In everyday language, 'pure' might mean clean or natural (e.g. 'pure orange juice').
In CHEMISTRY, a PURE SUBSTANCE has a precise meaning:
A pure substance contains only ONE type of element or compound — nothing else mixed in.
A pure substance has FIXED, SHARP melting and boiling points.
Examples:
Pure water: boils at exactly 100°C, melts at exactly 0°C (at standard pressure).
Pure iron: melts at exactly 1538°C.
Pure ethanol: boils at exactly 78.4°C.
IMPURE substances (mixtures):
Melt and boil over a RANGE of temperatures — not at a fixed point.
Melting point is LOWER than that of the pure substance (melting point depression).
Boiling point is HIGHER than that of the pure substance (boiling point elevation).
This gives chemists a way to test purity.
Using Melting and Boiling Points to Test Purity
MELTING POINT TEST:
Heat a small sample slowly and record the temperature at which it starts and finishes melting.
PURE substance: melts at a SHARP, PRECISE temperature — starts and finishes at the same temperature.
Example: pure aspirin melts at exactly 135°C.
IMPURE substance: melts over a RANGE of temperatures — starts melting below the expected temperature.
Example: aspirin with impurity might melt from 128°C to 133°C.
BOILING POINT TEST:
Pure substance: boils at a fixed temperature throughout.
Mixture: temperature changes during boiling as components evaporate at different rates.
IDENTIFYING SUBSTANCES:
Comparing measured melting/boiling point to known data tables identifies the substance.
A substance that melts at 135°C and matches pure aspirin data → likely aspirin.
This method is simple, quick and requires only a small sample — very useful in organic chemistry.
Purity in Context
PHARMACEUTICAL PURITY:
Medicines must be extremely pure — even tiny impurities could be toxic or reduce effectiveness.
Thalidomide showed how critically purity (and stereochemistry) matters — one form was therapeutic, another caused birth defects.
FOOD PURITY:
Food standards require ingredients at specific purities.
Sugar (sucrose) in food must meet purity standards — impurities affect flavour and safety.
INDUSTRIAL PURITY:
Some industrial processes require specific purity levels.
Semiconductors (silicon chips) need extremely high purity silicon — impurities disrupt electrical properties.
CHEMICAL ANALYSIS:
Chemists use melting and boiling points alongside other techniques (chromatography, spectroscopy) to confirm identity and purity of compounds.
⚠️ Common Mistake
In chemistry, 'pure' does NOT mean natural or healthy — it means containing only ONE substance with no impurities. A mixture of two pure chemicals is NOT a pure substance. Orange juice (even 100% natural) is a MIXTURE — not a pure substance in the chemical sense.
📌 Key Note
Pure substance: one element or compound only — sharp fixed melting and boiling points. Impure (mixture): melts/boils over a range of temperatures. Impurity lowers melting point and raises boiling point. Use melting/boiling point data to identify substances and test purity.
🎯 Matching Activity — Pure or Impure?
Match each observation to pure substance or impure mixture. — drag the symbols on the right to match the component names on the left.
Pure substance
Drop here
Impure mixture
Drop here
Pure substance
Drop here
Impure mixture
Drop here
Pure substance
Drop here
Has a fixed composition — contains only one type of compound
Boiling point higher than expected for the pure substance
Boils at a constant temperature throughout
Melts at exactly one temperature — sharp melting point
Melts over a range of temperatures — starts below expected melting point
⭐ Higher Tier Only
Melting point depression: impurities disrupt lattice, requiring less energy to melt, lowering MP and broadening the range. Use melting point data to assess purity quantitatively. Compare measured values with data table values. Understand the role of melting point in pharmaceutical quality control.
🎯 Test Yourself
Question 1 of 2
1. A student heats a solid and records that it melts between 118°C and 125°C. Pure samples of this compound melt at 135°C. What does this suggest?
2. Why is 'pure orange juice' not a pure substance in the chemical sense?
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