Chemical identification of anions

An anion can be identified by adding a suitable reagent and observing the result. The reagent and observation must be considered together: a precipitate or colour change on its own does not identify an ion.

Anion tests that produce a gas

Carbonate ions: Add dilute acid to the sample. Carbonate ions react with the acid and release carbon dioxide, producing bubbles. Pass the gas through limewater: carbon dioxide turns it milky.

Nitrate ions: Add aqueous sodium hydroxide and aluminium foil to the sample, then warm gently. Aluminium reduces nitrate ions to ammonia. The ammonia turns damp red litmus paper blue.

Anion tests that produce a precipitate

Halide ions: Acidify the solution with dilute nitric acid, then add aqueous silver nitrate. The colour of the silver halide precipitate distinguishes the ions:

Chloride, $\ce{Cl-}$ : white silver chloride, $\ce{AgCl}$ .
Bromide, $\ce{Br-}$ : cream silver bromide, $\ce{AgBr}$ .
Iodide, $\ce{I-}$ : yellow silver iodide, $\ce{AgI}$ .

Sulfate ions: Acidify the solution with dilute nitric acid, then add aqueous barium nitrate. Sulfate ions produce a white precipitate of barium sulfate, $\ce{BaSO4}$ .

Sulfite test: colour change

Add purple acidified aqueous potassium manganate(VII) to the sample. Sulfite ions reduce the manganate(VII) ions, causing the purple solution to become colourless.

Identification of aqueous cations with hydroxide and ammonia

An aqueous cation can be identified by adding a reagent and observing whether an insoluble solid, called a precipitate, forms. For the metal ions in these tests, the precipitates are metal hydroxides.

Test separate portions of the solution with aqueous sodium hydroxide and aqueous ammonia. Add each reagent drop by drop, then add it in excess. Record the precipitate’s colour and whether it dissolves in excess reagent. These observations together distinguish the ions.

Cation	Aqueous sodium hydroxide	Aqueous ammonia
Aluminium	White precipitate; dissolves in excess.	White precipitate; insoluble in excess.
Calcium	White precipitate.	No precipitate.
Chromium(III)	Grey-green precipitate; dissolves in excess.	Grey-green precipitate; insoluble in excess.
Copper(II)	Light-blue precipitate; insoluble in excess.	Light-blue precipitate; dissolves in excess to form a deep-blue solution.
Iron(II)	Green precipitate; insoluble in excess.	Green precipitate; insoluble in excess.
Iron(III)	Red-brown precipitate; insoluble in excess.	Red-brown precipitate; insoluble in excess.
Zinc	White precipitate; dissolves in excess to form a colourless solution.	White precipitate; dissolves in excess to form a colourless solution.

For example, aluminium and zinc both give white precipitates with either reagent. Excess aqueous ammonia distinguishes them: the aluminium precipitate remains, while the zinc precipitate dissolves.

Testing for ammonium ions

Ammonium ions, $\ce{NH4+}$ , are identified by a gas test rather than a metal hydroxide precipitate. Add aqueous sodium hydroxide and warm the mixture. If ammonium ions are present, ammonia gas, $\ce{NH3}$ , is released and turns damp red litmus paper blue.

There is no corresponding precipitate test with aqueous ammonia that distinguishes ammonium ions.

Chemical identification of gases

A gas can be identified by applying a suitable test and observing its characteristic result. The test reagent and the result must be considered together.

Gas tests using indicators and reagents

Ammonia turns damp red litmus paper blue. The gas dissolves in the water on the paper and forms an alkaline solution, which changes the litmus colour.

Carbon dioxide turns limewater milky when bubbled through it. The cloudy appearance is caused by a white precipitate of calcium carbonate, $\ce{CaCO3}$ , forming in the limewater.

Chlorine bleaches damp litmus paper, making its colour disappear. Blue litmus paper may first turn red before it is bleached.

Sulfur dioxide decolourises purple acidified aqueous potassium manganate(VII). It does this by reducing the manganate(VII) ions, $\ce{MnO4^-}$ , in the solution.

Gas tests using splints

Hydrogen burns with a squeaky pop when a lighted splint is placed at the gas.

Oxygen relights a glowing splint because oxygen supports combustion.

Flame tests for metal cations

A flame test helps identify certain metal cations by the colour they produce in a flame. A small sample is placed on a clean test wire and introduced into a non-luminous flame. The observed colour is compared with the characteristic flame colours of the cations.

The wire must be cleaned before testing a sample. Material left from a previous sample could produce its own flame colour and obscure the result.

Characteristic flame colours:

Lithium: crimson red
Sodium: yellow
Potassium: lilac
Calcium: orange-red
Barium: apple green
Copper(II): blue-green

A flame test identifies these cations by a colour in the flame. This is a different observation from the formation of a precipitate in an aqueous test.