Tests for gases and ions; instrumental methods

Chemistry · Chemical analysis. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Chemists identify a substance by tests that give a result you can see: a colour, a precipitate, a gas or a sound. You need to know each test and its positive result.

An ionic compound has a positive metal ion and a negative ion, and each is found with a separate test. Metal ions are found with a flame test or with sodium hydroxide solution, which gives a coloured precipitate. Carbonate, halide and sulfate ions each have their own test.

Instrumental methods use machines in place of these tests. They are accurate, sensitive and rapid, and they work on very small samples. Flame emission spectroscopy is one example.

Rules to remember

Gases: hydrogen burns with a squeaky pop (lighted splint); oxygen relights a glowing splint; carbon dioxide turns limewater milky; chlorine bleaches damp litmus paper white.
Flame colours: lithium crimson, sodium yellow, potassium lilac, calcium orange-red, copper green.
With sodium hydroxide solution: Cu²⁺ blue precipitate, Fe²⁺ green, Fe³⁺ brown; Al³⁺, Ca²⁺ and Mg²⁺ white, and only the aluminium precipitate dissolves in excess.
Carbonate: add dilute acid; carbon dioxide is given off, which turns limewater milky.
Halides: add dilute nitric acid, then silver nitrate solution. Chloride gives a white precipitate, bromide cream, iodide yellow.
Sulfate: add dilute hydrochloric acid, then barium chloride solution; a white precipitate forms.
Flame emission spectroscopy: the line spectrum identifies the metal ions, even in a mixture, and the intensity of the light gives their concentration.

Common mistake

Students mix up the two acid and solution pairs. Silver nitrate (with nitric acid) is for halides; barium chloride (with hydrochloric acid) is for sulfates.

Worked example

A solid gives a green flame in a flame test. Its solution gives a blue precipitate with sodium hydroxide solution, and a cream precipitate with dilute nitric acid followed by silver nitrate solution. Name the solid.

A green flame and a blue precipitate with sodium hydroxide both show copper(II) ions, Cu²⁺.
Nitric acid then silver nitrate is the halide test, and a cream precipitate shows bromide ions, Br⁻.
Put the positive ion and the negative ion together to name the compound.

Answer: The solid is copper(II) bromide.

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