States of Matter and State Symbols

Every substance can exist in three states: solid, liquid and gas. The particle model explains the differences by representing the particles as small solid spheres and describing how they are arranged and how they move.

State	Arrangement	Movement	Shape and volume
Solid	Regular, tightly packed, touching	Vibrate about fixed positions	Fixed shape, fixed volume
Liquid	Close together, irregular	Move around each other	Takes the container's shape, fixed volume
Gas	Far apart, random	Move quickly in all directions	Fills the container, no fixed shape or volume

As you go from solid to liquid to gas, the particles gain energy, move more, and spread further apart.

The particle model treats particles as identical small spheres. It is a simplification, but it explains states, changes of state and why substances differ in melting and boiling point.

The same particles are present in all three states of a substance; only their arrangement and energy change.

A change of state is a physical change: the particles themselves are unchanged, only their arrangement and energy alter. Each change has a name and happens at a fixed temperature for a pure substance.

• Melting (solid → liquid) and freezing (liquid → solid) both happen at the melting point.
• Boiling (liquid → gas) and condensing (gas → liquid) both happen at the boiling point.

        melting            boiling
 SOLID  ------->  LIQUID  ------->  GAS
        <-------          <-------
        freezing          condensing

To change state, energy must be supplied (heating) or removed (cooling). Heating a solid to its melting point gives the particles enough energy to break free of their fixed positions; heating a liquid to its boiling point lets particles escape into the gas state.

Melting and freezing are the same temperature (the melting point), just in opposite directions. The same is true of boiling and condensing (the boiling point).

Because the particles are not changed, a change of state can be reversed by reversing the energy transfer.

You can work out the state of any substance at a given temperature if you know its melting and boiling points. The rule is simple.

• Below the melting point → solid.
• Between the melting and boiling points → liquid.
• Above the boiling point → gas.

Worked example. Substance X has a melting point of −7 °C and a boiling point of 59 °C. What state is it in at 25 °C?

25 °C is above the melting point (−7 °C) and below the boiling point (59 °C), so X is a liquid at 25 °C.

Worked example. Substance Y has a melting point of 801 °C and a boiling point of 1465 °C (these are the values for sodium chloride). What state is it in at room temperature, 20 °C?

20 °C is well below the melting point of 801 °C, so Y is a solid at room temperature.

Compare the given temperature to both the melting point and the boiling point. The state depends on which side of each value the temperature falls.

Different substances melt and boil at very different temperatures. The reason is the strength of the forces between the particles, which depends on the substance's bonding and structure.

Changing state means overcoming the forces holding the particles together. The stronger these forces, the more energy is needed, so the higher the melting and boiling points.

Substance	Bonding / structure	Melting point	Reason
Oxygen (O₂)	Small molecules, weak forces between them	−218 °C	Little energy needed to separate the molecules
Water (H₂O)	Small molecules, stronger forces between them	0 °C	More energy needed than for oxygen
Sodium chloride	Giant ionic lattice, strong forces	801 °C	Strong electrostatic forces throughout the lattice

Explaining a change-of-state temperature in the exam means naming the forces and the energy involved: for example, "sodium chloride has a high melting point because the strong electrostatic forces between the oppositely charged ions require a large amount of energy to overcome."

Note carefully: when a molecular substance melts or boils, it is the weak forces between molecules that break, not the strong covalent bonds inside them.

Balanced chemical equations use state symbols to show the physical state of each substance. They are written in brackets straight after each formula.

Symbol	Meaning
(s)	solid
(l)	liquid (a pure liquid)
(g)	gas
(aq)	aqueous (dissolved in water)

The difference between (l) and (aq) matters: (l) means a pure liquid, while (aq) means the substance is dissolved in water. Water itself is written H₂O(l), but sodium chloride solution is NaCl(aq).

Worked example. Magnesium (a solid) reacts with hydrochloric acid (a solution) to give magnesium chloride solution and hydrogen gas:

$\text{Mg(s)}+2\text{HCl(aq)}\to {\text{MgCl}}_2\text{(aq)}+{\text{H}}_2\text{(g)}$

Worked example. Solid calcium carbonate decomposes on heating into solid calcium oxide and carbon dioxide gas:

${\text{CaCO}}_3\text{(s)}\to \text{CaO(s)}+{\text{CO}}_2\text{(g)}$

Add the correct state symbol to every substance in an equation when asked. Getting (aq) and (l) the wrong way round is a common and avoidable error.

(Higher Tier only) The limitations of the simple particle model are assessed at Higher Tier.

The simple particle model represents particles as small solid spheres, which is useful but not fully accurate. There are two limitations you must be able to state.

• It does not show the forces between the particles. The model draws particles as separate spheres with nothing between them, but in reality there are forces of attraction between particles, and these forces are what determine melting and boiling points.
• It treats all particles as solid, inelastic spheres. Real particles (atoms, molecules and ions) are not simple solid balls; they are not all the same, and they are not inelastic. Modelling them as identical hard spheres ignores their real nature.

A further point the spec makes is that atoms do not have bulk properties. A single atom is not "shiny", "hot" or "a solid" — those are properties of large numbers of particles together, not of one particle.

A single molecule of water is not "wet" and a single copper atom is not "malleable". Bulk properties emerge only from huge numbers of particles interacting.

Recognising these limitations shows you understand the model is an aid to thinking, not a literal description of reality.

1. Mixing up (l) and (aq)

(l) is a pure liquid; (aq) means dissolved in water. Hydrochloric acid is HCl(aq), not HCl(l). Water is H₂O(l).

2. Saying covalent bonds break when a molecular substance boils

When a small molecule boils, the weak forces between molecules are overcome, not the strong covalent bonds inside them. Confusing the two gives the wrong reason for a low boiling point.

3. Forgetting which temperature controls which change

Melting and freezing happen at the melting point; boiling and condensing happen at the boiling point. Do not use "boiling point" for a solid melting.

4. Claiming a single atom has bulk properties

Properties like melting point, colour and conductivity belong to large collections of particles. Do not describe a single atom as a solid or as shiny.

5. Giving the model limitations without the detail

For the Higher Tier limitations question, state both points precisely: the model shows no forces between particles, and it assumes all particles are solid, inelastic spheres.