2.1–2.3The three states of matter
The three states of matter are solid, liquid and gas. Melting and freezing take place at the melting point; boiling and condensing take place at the boiling point.
The three states of matter can be represented by a simple model. In this model, particles are represented by small solid spheres. Particle theory can help to explain melting, boiling, freezing and condensing.
The amount of energy needed to change state from solid to liquid, and from liquid to gas, depends on the strength of the forces between the particles of the substance. The nature of the particles involved depends on the type of bonding and the structure of the substance. The stronger the forces between the particles, the higher the melting point and boiling point of the substance.
That sentence is the hinge of the whole topic. Every property in the rest of this sub-topic — why sodium chloride melts at 801 °C and oxygen at −218 °C — is the same argument applied to different particles and different forces.
Worked example
A substance has a melting point of −7 °C and a boiling point of 59 °C. Predict its state at 20 °C and at 100 °C.
- Below the melting point the substance is solid; between melting and boiling point it is liquid; above the boiling point it is gas.
- 20 °C is above −7 °C, so it has melted.
- 20 °C is below 59 °C, so it has not yet boiled.
- 100 °C is above 59 °C, so it has boiled.
Answer: Liquid at 20 °C and gas at 100 °C. (The substance is bromine.)
You should be able to predict the states of substances at different temperatures given appropriate data, explain the different temperatures at which changes of state occur in terms of energy transfers and types of bonding, and recognise that atoms themselves do not have the bulk properties of materials.
That last point matters more than it looks. Melting point, hardness and conductivity belong to the structure, not to the atom — a single copper atom is not shiny, malleable or a conductor. Those are properties of many atoms bonded together.
Common mistake
When a molecular substance boils, the covalent bonds inside the molecules are not broken — only the forces between the molecules. Water vapour is still H₂O. Saying "the covalent bonds break when water boils" loses the mark.
- States of matter
- solid, liquid and gas; melting and freezing take place at the melting point, boiling and condensing at the boiling point
2.1Limitations of the simple particle modelHigher tier
Limitations of the simple model above include that in the model there are no forces, that all particles are represented as spheres, and that the spheres are solid.
You should be able to explain the limitations of particle theory in relation to changes of state when particles are represented by solid inelastic spheres which have no forces between them.
Common mistake
The limitation is not that the drawing is small or two-dimensional. Answer in terms of the three assumptions the specification names — no forces, spheres, solid spheres — and say what each one prevents the model from explaining.
State symbols
In chemical equations, the three states of matter are shown as (s), (l) and (g), with (aq) for aqueous solutions.
You should be able to include appropriate state symbols in chemical equations for the reactions in this specification.
- (s)
- Solid.
- (l)
- Liquid — a pure liquid, such as water itself.
- (g)
- Gas.
- (aq)
- Aqueous — dissolved in water.
Worked example
Add state symbols to: Mg + 2HCl → MgCl₂ + H₂, where the acid and the salt solution are in water.
- Magnesium is a metal added as a solid ribbon or powder: (s).
- Hydrochloric acid is dissolved in water: (aq).
- Magnesium chloride stays dissolved in the solution: (aq).
- Hydrogen bubbles off as a gas: (g).
Answer: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
Common mistake
(l) and (aq) are not interchangeable. (l) means the pure substance is a liquid; (aq) means it is dissolved in water. Hydrochloric acid is (aq), not (l), because it is hydrogen chloride dissolved in water.
2.4Predicting the state of a substance
Given a melting point, a boiling point and a temperature, you can say what state a substance is in. There are only three cases, and the comparisons are with the temperature you are asked about:
• below the melting point — solid • between the melting point and the boiling point — liquid • above the boiling point — gas.
The trap is negative numbers. −39 °C is below −15 °C, and a question about a substance at room temperature is asking you to compare with 20 °C or 25 °C — the question will tell you which.
Worked example
Bromine melts at −7 °C and boils at 59 °C. What is its state at room temperature, 20 °C? What about at −20 °C?
- At 20 °C: compare with the melting point first — 20 is above −7, so it has melted.
- Compare with the boiling point — 20 is below 59, so it has not boiled.
- At −20 °C: −20 is below the melting point of −7.
Answer: Liquid at 20 °C; solid at −20 °C.
Common mistake
A substance exactly at its melting point is changing state, not in one state — solid and liquid are both present. If a question gives you a temperature equal to the melting or boiling point, say so rather than picking one.
Before you practise
- Solid: regular arrangement, vibrating in fixed positions, least energy. Liquid: close but disordered, moving past one another. Gas: far apart, random and fast, most energy.
- Melting, boiling, freezing and condensing are physical changes — no new substance is made and the change can be reversed.
- Energy is put in to overcome the forces between particles when melting or boiling, and given out when freezing or condensing.
- Sublimation is solid straight to gas, with no liquid stage.
- State symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
- Below the melting point it is a solid; between melting and boiling point a liquid; above the boiling point a gas — watch the negative numbers.
- The simple model treats particles as solid spheres with no forces between them, which is why it cannot explain everything.