MYP Chemistry: States of Matter & Changes of State FAQs

Solid, liquid and gas are the three states covered in MYP Chemistry, defined by how closely particles pack together and how freely they move. Solids hold a fixed shape, liquids flow but keep a fixed volume, and gases expand to fill whatever container holds them.

The particle model treats all matter as tiny particles in constant motion, where the state depends on particle energy and the forces holding them together. Heat a substance and particles gain kinetic energy, weakening those forces until particles slide past each other (melting) or break free entirely (evaporating or boiling).

Melting turns a solid to liquid at a fixed melting point; boiling turns a liquid to gas throughout the whole liquid at a fixed boiling point; evaporation turns liquid to gas only at the surface, at any temperature below the boiling point. Examiners love asking you to tell evaporation and boiling apart.

Temperature plateaus during melting or boiling because the energy you add stops speeding particles up and instead breaks the intermolecular forces holding them together — this stored energy is called latent heat. Once the change of state finishes, extra energy goes back into particle motion and temperature starts rising again.

Sublimation is a solid changing directly into a gas without ever becoming liquid; the reverse process, gas to solid, is called deposition. The classic MYP example is solid carbon dioxide, "dry ice", subliming at room temperature — a favourite practical demonstration because there's no messy liquid stage to watch.

Start by identifying the command term — Describe wants what happens, Explain wants why, using particle theory. Name the state change, mention particle arrangement and energy, and use precise vocabulary like melting point and kinetic energy. Full marks usually need you to link energy input to particle movement, not just name the change.

A heating curve plots temperature against time as heat is added at a constant rate: the line rises while a substance stays in one state, then flattens into a plateau at the melting or boiling point while the change happens, then rises again once the change finishes. Interpreting it means matching each section to a state or a change of state.

The command terms used most for this topic are Identify, Describe, Explain and Analyse, each demanding more depth than the last. Identify wants a single fact; Describe wants a full, accurate account; Explain wants reasoning grounded in particle theory; Analyse wants you to break a heating curve or dataset into its parts.

The mistake I see most is students writing that "particles melt" or "particles expand" when it's the substance changing state and the gaps between particles changing, not the particles themselves. A close second is confusing evaporation with boiling, and forgetting temperature holds constant during a change of state rather than rising smoothly.

Practise sketching a full heating curve from memory, then explain each segment aloud in terms of particle energy and forces — if you can teach it, you actually understand it. Pair this with past MYP-style questions and mark your own answers against the command term expected, not just the content covered.

States of matter questions are usually assessed against Criterion A: Knowing and understanding, since you're applying the particle model to explain phenomena, and sometimes Criterion C: Processing and evaluating if you're interpreting heating-curve data from an experiment. The command term on the task sheet signals which strand is in play.

Yes — states of matter and the particle model are core Chemistry content that regularly appear in the on-screen MYP eAssessment for Sciences, often as interactive diagrams or heating-curve tasks where you drag labels onto the correct segment. According to the IB, eAssessment uses on-screen, adaptive-style tasks rather than paper diagrams, so practising in digital format matters.

The particle model your child learns in MYP 4-5 is the direct foundation for DP Chemistry's structure and bonding topics, where phase changes return with quantitative energetics — enthalpy of fusion and vaporisation using q = mcΔT and molar heat. Students who understand why temperature plateaus in MYP rarely struggle with the DP maths.

For MYP 4-5 Chemistry, the resources that help most have clear heating-curve diagrams, particle-arrangement visuals and short questions written to the exact command term used in class, rather than long textbook chapters. Choose materials pitched at MYP 4-5 specifically — DP-level content skips the particle-model groundwork your child still needs.