Pure Substance

In IGCSE chemistry, a pure substance consists of a single element or a single compound that is not mixed with any other substance ¹.

Pure Element            Pure Compound              Mixture
  (A) (A)                  (A-B) (A-B)             (A) (B) (A-B)
  (A) (A)                  (A-B) (A-B)            (A-B) (A) (B)
[Single type of atom]   [Fixed ratio bonded]   [Physically mixed]

Criteria of Purity

Purity is assessed through physical constants and separation techniques ¹:

1. Fixed Melting and Boiling Points:
   • A pure substance has a sharp, fixed melting point and boiling point (e.g., pure water melts at $0\text{ }^\circ\text{C}$ and boils at $100\text{ }^\circ\text{C}$ at $1\text{ atm}$).
   • Impurities lower (depress) the melting point and elevate the boiling point, causing phase transitions across a temperature range.

Temp ^        Pure (Sharp Plateau)         Impure (Range)
     |               / Liquid                     /
M.P. +==============+                      ......' (No flat line)
     |             /                      /
     +----------------------> Heat       +---------------------->

2. Chromatography:
   • On a paper chromatogram, a pure substance produces only a single spot, whereas mixtures separate into multiple spots ¹.

Importance

Purity is critical in food, water supplies, and pharmaceutical manufacturing, where trace impurities can cause adverse physiological effects or alter chemical reactivity ¹.

Related

• Boiling - Bulk liquid-to-gas transition occurring at a fixed boiling point for pure substances.
• Freezing - Solidification phase change at a fixed freezing point.
• Lattice - Regular repeating framework in crystalline pure solids.
• Melting - Solid-to-liquid transition characterized by sharp melting plateaus.
• Chemistry - Foundational science domain.