8. Chemical Analysis

• Analysts use qualitative tests to detect specific chemicals by observing gas properties, color changes, or precipitates.

• Instrumental methods are fast, sensitive, and accurate, useful when analyzing very small amounts of chemicals.

• Forensic and drug control scientists rely on instrumental analysis for precise identification.

5.8.1 Purity, Formulations and Chromatography

Pure substances

• Pure substances are single elements or compounds with no mixtures.

• Pure substances have specific melting and boiling points.

• Melting/boiling point data can distinguish pure substances from mixtures.

• In everyday language, “pure” means unadulterated, e.g., pure milk.

Formulations

• Formulations are mixtures designed for a specific purpose.

• Each component in a formulation has a particular role.

• Examples: fuels, cleaning agents, paints, medicines, alloys, fertilizers, foods.

• Formulations are made by mixing components in precise quantities.

Chromatography

• Technique to separate mixtures and identify substances.

• Involves stationary phase and mobile phase.

• Separation depends on how substances distribute between these phases.

• Rf value = distance moved by substance ÷ distance moved by solvent.

• Different compounds have unique Rf values in different solvents.

• Pure compounds produce a single spot in all solvents; mixtures produce multiple spots.

• Paper chromatography is a common method for colored substances.

5.8.2 Identification of Common Gases

Hydrogen

• Test: Burning splint held at gas.

• Result: Burns rapidly with a pop sound.

Oxygen

• Test: Glowing splint inserted into gas.

• Result: Splint relights.

Carbon Dioxide

• Test: Bubble gas through limewater (calcium hydroxide solution).

• Result: Limewater turns milky/cloudy.

Chlorine

• Test: Damp litmus paper placed in gas.

• Result: Litmus paper bleaches and turns white.