Skip to main content

O-Level Chemistry · Secondary 3 and 4

Qualitative Analysis

Qualitative analysis is the set of chemical tests used to identify the ions in an unknown salt and the gases given off in a reaction. In O-Level Chemistry you need to know the standard reagents, the exact observations and what each result tells you. These tests appear in practical work and in written papers, where marks depend on precise wording such as 'white precipitate, soluble in excess, giving a colourless solution'. The topic draws on acids, salts and redox, so it is a good test of joined-up understanding.

What the syllabus expects

  • Identify the cations Al³⁺, NH₄⁺, Ca²⁺, Cu²⁺, Fe²⁺, Fe³⁺ and Zn²⁺ using aqueous sodium hydroxide and aqueous ammonia
  • Identify the anions carbonate, chloride, iodide, nitrate and sulfate
  • Test for the gases ammonia, carbon dioxide, chlorine, hydrogen, oxygen and sulfur dioxide
  • Record observations precisely, including colour, precipitate formation and solubility in excess reagent
  • Write ionic equations for the formation of precipitates

Testing cations with aqueous sodium hydroxide

Add aqueous sodium hydroxide a few drops at a time, then in excess, and note both the first result and what happens in excess. Al³⁺ gives a white precipitate that dissolves in excess to give a colourless solution. Zn²⁺ gives exactly the same result. Ca²⁺ gives a white precipitate that is insoluble in excess. Cu²⁺ gives a light blue precipitate, insoluble in excess. Fe²⁺ gives a green precipitate, insoluble in excess, which slowly turns reddish-brown at the surface on standing as it is oxidised by air. Fe³⁺ gives a reddish-brown precipitate, insoluble in excess. NH₄⁺ gives no precipitate, but on warming, ammonia gas is given off, which turns damp red litmus paper blue. The precipitates are metal hydroxides, for example Cu²⁺ + 2OH⁻ gives Cu(OH)₂ and Fe³⁺ + 3OH⁻ gives Fe(OH)₃.

Testing cations with aqueous ammonia

Aqueous ammonia separates the ions that sodium hydroxide cannot. Al³⁺ gives a white precipitate that is insoluble in excess. Zn²⁺ gives a white precipitate that dissolves in excess to give a colourless solution, so ammonia distinguishes zinc from aluminium. Ca²⁺ gives no precipitate. Cu²⁺ gives a light blue precipitate that dissolves in excess to give a dark blue solution. Fe²⁺ gives a green precipitate and Fe³⁺ a reddish-brown precipitate, both insoluble in excess. A useful summary: with sodium hydroxide, the white precipitates of Al³⁺ and Zn²⁺ dissolve in excess but Ca²⁺ does not, and with ammonia, only Zn²⁺ and Cu²⁺ precipitates dissolve in excess.

Testing anions

Carbonate, CO₃²⁻: add a dilute acid. Effervescence is seen, and the gas gives a white precipitate with limewater, showing carbon dioxide. Chloride, Cl⁻: acidify with dilute nitric acid, then add aqueous silver nitrate. A white precipitate of silver chloride forms. Iodide, I⁻: acidify with dilute nitric acid, then add aqueous silver nitrate. A yellow precipitate of silver iodide forms. Sulfate, SO₄²⁻: acidify with dilute nitric acid, then add aqueous barium nitrate. A white precipitate of barium sulfate forms. Nitrate, NO₃⁻: add aqueous sodium hydroxide and aluminium foil, then warm. Ammonia is given off, which turns damp red litmus paper blue. The acid in the halide and sulfate tests removes any carbonate ions, which would otherwise also give a white precipitate. Never acidify with hydrochloric acid in the chloride test or sulfuric acid in the sulfate test, because you would be adding the very ion you are testing for.

Testing gases

Ammonia turns damp red litmus paper blue. Carbon dioxide gives a white precipitate when bubbled through limewater. Chlorine bleaches damp litmus paper. Hydrogen extinguishes a lighted splint with a 'pop'. Oxygen relights a glowing splint. Sulfur dioxide turns acidified aqueous potassium manganate(VII) from purple to colourless. Notice the difference between the two splint tests: a lighted splint is used for hydrogen and a glowing splint for oxygen. Litmus paper must be damp, because gases need to dissolve in water before they can change its colour.

Writing observations that earn marks

Examiners look for exact observations. Write 'white precipitate', not 'white solid' or 'cloudy'. A solution with no colour is 'colourless', not 'clear', since a coloured solution can also be clear. Always say what happens in excess, such as 'precipitate dissolves in excess to give a colourless solution' or 'insoluble in excess'. For a gas, give both what you see and the test result, such as 'effervescence, gas gives a white precipitate with limewater', rather than just 'bubbles'. Only name a conclusion when the observations allow it. A white precipitate with sodium hydroxide that dissolves in excess could be Al³⁺ or Zn²⁺, so you need the ammonia test before deciding.

Worked example

Solution X gives a green precipitate with aqueous sodium hydroxide that is insoluble in excess. A separate sample is acidified with dilute nitric acid, and aqueous barium nitrate is added, giving a white precipitate. Identify X and write ionic equations for both precipitates.

  1. A green precipitate insoluble in excess sodium hydroxide is the standard result for Fe²⁺.
  2. Ionic equation: Fe²⁺ + 2OH⁻ gives Fe(OH)₂.
  3. The nitric acid removes any carbonate ions, so a white precipitate with barium nitrate must be barium sulfate, showing SO₄²⁻.
  4. Ionic equation: Ba²⁺ + SO₄²⁻ gives BaSO₄.
  5. Combine the cation and anion to name the salt.

Answer: X is iron(II) sulfate, FeSO₄. Fe²⁺ + 2OH⁻ gives Fe(OH)₂ and Ba²⁺ + SO₄²⁻ gives BaSO₄.

Common mistakes

  • Writing 'white solution' or 'cloudy' instead of 'white precipitate', or 'clear' instead of 'colourless'.
  • Forgetting to add the reagent in excess, or not stating what happens in excess. This is the only way to separate Al³⁺, Zn²⁺ and Ca²⁺.
  • Acidifying with hydrochloric acid before adding silver nitrate. This adds Cl⁻ ions, so you always get a white precipitate.
  • Using dry litmus paper, or blue litmus paper, to test for ammonia. Use damp red litmus, which turns blue.
  • Mixing up the splint tests. Hydrogen pops with a lighted splint, while oxygen relights a glowing splint.
  • Describing the Cu²⁺ result with ammonia as a 'blue precipitate that stays'. The light blue precipitate dissolves in excess to give a dark blue solution.

Exam tips

  • Learn the cation results as a two-column table (sodium hydroxide and aqueous ammonia) for the seven cations, and practise writing it out from memory until every colour and excess result is exact.
  • In practical answers, record what you see before you conclude. Observations and conclusions are often marked separately.
  • Name the acid you use to acidify. Dilute nitric acid is the safe choice for the chloride, iodide and sulfate tests.

Practice questions

1. Easy: Describe the test for carbon dioxide and its positive result.

Show answer

Bubble the gas through limewater (aqueous calcium hydroxide). A white precipitate forms, so the gas is carbon dioxide.

2. Medium: Describe one test that distinguishes aqueous zinc sulfate from aqueous aluminium sulfate.

Show answer

Add aqueous ammonia dropwise and then in excess. Both give a white precipitate at first. The zinc precipitate dissolves in excess to give a colourless solution, while the aluminium precipitate stays insoluble in excess. (Sodium hydroxide cannot separate them, since both precipitates dissolve in excess.)

3. Harder: Solution Z gives no precipitate with aqueous sodium hydroxide, but on warming a gas is given off that turns damp red litmus paper blue. When Z is acidified with dilute nitric acid and aqueous silver nitrate is added, a yellow precipitate forms. Identify Z and give an ionic equation for the precipitate.

Show answer

Z is ammonium iodide, NH₄I. Ammonia released on warming with sodium hydroxide shows NH₄⁺. A yellow precipitate with acidified silver nitrate shows I⁻. Ionic equation: Ag⁺ + I⁻ gives AgI.

Related topics

Want a tutor for Chemistry?

Shortlist on WhatsApp within 16 hours. You choose the tutor. About O-Level Chemistry tuition

Get matched with a tutor

Or ask on WhatsApp