Chemical Tests for Ions & Gases

Why test for ions and gases?

Chemists often need to find out what an unknown substance contains. Instead of guessing, we use qualitative analysis — a set of simple, reliable tests that each give a clear, visible result. In this chapter you will learn how to identify metal cations, common anions, several gases, and even water itself.

Key terms

Cation — a positively charged ion (e.g. $Na^+$, $Cu^{2+}$).

Anion — a negatively charged ion (e.g. $Cl^-$, $SO_4^{2-}$).

Precipitate (ppt) — an insoluble solid that forms when two solutions are mixed.

The golden rule in the exam: always state both the test and the positive result. "Add silver nitrate" is worthless on its own — you must also say what colour appears.

Flame tests for metal ions

Some metal ions give a characteristic colour when held in a hot flame. The method is the same every time:

1. Dip a clean nichrome wire (or splint) into concentrated hydrochloric acid to clean it.
2. Dip the wire into the solid sample.
3. Hold it in the edge of a blue Bunsen flame.
4. Observe and record the flame colour.

Exam tip

A strong sodium flame is yellow and will mask other colours, so the wire must be properly cleaned between tests. Remember the order Li → Na → K by colour: red, yellow, lilac as you go down Group 1.

Testing cations with sodium hydroxide solution

Adding a few drops of sodium hydroxide solution ($NaOH$) to a solution of a metal salt often produces a coloured metal hydroxide precipitate. The colour identifies the cation.

For example:

$$Cu^{2+}(aq) + 2OH^-(aq) \rightarrow Cu(OH)_2(s)$$

The ammonium ion, $NH_4^+$, behaves differently. There is no coloured precipitate. Instead:

Add sodium hydroxide solution and warm the mixture gently.
Ammonia gas ($NH_3$) is given off.
Hold damp red litmus paper at the mouth of the tube — it turns blue.

$$NH_4^+(aq) + OH^-(aq) \rightarrow NH_3(g) + H_2O(l)$$

Watch out

Iron(II) hydroxide slowly turns brown at its surface as it is oxidised to iron(III) in air. Read the colour quickly — the first colour you see (green) is the right answer.

Testing for anions

#### Carbonate ions ($CO_3^{2-}$)

Add a dilute acid (such as hydrochloric acid). Carbonates fizz, releasing carbon dioxide. Bubble the gas through limewater — it turns milky (cloudy white).

$$CO_3^{2-} + 2H^+ \rightarrow CO_2 + H_2O$$

#### Sulfate ions ($SO_4^{2-}$)

1. Add dilute hydrochloric acid (to remove carbonate ions).
2. Then add barium chloride solution.
3. A white precipitate of barium sulfate forms.

$$Ba^{2+}(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s)$$

#### Halide ions ($Cl^-$, $Br^-$, $I^-$)

1. Add dilute nitric acid (to remove carbonate ions).
2. Then add silver nitrate solution.
3. A coloured silver halide precipitate forms.

$$Ag^+(aq) + Cl^-(aq) \rightarrow AgCl(s)$$

Exam tip

The acid added before the main test is not optional. For sulfates use dilute hydrochloric acid; for halides use dilute nitric acid. The acid removes carbonate impurities that would otherwise give a false white precipitate. Choosing the wrong acid is a common way to lose marks.

Worked example

An unknown white solid dissolves in water. A flame test gives a yellow flame. Adding dilute nitric acid then silver nitrate gives a cream precipitate. Name the compound.

The yellow flame shows sodium ($Na^+$). The cream silver-halide precipitate shows bromide ($Br^-$). The compound is sodium bromide, $NaBr$.

Tests for gases

Each common gas has a quick, memorable test.

Real world

The "relights a glowing splint" test for oxygen is the same principle used by paramedics and pilots to check oxygen cylinders work. A higher oxygen concentration makes things burn far more vigorously — which is why no naked flames are allowed near hospital oxygen supplies.

Tests for water

Two chemical tests show that a liquid contains water:

Anhydrous copper(II) sulfate is white. Add water and it turns blue.
Cobalt chloride paper is blue when dry. Add water and it turns pink.

Both colour changes happen because the substances absorb water to form hydrated crystals.

However, these tests only prove water is present — they do not prove the water is pure. Sea water would pass both tests.

To show water is pure, measure its boiling point: pure water boils at exactly 100°C (at normal atmospheric pressure). It also freezes at exactly 0°C. Dissolved impurities raise the boiling point and lower the freezing point, so any deviation means the water is not pure.

Watch out

A common exam trap: "How would you show this water is pure?" Do not answer with copper sulfate or cobalt chloride — those only detect the presence of water. The correct answer is to check that it boils at exactly 100°C.

Quick recap

Flame tests: Li red, Na yellow, K lilac, Ca orange-red, Cu blue-green.
NaOH precipitates: Cu blue, Fe(II) green, Fe(III) brown; ammonium gives ammonia that turns damp red litmus blue on warming.
Anions: carbonate fizzes (CO₂ → milky limewater); sulfate gives a white ppt with HCl + barium chloride; halides give white/cream/yellow ppts with nitric acid + silver nitrate.
Gases: H₂ squeaky pop, O₂ relights splint, CO₂ milky limewater, NH₃ red litmus → blue, Cl₂ bleaches litmus.
Water: white anhydrous copper sulfate → blue, blue cobalt chloride → pink; pure water boils at 100°C.