OCR GCSE Chemistry A: The particle model (J248)

This study guide covers sub-topic C1.1 The particle model, from Topic C1 Particles, in OCR GCSE (9–1) Chemistry A (Gateway Science). It follows the J248 specification (Version 4.0, August 2026) and teaches learning outcomes C1.1a to C1.1c, the mathematical learning outcome CM1.1i and the practical suggestion listed for this sub-topic. C1.1a and C1.1b are assessed on both tiers. C1.1c is Higher Tier only. Foundation candidates meet Topic C1 in Paper 1 and Higher candidates in Paper 3; Papers 2 and 4 assume you know it.

After this guide, use the particle model revision notes for recall and the particle model practice set to check yourself. Next comes atomic structure, C1.2. Course-wide pages: the J248 printable checklist and the OCR GCSE Chemistry hub page. Not sure what you already know? Our free diagnostics take about ten minutes.

What this sub-topic covers

Spec reference	What you must be able to do	Tier
C1.1a	Describe the main features of the particle model in terms of states of matter and change of state	Both tiers
C1.1b	Explain, using the particle model, the difference between physical changes and chemical changes	Both tiers
C1.1c	Explain the limitations of the particle model for changes of state when particles are shown as inelastic spheres (like bowling balls): it ignores the forces of attraction between particles, the size of the particles and the space between them	Higher Tier only
CM1.1i	Represent three-dimensional shapes in two dimensions and vice versa when looking at chemical structures, for example allotropes of carbon (maths skill M5b)	Both tiers
Practical suggestion	Observations of change of state, compared with chemical changes	Both tiers

The specification also links this content to working scientifically skills Working Scientifically 1.1a, Working Scientifically 1.1b and Working Scientifically 1.1c (theories developing, using models, and the power and limitations of science).

C1.1a: The particle model and states of matter

In the particle model, everything is built from tiny particles, which may be atoms, molecules or ions. It explains the three states by how the particles are arranged, how they move and how close they are.

	Solid	Liquid	Gas
Arrangement	Regular, fixed pattern	Random, irregular	Random
Spacing	Touching, closely packed	Mostly touching, close	Far apart, with empty space between
Movement	Vibrate on the spot	Slide past one another	Dart about fast and randomly
Particle energy	Least	Middle	Greatest
Shape and volume	Fixed shape, fixed volume	Takes the container’s shape, fixed volume	Fills the container
Compressible?	No	Hardly	Yes, easily

Two ideas trip people up. First, the space between gas particles is empty, not filled with air. Second, the particles themselves do not change when a substance is heated. They do not swell or melt. They gain energy and move faster. A metal bar expands because its particles vibrate more and take up more room, not because each particle gets bigger.

Changes of state

Process	Start state → end state	Energy
Melting	solid → liquid	taken in
Freezing	liquid → solid	given out
Boiling or evaporating	liquid → gas	taken in
Condensing	gas → liquid	given out
Subliming	solid → gas directly	taken in

Evaporation happens at a liquid’s surface at any temperature; boiling happens throughout the liquid at its boiling point.

Changes of state are reversible. Cool water vapour and you get liquid water; cool that and you get ice. The particles are the same water molecules throughout, and the mass does not change, because no particles are gained or lost.

Worked example 1: deciding a state from data

Use these values. At what state is each substance at 20 °C, and which one changes state if it is warmed to 60 °C? Describe what happens to its particles.

Substance	Melting point / °C	Boiling point / °C
P	−62	48
Q	115	445
R	−218	−183
S	770	1420

Step 1. Below the melting point a substance is solid; between the melting and boiling points it is liquid; above the boiling point it is gas.

Step 2. At 20 °C: P is a liquid (20 is between −62 and 48). Q is a solid (20 is below 115). R is a gas (20 is above −183). S is a solid.

Step 3. At 60 °C, P is above its boiling point, so P boils and becomes a gas. Q, R and S stay in the same state.

Step 4. For P, the particles take in energy, move faster and spread far apart, moving quickly and randomly. The particles themselves stay the same.

Watch the negative numbers: −183 is higher than −218.

C1.1b: Physical changes and chemical changes

The particle model gives you a clear test. Ask what happens to the particles themselves.

• In a physical change, the particles stay the same. Only their arrangement, spacing or movement changes. No new substance forms. Changes of state and dissolving are physical changes. They are usually easy to reverse.
• In a chemical change, atoms are rearranged to make new particles. Bonds between atoms break and new bonds form, so at least one new substance forms with different properties. The atoms are not created or destroyed, so the total mass of everything in a closed system stays the same.

The particle model also explains what elements and compounds are. An element is made of only one type of atom. In a chemical change, atoms of different elements can join to form a compound, and a compound’s atoms can separate and recombine into different substances.

Do not use “can it be reversed?” as your only test. Some chemical changes can be reversed. The deciding question is whether new particles have formed.

Worked example 2: one candle, two kinds of change

A candle is lit. The wax near the wick turns to a clear liquid, some liquid wax rises up the wick and becomes a vapour, and the vapour burns. When the candle is blown out, the clear liquid sets white again. Classify each change and explain using particles.

Melting of the wax (physical). The wax particles gain energy, leave their fixed positions and move past each other. When the candle is blown out they lose energy and the wax sets. Same particles, so no new substance.

Wax liquid becoming vapour (physical). The wax particles gain more energy and move apart as a gas. They are still wax particles.

Burning of the vapour (chemical). The wax molecules react with oxygen from the air. Their atoms are rearranged into new particles: carbon dioxide molecules and water molecules. These are new substances, and the change gives out energy as heat and light. You cannot get the wax back by cooling the products.

The practical suggestion: observing both kinds of change

The specification suggests observing changes of state and comparing them with chemical changes. A typical lesson setup:

1. Gently warm a small piece of wax in a test tube in a water bath. Watch it melt, then let it cool and set. Note that it looks and behaves the same afterwards.
2. Heat a small amount of green copper(II) carbonate in a test tube over a Bunsen burner. It turns black, and the gas given off turns limewater cloudy. On cooling, the black solid does not turn green again.

The wax shows a physical change: same substance before and after. The copper(II) carbonate shows a chemical change: a new black solid (copper(II) oxide) and a new gas (carbon dioxide) form. Wear eye protection, and take the delivery tube out of the limewater before the heating stops, so no suck-back happens.

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

The simple model draws particles as inelastic spheres, like bowling balls: hard, solid, all alike and bouncing without changing. This is useful for describing the states, but OCR names three things it does not take into account.

1. Forces of attraction between particles. Bowling balls do not pull on each other. Real particles do attract each other. The model therefore cannot explain why energy is needed to melt or boil a substance, or why different substances melt and boil at different temperatures. A substance with stronger attractions between its particles needs more energy to separate them.
2. The size of the particles. The model shows every particle as the same size. Real particles have different sizes. A molecule of wax is much larger than a molecule of water, and an ion is a different size from an atom.
3. The space between particles. Diagrams usually draw gas particles only two or three particle-widths apart, so that they fit on the page. In reality the gaps are far larger.

Nor does the model explain why different materials have different properties. That needs the idea that the particles, and how they are held together, differ between substances (Topic C2).

Worked example 3: how far apart are gas particles?

1 g of liquid water takes up about 1 cm³. As steam at 100 °C and normal atmospheric pressure, the same 1 g takes up about 1,700 cm³. A textbook diagram shows water particles as circles 4 mm across. Estimate how far apart the centres of steam particles should be on a to-scale diagram, and comment on the textbook diagram.

Step 1. Volume ratio = 1700 ÷ 1 = 1,700.

Step 2. Volume depends on distance cubed, so the distance between particles grows by the cube root: ∛1700 ≈ 11.9, or about 12.

Step 3. In the liquid, particles are roughly touching, so their centres are about one particle-width apart. In the steam, centres are about 12 particle-widths apart: 12 × 4 mm = 48 mm, nearly 5 cm.

Step 4. Across a 210 mm wide sheet of A4, only about four steam particles would fit in a row. A diagram with gas particles a few millimetres apart shows the spaces far too small. The particles themselves fill only about 1/1700 of the volume, roughly 0.06%.

CM1.1i: Moving between three dimensions and two

Real substances are three-dimensional, but you draw them on flat paper. You need to read a 2D diagram as a 3D structure and the other way round.

Worked example 4: a model crystal

A model of a solid is made as a cube of balls, 4 balls along each edge. How many balls are there? How many does a 2D drawing of one face show? How many balls are completely hidden inside?

Step 1. Total balls = 4 × 4 × 4 = 64.

Step 2. One face is a 4 × 4 square, so a 2D drawing shows 16. A 2D diagram of a solid shows only one layer.

Step 3. The hidden inner cube is 2 balls along each edge (remove one layer from each side): 2 × 2 × 2 = 8. So 64 − 8 = 56 balls are on the surface.

The same skill applies to the allotropes of carbon named in CM1.1i. A flat drawing of graphite shows one layer of hexagons, but graphite is many stacked layers. A 2D sketch of diamond hides the fact that its carbon atoms are joined in three dimensions. You study these in Topic C2.

How the model was developed

Evidence for particles built up over time (Working Scientifically 1.1a). In 1827 the botanist Robert Brown saw tiny particles from pollen grains jiggling randomly in water under a microscope. In 1905 Albert Einstein explained this as the particles being struck by moving water molecules, and Jean Perrin’s experiments in 1908–1909 confirmed his predictions. New instruments gave new evidence, and the model gained support. The model is also used to make predictions (Working Scientifically 1.1b), such as which state a substance will be in at a given temperature.

Common errors

• Saying particles “expand”, “melt” or “get hotter” when a substance is heated. The particles gain energy and move more; the substance expands.
• Saying there is air between gas particles, or that liquid particles are “far apart”.
• Calling dissolving a chemical change.
• Using reversibility alone to decide between physical and chemical change.
• For Higher Tier, listing limitations of the model without saying what the model leaves out: forces, sizes, spaces.

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.