Purity, Formulations & Chromatography

Pure substances

• In chemistry a pure substance is a single element or compound, not mixed with anything else. In everyday language "pure" means nothing added, like pure orange juice, which is chemically a mixture
• A pure substance melts and boils at a specific, sharp temperature. A mixture melts and boils over a range of temperatures, and an impurity lowers the melting point and raises the boiling point
• So melting point data tells you purity: a sample that melts sharply at the known value is pure; one that melts over several degrees, below the known value, is impure

Formulations

• A formulation is a mixture designed as a useful product. Each component has a particular purpose, and they are mixed in carefully measured quantities so the product has the properties needed
• Examples: fuels, cleaning agents, paints, medicines, alloys, fertilisers, foods
• Recognise one in a question by the description: several components, each with a job, in fixed proportions

Chromatography

• Paper chromatography separates a mixture and helps identify the substances in it
• Two phases: the stationary phase (the paper) does not move; the mobile phase (the solvent) moves up through it
• Separation depends on how each substance is distributed between the two phases: a substance that is more soluble in the solvent and less attracted to the paper travels further

$$R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent}}$$

• Measure from the start line to the centre of the spot, and from the start line to the solvent front. $R_f$ has no units and is always less than 1
• Worked example: a spot moves 3.6 cm and the solvent 8.0 cm. $R_f = \frac{3.6}{8.0} = 0.45$
• A substance has a different $R_f$ in different solvents, but the same $R_f$ in the same solvent, so $R_f$ values can identify compounds when compared with known values
• A pure compound gives a single spot in every solvent. A mixture gives more than one spot, and its spots may separate differently in different solvents

Required Practical 6: paper chromatography

• Draw a start line in pencil about 1 cm from the bottom of the chromatography paper. Pencil does not dissolve; ink would run
• Put a small spot of each ink or dye on the line, and let it dry. Keep spots small so they do not spread
• Pour solvent (water, or ethanol for insoluble dyes) into a beaker to a depth below the start line, so the spots do not wash off
• Stand the paper in the solvent, held by a lid or a glass rod, and cover with a lid to stop the solvent evaporating
• Remove the paper when the solvent is near the top and mark the solvent front in pencil at once, before it dries. Let the paper dry
• Measure the distances and calculate $R_f$ for each spot. Compare with the spots from known pure dyes run alongside