States of Matter and Changes of StateAQA GCSE Chemistry: Revision notes

The three states of matter — solid, liquid and gas — can be explained using particle theory, based on how particles are arranged and how much energy they have:

Changes of state (melting, freezing, boiling, condensing, subliming) involve a transfer of energy and depend on the strength of the forces (bonding) holding the particles together.

State symbols are used in balanced symbol equations to show the physical state of each substance:

For example: NaCl(s) + aq → Na+(aq) + Cl−(aq)

Ionic compounds form giant ionic lattices, held together by strong electrostatic forces of attraction acting in all directions between oppositely charged ions.

Because so many strong forces must be overcome, ionic compounds have high melting and boiling points. They only conduct electricity when molten or dissolved in water, because only then are the ions free to move and carry charge — in the solid state, the ions are held in fixed positions.

Substances made of small molecules (simple molecular substances) have only weak intermolecular forces between molecules, even though the covalent bonds within each molecule are strong.

Because only the weak intermolecular forces need to be overcome to change state, these substances have low melting and boiling points. Larger molecules have higher melting and boiling points, since their intermolecular forces are stronger.

Simple molecular substances do not conduct electricity, as there are no free ions or delocalised electrons.

In giant covalent structures (e.g. diamond, silicon dioxide), every atom is joined to its neighbours by strong covalent bonds throughout the whole structure — not confined to small, separate molecules.

Because so many strong covalent bonds must be broken to melt the substance, giant covalent structures have very high melting points, often the highest of any substance type.