Chemical analysis

• A pure substance 纯净物 in chemistry means one element or compound — melting and boiling points prove it; formulation 配方s are deliberate mixtures.
• Chromatography separates by distribution between phases — Rf values identify the spots.
• Gas tests (H₂, O₂, CO₂, Cl₂) and (chemistry only) ion tests — flame colours, hydroxide precipitate 沉淀s, carbonates, halides, sulfates — build the analyst's toolkit.

Pure substance — a single element or compound, not mixed with anything. Pure substances melt and boil at specific, sharp temperatures; mixtures melt over a range — use melting/boiling point data to tell them apart. (In everyday speech "pure" means unadulterated — pure milk — a different sense.)

Formulations — mixtures designed as useful products, each component in carefully measured amounts for a purpose: fuels, cleaning agents, paints, medicines, alloys, fertilisers, foods. Identify one from given information.

Chromatography 色谱法 — a stationary phase 固定相 (the paper) and a mobile phase 流动相 (the solvent); separation depends on the distribution of each substance between the phases — those more attached to the stationary phase stay lower. Measure:

Different compounds have different Rf values in different solvents — use them to identify; a pure compound gives a single spot in every solvent.

Flame tests (cations): lithium — crimson; sodium — yellow; potassium — lilac; calcium — orange-red; copper — green. In a mixture some colours mask others.

Metal hydroxides (sodium hydroxide solution): Al³⁺, Ca²⁺, Mg²⁺ give white precipitates — but only Al(OH)₃ dissolves in excess NaOH; Cu²⁺ blue, Fe²⁺ green, Fe³⁺ brown precipitates.

Carbonates: add dilute acid — effervescence; the gas turns limewater milky.

Halides: add dilute nitric acid then silver nitrate — AgCl white, AgBr cream, AgI yellow precipitates.

Sulfates: add dilute hydrochloric acid then barium chloride — a white precipitate of BaSO₄.

(HT) Flame emission spectroscopy 光谱法: instrument draws the line emission spectrum; the positions of the lines identify the metal, their intensity its concentration. Advantages over flame test 焰色试验s: very sensitive, works on mixtures, rapid.

• Pure vs mixture by melting point; name five formulations.
• Rf calculated from a chromatogram; why a pure compound gives one spot.
• The four gas tests with results.
• (Chem) flame colours; hydroxide colours with the aluminium-in-excess trick; carbonate/halide/sulfate reagents and precipitate colours; (HT) spectroscopy advantages.