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 = 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

Testing purity by melting and boiling behaviour

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
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

Exam tip

Physical test to confirm a liquid is pure water

What comes up: Describe or state a physical test to show a colourless liquid is pure water (2 marks).

Write (two marks): (1) Measure its boiling point. (2) A pure sample will boil at exactly 100 °C. Alternatively: measure the freezing point and confirm it is exactly 0 °C — but you must match the stated value to the stated property.

Watch out: Chemical tests (anhydrous copper sulfate turns blue, cobalt chloride paper turns pink) identify water but do not show it is pure. The question asks for a physical test of purity, so the physical boiling/freezing-point answer is required.