Purity
Summary: A pure substance contains only one type of element or compound with a fixed composition. Purity can be assessed by measuring melting point and boiling point — pure substances have sharp, fixed melting/boiling points, while impurities lower the melting point and broaden the melting range. Tags: igcse chemistry definition practical-skills Created: 2026-07-14 Last Updated: 2026-07-23
In chemistry, a pure substance is a single element or compound that is not mixed with any other substance. Purity is an important concept in both chemical analysis and practical techniques.
What Is a Pure Substance?
A pure substance:
- Contains only one type of element or compound
- Has a fixed composition
- Has characteristic physical properties (melting point, boiling point, density)
Examples of pure substances:
- Pure water (H₂O only — not tap water which contains dissolved minerals)
- Pure sodium chloride (NaCl only)
- Pure ethanol (C₂H₅OH only)
Not pure:
- Tap water (contains dissolved salts, chlorine, minerals)
- Air (mixture of N₂, O₂, Ar, CO₂)
- Petroleum (mixture of many hydrocarbons)
Assessing Purity Using Melting Point
Melting point is the most common way to assess purity at IGCSE:
Pure Substance
- Has a sharp, fixed melting point
- Melts completely within 1-2°C range
- e.g. Pure water melts exactly at 0°C
Impure Substance
- Melts over a range of temperatures (broader range)
- Melting point is lower than the pure substance
- e.g. Impure water might melt between −2°C and 2°C
Why Impurities Lower the Melting Point
Impurities disrupt the regular crystal lattice, making it easier for the solid to break down into liquid → less energy needed → lower melting point.
Assessing Purity Using Boiling Point
Pure Substance
- Has a sharp, fixed boiling point
- Boils at one specific temperature
- e.g. Pure water boils at 100°C (at standard pressure)
Impure Substance
- Boils over a range of temperatures
- Boiling point is typically higher than the pure substance
- e.g. Salt water boils above 100°C
Purity and Chromatography
Paper chromatography can also assess purity:
- A pure substance produces one single spot on the chromatogram
- An impure substance (mixture) produces multiple spots
- Each spot represents a different component of the mixture
Key Facts
- Pure substance = one element/compound only — not mixed with anything else
- Pure substances have sharp, fixed melting and boiling points
- Impurities lower the melting point and broaden the melting range
- Impurities raise the boiling point and broaden the boiling range
- On a chromatogram: one spot = pure; multiple spots = impure/mixture
- Tap water is NOT pure — it contains dissolved minerals
See Also
This is a key term definition page — part of the IGCSE Chemistry (0620/0971) keyword glossary.