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A pure substance melts at a sharp, fixed temperature, while an impure one melts over a range, usually starting lower. Free notes and 9 questions answered.
Using Melting and Boiling Points to Test Purity is part of Purity, formulations and chromatography in GCSE AQA Higher Chemistry.
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A pure substance melts at a sharp, fixed temperature, while an impure one melts over a range, usually starting lower. Free notes and 9 questions answered.
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Use this lesson to stay inside Purity, formulations and chromatography while connecting the detail back to the wider Topic 8: Chemical analysis topic.
Read the core explanations from Using Melting and Boiling Points to Test Purity before testing yourself from memory.
Here's a puzzle: you heat ice and it melts at exactly 0°C. You heat pure water and it boils at exactly 100°C. But add salt to that water, and suddenly it boils at 102°C. What changed?
This isn't just kitchen chemistry - it's how forensic scientists identify unknown substances and how pharmaceutical companies check their medicines are pure. The secret lies in understanding that pure substances behave predictably, while mixtures don't.
Think of melting and boiling points as a substance's ID card. Pure water always melts at 0°C and boils at 100°C (at standard pressure). Pure ethanol melts at -114°C and boils at 78°C. These values never change for pure substances.
The key word is 'sharp'. When you plot temperature against time for a pure substance melting, you get a flat line - the temperature stays constant throughout melting. This happens because all the energy goes into breaking bonds, not increasing temperature.
Impurities are like party crashers - they disrupt the orderly process of melting and boiling. Instead of sharp, fixed temperatures, you get ranges. Instead of flat lines on temperature graphs, you get slopes.
Here's what happens at the molecular level: when pure ice melts, water molecules break free from their crystal structure at exactly 0°C. But if salt is mixed in, some water molecules are 'trapped' by salt ions. These need extra energy to escape, so melting happens over a range of temperatures.
Now for the detective work. You've got a white solid that might be pure aspirin. The data book says pure aspirin melts at 135°C. Your sample starts melting at 128°C and finishes at 134°C. What's your verdict?
Worked example: Analysing Melting Point Data for Purity
A sample thought to be pure benzoic acid melts over the range 118-121°C. Pure benzoic acid melts at 122°C. Is the sample pure?
Answer: The sample is impure because it melts over a range (not at a sharp point) and the melting starts below the literature value for pure benzoic acid.
The same logic applies to boiling points. If your 'pure' ethanol boils at 82°C instead of 78°C, it contains impurities that have raised the boiling point. The further from the literature value, the less pure your sample.
Examiners love testing purity because it combines practical skills with data analysis. Here's how to nail these questions every time.
Watch out for the classic trap: students often say 'the melting point is wrong' instead of explaining what this tells us about purity. Always link your observations to the conclusion about whether the substance is pure or impure.
Exam precision note: In Using Melting and Boiling Points to Test Purity - Pure substances - Purity, formulations and chromatography, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.
Pure substances melt and boil at fixed temperatures because every particle experiences the same structure and bonding conditions. Impurities disrupt that regular arrangement. As a result, an impure sample usually melts over a range and often begins melting at a lower temperature than the pure substance. This is why melting-point tests are useful for checking purity in organic chemistry and in GCSE practical work. Students should compare both the start of melting and the width of the melting range when deciding whether a sample is pure.
A simple structure view is enough here: the key idea is that a regular particle arrangement gives a sharp melting point, while impurities disturb that arrangement.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Pure aspirin melts at 135°C. A sample that starts melting at 128°C and finishes at 134°C is impure, because it melts over a range and lower than the pure value.
Pure benzoic acid melts at 122°C. A sample melting over the range 118-121°C is impure, because it melts over a range rather than at a sharp point and starts melting 4°C lower than the pure value.
Impurities usually lower the melting point and make a substance melt over a range instead of at a sharp temperature. They can also change the boiling point; for example, ethanol that boils at 82°C instead of 78°C contains impurities, and salty water boils at 102°C.
A pure substance melts at a sharp, fixed temperature that matches its literature value, while an impure substance melts over a range of temperatures, usually starting lower. For example, a sample of benzoic acid melting at 118-121°C instead of 122°C is impure.
A pure substance has a sharp melting point because every particle experiences the same structure and bonding, so the particles all break free at the same temperature. Impurities disrupt this regular arrangement, so an impure sample melts over a range.
Pure water melts at 0°C and boils at 100°C at standard pressure, while pure ethanol melts at -114°C and boils at 78°C. These fixed values act like fingerprints for identifying pure substances.
On a graph of temperature against time, a pure substance gives a flat line while it melts, because the temperature stays constant throughout melting. An impure substance gives a slope instead, because it melts over a range of temperatures.
A melting point range is the temperature range over which an impure substance melts, from when melting begins to when it is completely liquid. Pure substances melt at a single, sharp temperature instead.
Adding salt raises the boiling point of water: pure water boils at exactly 100°C, but salty water can boil at 102°C. Salt in the water is one of the impurities, and impurities change the fixed melting and boiling points of pure substances.