MYP Science States of Matter: Frequently Asked Questions

Solid, liquid and gas are the three states MYP Science focuses on, though some units in MYP 3 mention plasma too. Each state is defined by how its particles are arranged, how much they move, and how much energy they hold — not by what the substance actually is.

Particle theory says all matter is made of tiny particles in constant motion, and the state of a substance depends on how much energy those particles have and how strongly they're attracted to each other. In MYP Science, almost every mark on states of matter questions comes from applying this theory correctly.

You explain a change of state by describing what happens to particle energy and spacing, in order. For melting: solid particles gain energy, vibrate faster, break free from fixed positions, and start to move past one another as the substance becomes liquid.

Boiling happens throughout the whole liquid at one fixed temperature (the boiling point), while evaporation happens only at the surface, at any temperature below boiling point, and is slower. Both are changes from liquid to gas — the difference is where and how fast the particles escape.

Heating gives particles more kinetic energy, so they vibrate or move faster and push further apart, increasing the space between them. The particles themselves don't get bigger — the gaps between them do. This is why the same mass of gas takes up far more volume than the same mass of liquid or solid.

Answer by linking the observation to particle behaviour: state what happens to particle energy, spacing and arrangement, then connect that to the property being asked about (shape, volume, compressibility). A full-mark answer names the state change, describes particle movement, and explains the energy transfer causing it — in that order.

The most common command terms are 'describe', 'explain' and 'compare'. Describe means state what happens with no reasoning; explain means give reasons using particle theory; compare means identify similarities and differences between two states or substances, not just list facts about each separately.

The biggest mistake is describing what's observed ('the ice melted') without explaining why, using particle behaviour. Others include confusing evaporation with boiling, saying particles themselves expand instead of the spaces between them, and forgetting that mass stays constant during a state change even though volume can.

States of matter is usually assessed against Criterion A (Knowing and understanding), which asks students to apply scientific knowledge and explain phenomena using scientific reasoning. According to the IB's MYP: Sciences guide, each of the four sciences criteria — A, B, C and D — is marked out of a maximum of 8 in each year of the programme.

States of matter is normally introduced in MYP 1, with particle theory, changes of state and simple particle diagrams as the core content. MYP 2 and 3 build on this with more detail on energy transfer, gas behaviour and pressure, and link it to chemistry topics like the kinetic theory used later in DP Sciences.

The most effective revision is active, not passive: get your child to draw particle diagrams for each state and change of state, then explain each one out loud using the words 'energy', 'spacing' and 'forces of attraction'. If they can't explain it verbally, they can't yet write it in an exam.

Particle theory from MYP states of matter is the direct foundation for kinetic theory in DP Physics, phase changes and enthalpy in DP Chemistry, and osmosis and diffusion in DP Biology. Students who understand particle spacing and energy properly in MYP 1-3 find these DP topics far easier to pick up quickly.