MYP Physics: The Particle Model & States of Matter — FAQ

The particle model explains that all matter is made of tiny particles — atoms or molecules — constantly in motion, with the spacing and forces between them determining whether a substance is solid, liquid or gas. In MYP Physics you use it to explain density, compressibility and how substances change state.

Solids have particles packed tightly in fixed positions, vibrating but not moving freely, giving a fixed shape and volume. Liquids have particles close together but able to slide past each other, so they take their container's shape. Gases have particles far apart, moving fast and randomly, filling any available space.

The particle model describes what particles look like and how they're arranged in solids, liquids and gases. Kinetic theory adds the maths — linking particle speed and collisions to temperature, pressure and energy. In MYP 4-5 you mostly use the particle model qualitatively; kinetic theory's quantitative side develops further in DP Physics.

Melting, boiling, freezing and condensing happen when energy is added or removed, changing how fast particles move and how strongly they're held together. Add enough energy and particles overcome the forces holding them in a fixed arrangement; remove energy and particles slow until forces pull them back into a more ordered structure.

This topic is assessed like every MYP Physics unit — through the four sciences criteria, not a single "topic test." Criterion A (Knowing and understanding) covers explaining particle behaviour and state changes. Criterion B and C apply if you design or evaluate an investigation, such as testing melting point. Criterion D covers real-world impacts of thermal properties.

Expect commands like "describe" (say what happens), "explain" (say why, using the particle model), "outline" (give a brief account) and "evaluate" (weigh strengths and limitations, if you're assessing an investigation). Command terms come from the IB's MYP command terms list — mixing them up costs marks even when your science is correct.

The biggest one I see: students describe particle arrangement ("gas particles are far apart") but never explain the cause, missing the "why" that examiners want for the top achievement levels. Others confuse particles expanding with the spaces between particles increasing — particles themselves don't get bigger when a substance is heated.

There's no single "topic grade" in MYP — but to hit the top achievement band (7-8 out of 8) on particle model questions, link observations to the model explicitly: state what particles are doing, explain the energy change involved, and connect it to a real property like density or expansion, using correct terminology throughout.

Conceptually, no — it's one of the more intuitive physics topics because you can see states of matter every day. Where students lose marks isn't understanding, it's precision: mixing up "particles expand" with "spaces between particles increase," or forgetting to mention the energy transfer when describing a state change.

According to the IB, the current DP Physics guide (first exams 2025) has a dedicated theme, "B: The particulate nature of matter," that builds directly on the qualitative particle model taught in MYP. Your child's MYP 4-5 groundwork — describing solids, liquids and gases — becomes the basis for DP's quantitative kinetic theory and gas laws.

Broadly, yes — solids, liquids, gases, particle arrangement and changes of state are near-universal in lower-secondary science, so a student moving between MYP and GCSE or a national curriculum won't find alien content. The difference is emphasis: MYP assesses understanding through inquiry-based criteria A-D rather than solely through recall-based exam questions.

Look for resources built around actual MYP command terms and the four sciences criteria, not generic worksheets — practice explaining state changes using the particle model, not just labelling diagrams. Topical worksheets that mix description questions ("outline") with explanation questions ("explain") mirror how Criteria A and C are actually marked.

Self-study content — revision notes, topical worksheets and mock papers — is typically far cheaper than private tutoring, since it's built once and reused across a whole cohort rather than delivered one-on-one. It also lets your child revisit a tricky topic like this as many times as needed without ongoing session costs.