Who this is for: Cambridge IGCSE Coordinated Science (0654) students who want criteria of purity — how to tell if a substance is pure — to become a reliable source of marks instead of a melting-point memorisation exercise.
What query it owns: how to understand and revise criteria of purity in Cambridge IGCSE Coordinated Science.
Why this is safe: this page owns the criteria-of-purity revision-guide angle, while Tutopiya’s Criteria Of Purity subtopic page owns the learning resource and the free Criteria Of Purity quiz owns the practice.

How do you know a substance is pure? Cambridge IGCSE Coordinated Science (0654) tests melting and boiling points, chromatography and the idea that a pure substance melts and boils at fixed, sharp temperatures. Impure substances melt over a range and below the expected value. This guide links each test to what examiners reward.

Key takeaways

• A pure substance melts and boils at one fixed temperature (sharp melting/boiling point).
• An impure substance melts over a range of temperatures and usually below the pure value.
• Chromatography separates mixtures; a single spot suggests purity (one component).
• Rf value = distance moved by substance ÷ distance moved by solvent front.
• Pure water boils at 100 °C and melts at 0 °C at standard pressure.

What are criteria of purity in Cambridge IGCSE Coordinated Science?

Criteria of purity are the tests used to decide whether a sample contains one substance or a mixture. Melting and boiling point measurements are the main chemical tests: a pure solid melts sharply at its literature value, while impurities lower the melting point and spread it over a range. Paper chromatography can show whether a sample contains one or several components.

You can read the full explanation, diagrams and notes on Tutopiya’s Criteria Of Purity subtopic page before you attempt questions.

Melting and boiling point — pure vs impure

Property	Pure substance	Impure substance
Melting point	Fixed, sharp (one temperature)	Lower than pure value; melts over a range
Boiling point	Fixed, sharp	Higher or less sharp; may vary
Example	Pure ice melts at exactly 0 °C	Salty ice melts below 0 °C over a range

Chromatography — interpreting results

Observation	What it suggests
Single spot	One component (may indicate purity)
Two or more spots	Mixture of at least two substances
Spot at same height as reference	That component is present
Rf value identical to known pure sample	Same substance likely present

Criteria of purity in past-paper wording: command words that matter

Command word / phrase	What the question wants	Typical purity stem
State	Give a fact or definition	”State what is meant by a pure substance.”
Explain	Link observation to purity	”Explain why the melting point is below 100 °C.”
Describe	Outline a test or method	”Describe how paper chromatography is used to test purity.”
Deduce	Conclude from data	”Deduce whether the sample is pure from the chromatogram.”

Worked exam-style stems (how to answer the wording)

1. “State what is meant by a pure substance.” A substance containing only one type of atom, molecule or ion (or one element or compound with no impurities). Mark-scheme reward: single substance / no other substances present.
2. “A solid melts between 78 °C and 82 °C. Pure benzoic acid melts at 122 °C. Is the sample pure?” No — it melts over a range and below the pure melting point, indicating impurities. Reward: range and comparison to pure value.
3. “In chromatography, a sample gives one spot. What does this suggest?” The sample contains one component (may be pure, but chromatography alone does not prove chemical purity). Reward: one substance / single component.

Test yourself with the Criteria Of Purity quiz once you can interpret melting point data and chromatograms.

How criteria of purity connect to the rest of Coordinated Science

Purity tests link to Methods Of Purification — you purify a substance and then check melting point to confirm success. They also underpin work on Elements, Compounds And Mixtures. The Cambridge IGCSE Coordinated Science resource hub links every Experimental Techniques subtopic.

Common mistakes students make

• Saying a substance is pure because it looks clean or dissolves in water.
• Forgetting that impure solids melt below the pure melting point.
• Confusing a sharp melting point (pure) with a range (impure).
• Thinking chromatography proves purity without considering other possible identical Rf values.
• Stating impure substances melt above the pure value (they melt lower).

When you need more support

If purity and chromatography questions keep costing marks, work through the Criteria Of Purity quiz, then get focused help from a Cambridge IGCSE Coordinated Science tutor.

Frequently asked questions

Is criteria of purity hard in Coordinated Science? The core ideas are straightforward — pure substances have sharp melting and boiling points; impurities cause a range and lower melting point.

How does impurity affect melting point? Impurities lower the melting point and cause the solid to melt over a range of temperatures rather than at one fixed value.

What does one spot on a chromatogram mean? It suggests one component in the sample, but further tests may be needed to confirm full chemical purity.

How do I revise criteria of purity effectively? Learn pure vs impure melting point patterns, practise Rf calculations, then take the Criteria Of Purity quiz.

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