OCR GCSE Chemistry A: The particle model (J248) – Revision Notes

These notes condense sub-topic C1.1 The particle model in OCR GCSE (9–1) Chemistry A (Gateway Science), following the J248 specification (Version 4.0, August 2026). The particle model study guide has the full explanations and longer examples. The notes cover C1.1a, C1.1b and C1.1c, the maths outcome CM1.1i and the practical suggestion. C1.1c is Higher Tier only; everything else is on both tiers. Topic C1 is examined in Paper 1 at Foundation Tier and Paper 3 at Higher Tier.

When you are ready, try the particle model practice set, then go on to atomic structure, C1.2. Tick off progress on the J248 checklist, browse the OCR GCSE Chemistry hub page, and find weak spots with our free diagnostics.

Key definitions

• Particle model: everything is built from tiny particles, which may be atoms, molecules or ions. Their arrangement, movement and spacing explain the states of matter.
• Change of state: a substance turning from one state to another, such as solid to liquid. It is a physical change and it is reversible.
• Physical change: the particles stay the same. No new substance forms.
• Chemical change: atoms are rearranged into new particles. At least one new substance forms.
• Element: a substance made of only one type of atom.
• Compound: a substance made of atoms of different elements joined together.
• Inelastic sphere model (Higher Tier only): drawing particles as hard, identical balls, like bowling balls.

C1.1a: States of matter at a glance

Feature	Solid	Liquid	Gas
Arrangement	Regular	Random	Random
Closeness	Touching	Close, mostly touching	Far apart
Movement	Vibrate in place	Slide past each other	Fast, in all directions
Energy	Least	More	Most
Shape	Fixed	Takes the container’s shape	Fills the container
Volume	Fixed	Fixed	Fills the container

The gaps between gas particles are empty space. Heating gives particles more energy, so they move faster. The particles do not get bigger.

Changes of state

Name	Direction	Energy taken in or given out
Melting	solid → liquid	taken in
Freezing	liquid → solid	given out
Boiling, evaporating	liquid → gas	taken in
Condensing	gas → liquid	given out
Subliming	solid → gas	taken in

Mass stays the same in a change of state because no particles are gained or lost.

Method in steps: state from melting and boiling points

1. Write the temperature next to the melting point (mp) and boiling point (bp).
2. Temperature below mp → solid.
3. Temperature between mp and bp → liquid.
4. Temperature above bp → gas.
5. Take care with negative values: −40 is higher than −80.

Worked reminder. A substance has mp 6 °C and bp 81 °C. At 2 °C it is solid; at 25 °C it is liquid; at 95 °C it is gas. On a cold day of 4 °C it would be solid.

Method in steps: describing a change of state with particles

1. Say whether energy is taken in or given out.
2. Say how the movement changes (faster or slower).
3. Say how the arrangement and spacing change.
4. Say that the particles themselves stay the same.

C1.1b: Physical or chemical?

	Physical change	Chemical change
Particles	Same particles, rearranged or spread out	New particles made from rearranged atoms
Atoms	Not rearranged into new combinations	Bonds broken and new ones made
New substance?	No	Yes
Mass in a closed system	Unchanged	Unchanged
Examples	Melting, boiling, condensing, dissolving	Burning, rusting, thermal decomposition

Must-know distinction. Reversibility is not the test. The test is whether new particles, and so new substances, form.

Signs of a chemical change you might observe: a gas given off, a permanent colour change, a new solid forming, or energy given out as heat or light.

Worked reminder. Natural gas (methane) can be cooled until it becomes a liquid for transport by ship. This is physical: the methane molecules lose energy and move closer together, but each one is still a methane molecule. When the same gas burns in a cooker, it is chemical: the atoms in methane and oxygen molecules are rearranged into new carbon dioxide and water molecules.

Misconceptions the specification highlights

• Matter is not continuous. It is made of separate particles with space between them.
• The space between gas particles is not filled with anything.
• Particles do not expand when heated.
• Changes of state are reversible: the same substance can be melted and frozen again and again.

Practical suggestion: change of state vs chemical change

Melt a little wax in a warm water bath, then let it set again. It is the same wax afterwards, so the change is physical. Then heat green copper(II) carbonate: it turns black and gives off a gas that turns limewater cloudy. The black solid stays black on cooling. New substances (copper(II) oxide and carbon dioxide) have formed, so the change is chemical.

C1.1c: Limitations of the model (Higher Tier only)

The inelastic sphere model does not take into account:

Missing idea	What is really true	What the model cannot explain
Forces of attraction	Particles attract each other	Why energy is needed to melt or boil, and why melting points differ
Size of particles	Particles differ in size	Why some substances are made of large molecules and others of small ones
Space between particles	Gas particles are very far apart	Diagrams show gases much too crowded

Estimating spacing. If a gas takes up N times the volume of the same mass of liquid, its particles are about ∛N times further apart. For example, N = 1,000 gives ∛1000 = 10.

The model also cannot explain why different substances have different properties. That needs ideas about bonding and structure from Topic C2.

CM1.1i: 3D to 2D

• A cube of n × n × n particles contains n³ particles.
• A drawing of one face shows n² particles.
• The fully hidden inner cube has (n − 2)³ particles.
• A flat drawing of a solid shows only one layer. Real structures, such as the carbon allotropes graphite and diamond, extend in three dimensions.

Worked reminder. A cube 5 particles along each edge: 5³ = 125 in total, 5² = 25 on one face, 3³ = 27 hidden inside.

How evidence built up (Working Scientifically 1.1a)

Robert Brown described the random jiggling of tiny particles from pollen grains in water in 1827. Einstein explained it in 1905 as the particles being hit by moving water molecules, and Perrin’s experiments in 1908–1909 confirmed this. A model gains support when it explains new observations.

Quick self-test

1. What is a gas turning into a liquid called?
2. Give two differences between the particles in a solid and in a liquid.
3. A substance has mp −64 °C and bp 61 °C. State its state at 90 °C.
4. True or false: the particles in a gas have air between them.
5. Classify each as physical or chemical: (a) chocolate melting; (b) bread turning black in a toaster.
6. Ice is melted in a sealed bag. Explain why the mass does not change.
7. Why is “it can be reversed” not enough to call a change physical?
8. In which state do the particles of a substance have the most energy?
9. (Higher Tier only) Name the three things the inelastic sphere model does not take into account.
10. (Higher Tier only) A gas occupies 125 times the volume of the same mass of liquid. About how many times further apart are its particles?
11. A cube model is 3 particles along each edge. How many particles are there, how many show on one face, and how many are hidden?

Answers

1. Condensing (condensation).
2. Any two: solid particles are in a regular pattern, liquid particles are random; solid particles only vibrate, liquid particles move past each other; solid particles have less energy.
3. Gas (90 °C is above 61 °C).
4. False. The space between gas particles is empty.
5. (a) Physical: the same particles, just able to move past each other. (b) Chemical: new substances, including carbon, form.
6. The same particles are present before and after; none are added or lost.
7. Some chemical changes can be reversed. The test is whether new particles form.
8. The gas.
9. The forces of attraction between particles, the size of the particles and the space between them.
10. ∛125 = 5 times.
11. 3³ = 27 in total; 9 on one face; (3 − 2)³ = 1 hidden.

Where marks are usually lost

• Writing that particles “expand”, “melt” or “get hot”. Write that they gain energy and move faster.
• Describing liquid particles as “far apart”. They are close together.
• Describing only arrangement when the question asks about movement and spacing too.
• Mixing up which changes of state take in energy and which give it out.
• Misreading negative temperatures when using mp and bp data.
• Saying a change is chemical “because it cannot be reversed”, with no mention of new substances or particles.
• Calling dissolving a chemical change.
• (Higher Tier only) Saying “the model is too simple” without naming the missing forces, sizes or spaces.
• (Higher Tier only) Multiplying instead of taking a cube root when going from a volume ratio to a spacing ratio.

Official syllabus

J248 is the code for OCR GCSE (9–1) Chemistry A (Gateway Science); this page follows its specification, Version 4.0 (August 2026), for first assessment in 2018 (publisher: OCR); unit C1.1 The particle model.