Chemical analysis flashcards

Topic 8: Chemical analysis

Pure substance (in chemistry), and how melting point shows purity

A single element or compound, not mixed with any other substance. A pure substance melts sharply at its known value; an impure one melts lower and over a range.

Formulation

A mixture designed as a useful product, with components in carefully measured quantities, each with a particular purpose.

Rf value, and a worked example: solvent 6.0 cm, spot 4.2 cm

Rf = distance moved by substance ÷ distance moved by solvent, both from the origin line. 4.2 ÷ 6.0 = 0.70. No units; always less than 1.

Chromatography (RP 6): why a pencil origin line, and where the solvent level goes

Pencil does not dissolve and run (ink would). The solvent level is below the origin line, so the spots do not dissolve into the solvent.

Tests for hydrogen and oxygen

Hydrogen: burning splint at the mouth of the tube → squeaky pop. Oxygen: glowing splint relights.

Tests for carbon dioxide and chlorine

Carbon dioxide: limewater turns milky. Chlorine: damp litmus paper is bleached white.

Flame test colours: Li⁺, Na⁺, K⁺, Ca²⁺, Cu²⁺

Li⁺ crimson, Na⁺ yellow, K⁺ lilac, Ca²⁺ orange-red, Cu²⁺ green.

Sodium hydroxide precipitates: Al³⁺, Ca²⁺, Mg²⁺, Cu²⁺, Fe²⁺, Fe³⁺

Al³⁺, Ca²⁺, Mg²⁺: white (only the aluminium one dissolves in excess). Cu²⁺: blue. Fe²⁺: green. Fe³⁺: brown.

Test for halide ions, and the results

Add dilute nitric acid, then silver nitrate solution. Cl⁻ white, Br⁻ cream, I⁻ yellow precipitate.

Tests for sulfate and carbonate ions

Sulfate: dilute hydrochloric acid, then barium chloride solution → white precipitate. Carbonate: dilute acid → fizzing; the gas turns limewater milky.

Why is nitric acid, not hydrochloric acid, used to acidify in the halide test?

Acid removes carbonate ions, which would also give a precipitate. Hydrochloric acid would add chloride ions and give a false positive for chloride.

Flame emission spectroscopy

The sample is put in a flame and the light is passed through a spectroscope to give a line spectrum. Lines identify the metal ions (even in mixtures); intensity gives the concentration.