M.1 Particle Theory & States of Matter

UNIT SYLLABUS

MYP

Everything around you is made of particles too small to see, and their arrangement and motion explain the difference between solids, liquids and gases. Particle theory is chemistry's first big model: solids have particles locked in fixed positions vibrating in place; liquids let particles slide past one another; gases have particles flying freely with huge spaces between them. Heating gives particles energy — melting, boiling, evaporation and diffusion all follow directly from this single picture.

Guiding Questions

? How can one simple model explain melting ice, boiling kettles and the smell of perfume across a room?
? What actually changes when a substance changes state — and what stays the same?

✓ What the IB expects you to master

Describe the arrangement, motion and energy of particles in solids, liquids and gases.
Use particle theory to explain melting, freezing, boiling, evaporation, condensation and sublimation.
Explain that temperature stays constant during a change of state as energy rearranges particles.
Distinguish evaporation (surface, any temperature) from boiling (throughout, at boiling point).
Explain diffusion in liquids and gases, and why it is faster at higher temperatures.
Calculate density with $\rho = \frac{m}{V}$ and predict floating or sinking.
Interpret heating and cooling curves, identifying the flat sections as state changes.

1 Key Formulas

Density

\rho = \frac{m}{V}

2 Exam Preparation & Topic Explanations

Particle diagrams that earn full marks (Criterion A)

When asked to draw particle diagrams: solids get regular, touching rows; liquids get touching but disordered particles; gases get widely spaced, random particles. Keep particle SIZE the same in all three — particles do not shrink or swell between states, only their spacing and arrangement change.

For explain questions, always connect three things: energy of particles → motion of particles → observable property.

Pro Exam Strategy

Never say particles "expand" when heated — the spacing grows, not the particles.
Evaporation vs boiling is a classic: surface-only vs throughout, any temperature vs fixed temperature.
On heating curves, label the plateaus with the state change happening there.
Density questions: watch the units — g/cm³ and kg/m³ differ by a factor of 1000.

3 MCQ Practice

Q1. During boiling, the temperature of a liquid stays constant because the energy supplied is used to:

Increase the speed of the particles
Overcome the forces between particles
Break the particles into atoms
Heat the container

Q2. A gas is compressed into half its volume at constant temperature. The particles now:

Move faster
Hit the container walls more often
Become smaller
Stick together

Q3. A drop of food colouring spreads through cold water in 40 minutes but through hot water in 5 minutes. This is because in hot water:

The dye particles dissolve better
The water particles move faster, so diffusion is quicker
Convection replaces diffusion
The dye particles become lighter

4 Short Answer Questions

PDF

Download the practice worksheet

All questions from this topic + answer key — free, printable.

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MYP Physics Guide →

The companion topic guide.

Criterion B Guide →

Design investigations that score.

Daily Question →

Practise one concept a day.

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M.2 Pure Substances, Mixtures & Separation

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