Analysis of inorganic compoundsEdexcel A-Level Chemistry: Revision notes
Section 1
Carbonate and hydrogencarbonate ions
Both carbonates and hydrogencarbonates react with dilute acid to give carbon dioxide, seen as effervescence.
- CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l)
- HCO₃⁻(aq) + H⁺(aq) → CO₂(g) + H₂O(l)
A solid carbonate such as CaCO₃(s) reacts in the same way: CaCO₃(s) + 2H⁺(aq) → Ca²⁺(aq) + CO₂(g) + H₂O(l).
To identify the gas, bubble it through limewater (calcium hydroxide solution), which turns milky/cloudy as insoluble calcium carbonate forms.
Quote the observation (effervescence) and the confirmation (limewater turns milky); the acid test alone does not tell carbonate from hydrogencarbonate.
Section 2
Sulfate ions
Add dilute hydrochloric acid and then barium chloride solution. A white precipitate of barium sulfate shows sulfate ions:
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
The acid is added first to remove carbonate ions (and sulfite), because barium carbonate is also a white precipitate. Barium sulfate does not dissolve in acid.
Acidifying with sulfuric acid before the barium test. It supplies sulfate ions and gives a precipitate every time.
Section 3
Ammonium ions
Warm the sample with sodium hydroxide solution. Ammonia gas is given off:
NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l)
Ammonia is the only common alkaline gas: it turns damp red litmus paper blue and has a sharp smell. With a rod dipped in concentrated hydrochloric acid it gives white smoke of ammonium chloride.
Litmus paper must be damp, as ammonia has to dissolve to make the alkaline solution that changes the colour.
Section 4
Halide, Group 1 and Group 2 tests
The tests for halide ions use nitric acid then silver nitrate (white AgCl, cream AgBr, yellow AgI, checked with ammonia). Group 1 and Group 2 ions are identified by flame tests and by the reactions of their compounds. These are covered in the sections on the halogens and on Groups 1 and 2.
Take care that the acid you use does not contain the ion you are testing for. Use nitric acid for halides and hydrochloric acid for sulfate.
Section 5
Core practical 7: analysing unknowns
In a systematic analysis of unknowns, record observations at each step and use them to rule ions in or out.
- Test for carbonate first, as carbonate gives false positives with barium and silver ions.
- Then test sulfate (HCl, then BaCl₂) and halides (HNO₃, then AgNO₃ and ammonia).
- Warm with sodium hydroxide to test for ammonium.
- Organic unknowns: bromine water is decolourised by alkenes; acidified potassium dichromate(VI) turns from orange to green with primary or secondary alcohols; carboxylic acids give carbon dioxide with carbonates.
Every positive conclusion needs a test and a stated observation.
Must know
- Carbonate and hydrogencarbonate: add acid, CO₂ turns limewater milky
- Sulfate: acidify with HCl then BaCl₂, white BaSO₄ precipitate
- Ammonium: warm with NaOH, NH₃ turns damp red litmus blue
- Halides: HNO₃ then AgNO₃, then ammonia
- Test carbonate first; do not use an acid that contains the ion being tested
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Analysis of inorganic compounds
- A school technician has labelled three white solids as sodium carbonate, sodium hydrogencarbonate and sodium sulfate, but the labels have come off. She has dilute hydrochloric acid, limewater and aqueous barium chloride.Write an ionic equation, with state symbols, for the reaction of hydrogencarbonate ions with dilute acid, and state how the gas formed is identified.2 marks
- An environmental analyst tests a sample of river water for dissolved sulfate ions using acidified aqueous barium chloride.Adding excess acidified barium chloride to 25.0 cm³ of the sample gives 0.466 g of dry barium sulfate (M = 233 g mol⁻¹). Calculate the amount of sulfate ions in the sample and their concentration in mol dm⁻³.2 marks
- A farm technician is analysing a fertiliser that is believed to be ammonium sulfate, (NH₄)₂SO₄.Describe how you would show that the fertiliser contains ammonium ions. Include the observation and an ionic equation with state symbols.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).