Pure Substances, Mixtures and Separation
MYP Year 5 Chemistry — Pure Substances, Mixtures and Separation
This topic is really one question asked six different ways: how do you know what you're looking at, and how do you pull it apart? Examiners test it almost entirely through data — a melting point range, a solubility curve, a chromatogram — so the skill isn't memorising definitions, it's reading evidence and matching it to the right classification or technique. Get the sharp-vs-range criterion for purity solid first; everything else in this chapter builds on it.
Overview: Purity, Mixtures and Separation
Chemistry starts by sorting matter into two boxes: pure substances (elements and compounds — one type of particle, fixed composition, constant physical properties) and mixtures (two or more substances physically combined in a variable ratio, each keeping its own properties). Everything else in this chapter — solubility, filtration, distillation, chromatography — is really just a toolkit for telling the two apart and for un-mixing mixtures back into their pure components.
- The single most-tested skill: use melting/boiling point behaviour (sharp value vs range) as evidence of purity, not appearance.
- Separation technique choice always comes back to one question: what physical property differs between the components? (particle size, magnetism, boiling point, solubility, colour/adsorption).
- MYP loves ‘classify and justify’ questions — a bare label (‘mixture’) with no reasoning tied to data scores zero for the justification mark.
The shape of the chapter
Command terms that drive the marks in this topic
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| Define | Give the precise scientific meaning, not an example. | Criterion A | Must include ‘fixed composition’ and ‘one type of particle/element or compound’ — an example alone (e.g. ‘like water’) earns 0. |
| Classify | Assign to a category (element / compound / mixture) using a stated criterion. | Criterion A | Category alone is often only half the mark — the criterion used (mp range, composition) must be named. |
| Justify / Explain | Give reasoning that links evidence to a conclusion. | Criterion A/D | ‘Because it's pure’ restates the conclusion — must cite the actual evidence (e.g. ‘the 0.5°C range matches a single substance’). |
| Deduce | Reach a conclusion using ONLY the data given in the question. | Criterion B | Bringing in outside numbers instead of the table provided loses the mark even if the final answer is numerically right. |
| Distinguish | State the point(s) of difference between two named things. | Criterion A | Describing each thing separately without a direct contrast scores lower than a stated discriminator. |
| Describe | Give a detailed, accurate account without justifying why. | Criterion A | Adding unrequested explanation doesn't gain extra credit but also isn't penalised — however skipping detail (units, conditions) loses marks. |
Key point
Overview