Tests for ions and functional groups

Tests for gases

• Hydrogen: a lighted splint gives a squeaky pop.
• Oxygen: relights a glowing splint.
• Carbon dioxide: turns limewater milky (cloudy).
• Chlorine: bleaches damp litmus paper white.

Flame tests (metal ions)

Dip a clean wire loop in the sample and hold it in a blue flame:

• lithium: crimson
• sodium: yellow
• potassium: lilac
• calcium: orange-red
• copper: green

A mixture of ions can mask colours, especially sodium's strong yellow.

Sodium hydroxide test (metal ions)

Add a few drops of sodium hydroxide solution:

• Cu^{2+}: blue precipitate
• Fe^{2+}: green precipitate
• Fe^{3+}: orange-brown precipitate
• Al^{3+}, Ca^{2+}, Mg^{2+}: white precipitate; only the aluminium one dissolves in excess sodium hydroxide.

Tests for negative ions

• Carbonate: add dilute acid; bubbles of CO_2 turn limewater milky.
• Sulfate: add dilute hydrochloric acid, then barium chloride; a white precipitate of barium sulfate forms.
• Halides: add dilute nitric acid, then silver nitrate. Chloride gives a white precipitate, bromide cream and iodide yellow.

The acid is added first to remove carbonate ions, which would otherwise also form a precipitate and give a false positive.

Cations

• Ammonium: warm with NaOH(aq); ammonia is released, turning damp red litmus blue.
• Group 2: hydroxide solubility increases down the group (Mg(OH)_2 barely soluble, Ba(OH)_2 quite soluble), while sulfate solubility decreases, so BaSO_4 precipitates.
• Transition metals: NaOH and NH_3(aq) give coloured hydroxide precipitates; with excess ammonia, Cu(OH)_2 dissolves to form deep blue \mathrm{[Cu(NH_3)_4(H_2O)_2]^{2+}}, a ligand-substitution reaction.

Anions and the order of testing

Test for carbonate first (acid, effervescence, limewater), then sulfate (BaCl_2 after acidifying), then halides (AgNO_3 after acidifying with nitric acid). Testing in this order avoids false positives: Ag_2SO_4 and Ag_2CO_3 can precipitate, and BaCO_3 is insoluble too. Use nitric acid for halides; hydrochloric acid would add chloride.

The silver halides are distinguished with ammonia:

• AgCl (white) dissolves in dilute NH_3;
• AgBr (cream) dissolves only in concentrated NH_3;
• AgI (yellow) is insoluble even in concentrated NH_3.

Organic functional groups

• Alkene: bromine water decolourises (orange to colourless).
• Primary/secondary alcohol or aldehyde: warm with acidified potassium dichromate(VI); orange turns green as Cr^{3+} forms. Tertiary alcohols give no change.
• Aldehyde vs ketone: Tollens' reagent gives a silver mirror, and Fehling's solution turns from blue to a brick-red precipitate of Cu_2O, with aldehydes only.
• Carbonyl (either): 2,4-dinitrophenylhydrazine gives an orange precipitate; its melting point after recrystallisation identifies the compound.
• Carboxylic acid: effervescence with sodium hydrogencarbonate (CO_2); phenols are too weak to react.
• Haloalkane: warm with aqueous ethanolic silver nitrate; the precipitate colour identifies the halogen and the rate reflects C–X bond strength (iodo fastest).

In modern labs these tests are complemented, not replaced, by infrared, NMR and mass spectrometry, which give structural detail that chemical tests cannot.