a material can change the state. Heating a solid material will cause it to melt from a solid to a liquid. Continued heating will cause the liquid to boil or evaporate to form a gas. In some instances, when heating the solid material, it can go straight to being a gas without being a liquid - this process is called sublimation.

Cooling a gas will cause it to condense from a gas to a liquid and cooling it further will cause it to then freeze from a liquid to a solid.

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

Boiling is an active process. It occurs at a certain temperature, called the boiling point. People actively apply energy to a liquid to turn it into a gas using a heater such as a kettle.

Evaporation, on the other, hand is a passive process. The liquid will slowly absorb energy from the surrounding area so that some of its particles will gain enough energy to escape from the surface of the liquid and it can occur at any temperature.

Throughout all of these changes, the number of particles has not changed, just 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.

curriculum-key-fact
With changes of state, mass is conserved. It does not change.

These changes in state are called physical changes because the process can be reversed (eg cooling instead of heating). This is different to the changes seen in a chemical reaction when the changes cannot be reversed so easily.

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