Purity, Formulations and Chromatography (C8.1)

Pure substances

In chemistry, a pure substance contains only one type of element or compound — no other substances mixed in. It has sharp, fixed melting and boiling points.

In everyday language, "pure" is used loosely (e.g. "pure orange juice") — this does not mean chemically pure.

Testing purity: if a substance is pure, it melts at a precise temperature (the melting point is a sharp point, not a range). Impurities cause:

• Melting point to be LOWER than the pure substance.
• Melting range to be BROADER (not a sharp point).
• Boiling point to be HIGHER than the pure substance.

Formulations

A formulation is a carefully designed mixture where each component has a specific purpose to achieve desired properties. Formulations are not simple mixtures — they are engineered products.

Examples:

• Medicines (tablets): active ingredient + binders + fillers + coatings.
• Alloys (e.g. steel, bronze): designed for strength, corrosion resistance.
• Fertilisers: specific NPK ratios.
• Paints: pigment + binder + solvent.
• Food products: emulsifiers, stabilisers, flavourings, preservatives.
• Fuels: additives to improve combustion.
• Cleaning products: surfactants + solvents.

Chromatography

Paper chromatography separates mixtures based on the different solubilities of components in the solvent (mobile phase) relative to the paper (stationary phase).

Procedure:

1. Draw a pencil baseline on chromatography paper (pencil, not pen — pen would run).
2. Place a small spot of the mixture on the baseline.
3. Dip the paper in solvent (mobile phase) — solvent front must be BELOW the spot.
4. Allow solvent to run up; remove and mark solvent front position.

Rf value:

Rf = distance moved by substance ÷ distance moved by solvent front

Rf values are between 0 and 1.

A pure substance produces a single spot on the chromatogram. A mixture produces multiple spots. Substances with the same Rf value in the same solvent are likely to be the same compound.

Gas chromatography (GC): separates mixtures of gases or volatile liquids. Carrier gas (mobile phase) passes over stationary phase (column). Separated components detected; retention time identifies the substance. Used in forensics, food analysis, doping tests.

Common exam errors

1. Drawing the baseline with a pen — pen ink would dissolve and ruin the chromatogram.
2. Dipping the paper so that the solvent covers the spot — the spots would dissolve into the solvent trough.
3. Saying Rf > 1 is possible — Rf can never be greater than 1 (substance cannot travel further than the solvent front).