Pure Substances and Mixtures — Elements, Compounds and Mixtures | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 5% of your exam marks.

stable

Rare

Stable5%

Definitions and separation techniques tested consistently across both papers.

In everyday English, "pure" usually means clean or natural
In chemistry, a pure substance = a sample containing only one element or only one compound, with nothing else present
"Pure" supermarket water is not chemically pure: it contains dissolved minerals, dissolved oxygen, sometimes fluoride
Chemically pure water contains only H₂O molecules

Pure substance: sharp, fixed melting point and boiling point
- Pure water melts at 0 °C, boils at 100 °C
- Pure NaCl melts at 801 °C
Mixture or impure substance: melts over a range of temperatures, not at a single value
- Impurities lower the melting point and raise the boiling point
Quality control for pharmaceuticals and foodstuffs routinely uses melting-point analysis
- A capillary tube of the sample is heated slowly in a melting-point apparatus
- Measured value is compared against the literature value
- Closer to the literature value, and narrower range = purer