Testing for ions and gases

Chemists are often asked to test a sample of a substance to find out if it contains specific ions or compounds. There are a variety of techniques used to do this.

Testing for ions – flame tests

The Group 1 elements in the periodic table are known as the alkali metals. They include lithium, sodium and potassium, which all react vigorously with air and water.

It is possible to use a flame test to detect the presence of an alkali metal ion.

A cleaned, moistened flame test wire is dipped into a solid sample of the compound. It is then put into the edge of a blue Bunsen flame. The flame colour produced indicates which alkali metal ion is present in the compound.

Flame colours and the alkali metal ion they represent

Flame colour	Ion present
Red	Lithium, Li+
Orange	Sodium, Na+
Lilac	Potassium , K+
Brick red	Calcium, Ca2+

Detecting ammonium ions

Ammonium ions (NH₄⁺) are present in dilute ammonia solution and any ammonium salt, such as ammonium chloride.

Ammonium ions can be identified in a solution by adding dilute sodium hydroxide solution and gently heating. If ammonium ions are present, they will be converted to ammonia gas. Ammonia has a characteristic choking smell. It also turns damp red litmus paper or damp universal indicator paper blue.

Hydroxide precipitates

Identifying transition metal ions

Transition metals form coloured compounds with other elements. Many of these are soluble in water, forming coloured solutions.

If sodium hydroxide solution is then added, a transition metal hydroxide is formed. Transition metal hydroxides are insoluble so they form solid precipitates. These precipitates often appear as small particles suspended in a solution.

Here are the equations for copper sulfate solution reacting with sodium hydroxide solution:

copper(II) sulfate + sodium hydroxide → copper(II) hydroxide + sodium sulfate

CuSO₄(aq) + 2NaOH(aq) → Cu(OH)₂(s) + Na₂SO₄(aq)

(blue solution + colourless solution → blue precipitate + colourless solution)Solutions containing copper(II) ions form a blue precipitate with sodium hydroxide

Different transition metals form different coloured precipitates:

Metal ion	Colour
Iron(II), Fe2+	Green – turns orange-brown when left standing
Iron(III), Fe3+	Orange-brown
Copper(II), Cu2+	Blue

Investigating transition metal ions

To tell whether an unknown substance contains iron(II) nitrate or iron(III) nitrate, add a few drops of sodium hydroxide solution:

• if you get a green precipitate, the unknown substance is iron(II) nitrate
• if you get an orange-brown precipitate, the unknown substance is iron(III) nitrate

Note that it is the metal that determines the colour. So you would get the same result whether you used copper(II) chloride or copper(II) nitrate – a blue precipitate in this example.

Tests for metal ions – white precipitates

Many metal ions produce white precipitates with sodium hydroxide. These are the two tests you would carry out to identify a metal ion:

1. add dilute sodium hydroxide to a solution of the metal ion
2. add dilute ammonia solution

The second test is necessary because the result of the first test can be the same for some metals. For instance, zinc and aluminium ions have the same reaction with sodium hydroxide as shown in these tables:

Observation with dilute NaOH

Ion	A few drops	Excess
Zn2+	White precipitate forms	Precipitate redissolves
Ca2+	White precipitate forms	No change
Al3+	White precipitate forms	Precipitate redissolves

Observation with dilute ammonia

Ion	A few drops	Excess
Zn2+	White precipitate forms	Precipitate redissolves
Ca2+	No precipitate formed	No precipitate formed
Al3+	White precipitate forms	No change

Tests for negative ions

Testing for carbonate ions

Carbonate ions, CO₃^2– can be detected whether in a solid compound or in solution. An acid, such as dilute hydrochloric acid, is added to the test compound.

Carbon dioxide gas bubbles if carbonate ions are present. Limewater is used to confirm that the gas is carbon dioxide. It turns from clear to milky when carbon dioxide is bubbled through.

Testing for sulfate ions

Sulfate ions in solution, SO₄^2–, are detected using barium chloride solution. The test solution is acidified using a few drops of dilute hydrochloric acid, and then a few drops of barium chloride solution are added. A white precipitate of barium sulfate forms if sulfate ions are present.

For example:

barium chloride + sodium sulfate → sodium chloride + barium sulfate

BaCl₂(aq) + Na₂SO₄(aq) → 2NaCl(aq) + BaSO₄(s)

The hydrochloric acid is added first to remove any carbonate ions that might be present – they would also produce a white precipitate, giving a false positive result.

Barium nitrate solution can be used instead of barium chloride solution. However, nitric acid is added first to acidify the test solution. Sulfuric acid cannot be used because it contains sulfate ions – these would interfere with the second part of the test.

Testing for halide ions

A test using silver nitrate

The halogens are the elements in Group 7 of the periodic table. Chlorine, bromine and iodine are halogens. Their ions are called halide ions, eg chloride, Cl^–.

Halide ions in solutions are detected using silver nitrate solutions. The test solution is acidified using a few drops of dilute nitric acid, and then a few drops of silver nitrate solution are added. Different coloured silver halide precipitates form, depending on the halide ions present:

• chloride ions give a white precipitate of silver chloride
• bromide ions give a cream precipitate of silver bromide
• iodide ions give a yellow precipitate of silver iodide

For example:

silver nitrate + sodium bromide → sodium nitrate + silver bromide

AgNO₃(aq) + NaBr(aq) → NaNO₃(aq) + AgBr(s)

One way to remember the colours is to think of ‘milk, cream, butter’ (white, cream, yellow).

The nitric acid is added first to remove any carbonate ions that might be present – they would produce a white precipitate of silver carbonate, giving a false positive result for chloride ions.

Testing for nitrate ions

Nitrate ions (NO₃^–) can be detected by reducing them to ammonia. This is done by:

• adding sodium hydroxide solution, then aluminium powder or foil
• heating strongly

If nitrate ions are present, ammonia gas is given off. This has a characteristic choking smell. It also turns damp red litmus paper or damp universal indicator paper blue.

Tests for gases

Hydrogen, oxygen, carbon dioxide, ammonia and chlorine can be identified using different tests.

Hydrogen

A lighted wooden splint makes a popping sound in a test tube of hydrogen.

Oxygen

A glowing wooden splint relights in a test tube of oxygen.

Carbon dioxide

Carbon dioxide turns limewater milky. A lighted wooden splint goes out in a test tube of carbon dioxide but this happens with other gases, too. So the limewater test is a better choice.

Ammonia

Ammonia has a characteristic sharp, choking smell. It also makes damp red litmus paper turn blue. Ammonia forms a white smoke of ammonium chloride when hydrogen chloride gas, from concentrated hydrochloric acid, is held near it.

Chlorine

Chlorine has a characteristic sharp, choking smell. It also makes damp blue litmus paper turn red, and then bleaches it white. Chlorine makes damp starch-iodide paper turn blue-black.