Qualitative Analysis

Summary: Systematic identification of ions: flame tests for metal cations (Li+ red, Na+ yellow, K+ lilac, Ca2+ orange-red, Cu2+ blue-green); NaOH and NH3(aq) tests for cation precipitates; specific reagent tests for anions (halides, sulfate, carbonate, nitrate). Also covers tests for common gases. Tags: igcse chemistry acids-bases Created: 2026-07-14 Last Updated: 2026-07-16

Learning Objectives

By the end of this topic, you should be able to:

• Carry out flame tests to identify Li+, Na+, K+, Ca2+, Cu2+
• Use aqueous sodium hydroxide and aqueous ammonia to identify Al3+, Ca2+, Cu2+, Fe2+, Fe3+, Zn2+, NH4+
• Describe tests to identify anions: Cl-, Br-, I- (with HNO3 + AgNO3), SO4 2- (with HCl + BaCl2), CO3 2- (with acid + limewater), NO3- (with Al/NaOH + warming)
• Describe tests for gases: H2, O2, CO2, Cl2, NH3, SO2
• Describe the chemical test for water using anhydrous CuSO4 or anhydrous CoCl2
• Devise a sequence of tests to identify unknown ionic compounds

Content

1. What is Qualitative Analysis?

Qualitative analysis tells you what is present (which ions), as opposed to quantitative analysis which tells you how much is present.

In IGCSE Chemistry, qualitative analysis focuses on identifying:

• Gases (6 common gases: H2, O2, CO2, Cl2, NH3, SO2)
• Cations (positive ions: metal ions and ammonium ion)
• Anions (negative ions: halides, sulfate, carbonate, nitrate)

2. Tests for Gases

Test for water:

• White anhydrous CuSO4 turns blue (forms CuSO4.5H2O)
• Blue anhydrous CoCl2 turns pink (forms CoCl2.6H2O)
• Physical test: pure water boils at 100°C, freezes at 0°C

3. Flame Tests for Metal Cations

Method:

1. Clean a nichrome (or platinum) wire by dipping in concentrated HCl and heating in a non-luminous Bunsen flame until no colour is imparted
2. Dip the clean wire into the sample (solid or solution)
3. Hold the wire in the edge of the non-luminous flame
4. Observe the characteristic flame colour

4. Cation Tests Using Sodium Hydroxide Solution

Adding NaOH(aq) dropwise (then in excess) to a solution of a metal salt:

Amphoteric hydroxides: Al(OH)3 and Zn(OH)2 are amphoteric — they react with both acids and bases, which is why they dissolve in excess NaOH.

Test for NH4+: Add NaOH(aq) and warm gently. Hold damp red litmus paper at the mouth of the test tube. Litmus turns blue (NH3 gas produced). Equation: NH4+ + OH- → NH3 + H2O.

5. Cation Tests Using Aqueous Ammonia

Adding aqueous ammonia dropwise (then in excess):

Key distinguishing tests:

• Al3+ vs Zn2+: Both give white precipitates with NaOH that dissolve in excess. But with excess NH3, only Zn2+ dissolves; Al3+ does not.
• Cu2+ gives a blue precipitate with both, but dissolves only in excess NH3 (deep blue), not in excess NaOH.

6. Anion Tests

Why add acid first? Carbonate ions form white precipitates with both Ag+ (Ag2CO3) and Ba2+ (BaCO3). Adding acid decomposes carbonates to CO2 + H2O, removing the interference. The effervescence confirms that carbonate was present and has now been eliminated.

7. Systematic Approach to Identifying an Unknown Salt

Step 1 — Appearance: Note colour. Cu2+ compounds: blue/green; Fe2+ compounds: green; Fe3+ compounds: yellow/brown; most others: white/colourless.

Step 2 — Flame test: Identifies Li+, Na+, K+, Ca2+, Cu2+.

Step 3 — NaOH test: Precipitate colour + solubility in excess.

Step 4 — NH3 test: Precipitate colour + solubility in excess.

Step 5 — Anion tests: Carbonate (acid + limewater), sulfate (HCl + BaCl2), halide (HNO3 + AgNO3), nitrate (NaOH + Al + warm).

Step 6 — Confirm: Cross-reference all results to identify the compound.

Worked Examples (Exam-Style)

Example 1: Flame Test and Anion

Question: A white solid produces a brick-red flame. Adding HCl and then BaCl2 gives a white precipitate. Identify the solid and name the white precipitate. [3 marks]

Solution (model answer):

• Brick-red flame → Ca2+ ✓
• White precipitate with acidified BaCl2 → SO4 2- ✓
• Solid is calcium sulfate (CaSO4); the white precipitate is barium sulfate (BaSO4) ✓

Example 2: Distinguishing Al3+ and Zn2+

Question: Describe tests to distinguish between aqueous solutions of aluminium sulfate and zinc sulfate. [4 marks]

Solution (model answer):

• Add aqueous sodium hydroxide to both — both give white precipitates that dissolve in excess ✓
• Add aqueous ammonia to both — both give white precipitates ✓
• With excess aqueous ammonia, the aluminium precipitate does NOT dissolve, but the zinc precipitate DOES dissolve ✓
• This distinguishes them ✓

Example 3: Full Unknown Analysis

Question: A solid X gives a lilac flame test. Adding NaOH and warming produces a gas that turns damp red litmus blue. Adding HNO3 and AgNO3 gives a cream precipitate. Identify X. [4 marks]

Solution (model answer):

• Lilac flame → K+ ✓
• Gas on warming with NaOH turning red litmus blue → NH4+ present OR… Actually, the lilac flame confirms K+ only. The NH3 gas on warming comes from NH4+ in the compound. But one compound has one cation. Re-examining: If NaOH + Al + warming → NH3, that indicates NO3-.
• Cream precipitate with HNO3 + AgNO3 → Br- ✓
• The compound is potassium nitrate… No, nitrate was detected via the Al/NaOH test. But the question says “adding NaOH and warming” (no Al mentioned), producing NH3 → that indicates NH4+.
• So: The compound contains NH4+ (cation) and Br- (anion): ammonium bromide.
• But the lilac flame test is from K+. So either KBr (contaminated with NH4+), or the question has both K+ and NH4+.

Let me rewrite this example more cleanly:

Alternative clean example: Question: A green solid Z dissolves in water to give a green solution. Adding NaOH(aq) gives a green precipitate. Adding HNO3 and AgNO3 gives a yellow precipitate. Suggest the identity of Z. [4 marks]

Solution (model answer):

• Green solid giving green solution → Fe2+ compound ✓
• Green precipitate with NaOH confirms Fe2+ (Fe(OH)2 forms) ✓
• Yellow precipitate with HNO3 + AgNO3 → I- (AgI is yellow) ✓
• The solid is iron(II) iodide (FeI2) ✓

Example 4: Sulfate vs Carbonate

Question: A white solid effervesces when dilute HCl is added, producing a gas that turns limewater milky. Adding HCl and BaCl2 gives a white precipitate. Identify TWO anions present. [3 marks]

Solution (model answer):

• Effervescence with acid, gas turns limewater milky → CO3 2- ✓
• White precipitate with acidified BaCl2 → SO4 2- ✓
• Both carbonate and sulfate ions are present ✓

Practice Questions

1. State the flame colour produced by: (a) sodium ions, (b) potassium ions, (c) copper(II) ions. [3 marks]

2. Describe what is observed when aqueous sodium hydroxide is added dropwise (then in excess) to a solution containing iron(II) ions. [2 marks]

3. A solution gives a white precipitate with acidified barium chloride. Name the anion present and give the formula of the precipitate. [2 marks]

4. Explain why dilute nitric acid is added before silver nitrate solution when testing for halide ions. [2 marks]

5. Describe a sequence of chemical tests to distinguish between solid sodium chloride and solid sodium carbonate. [4 marks]

6. A solution gives a blue precipitate with NaOH that does not dissolve in excess. When aqueous ammonia is added, a blue precipitate forms that dissolves in excess to give a deep blue solution. Identify the cation. [2 marks]

Key Facts to Memorise

Flame Tests:

• Li+ = red, Na+ = yellow, K+ = lilac, Ca2+ = orange-red, Cu2+ = blue-green

Cation Precipitates with NaOH:

• Al3+ = white (dissolves in excess), Ca2+ = white, Cu2+ = blue, Fe2+ = green, Fe3+ = red-brown, Zn2+ = white (dissolves in excess)
• NH4+ = NH3 gas on warming (turns damp red litmus blue)

Anion Tests:

• Cl- = white ppt with HNO3 + AgNO3
• Br- = cream ppt with HNO3 + AgNO3
• I- = yellow ppt with HNO3 + AgNO3
• SO4 2- = white ppt with HCl + BaCl2
• CO3 2- = effervescence with acid; gas turns limewater milky
• NO3- = NaOH + Al, warm; NH3 gas turns damp red litmus blue

Gas Tests:

• H2 = lighted splint → squeaky pop
• O2 = glowing splint → relights
• CO2 = limewater → milky
• Cl2 = damp blue litmus → red then bleaches white
• NH3 = damp red litmus → blue
• SO2 = acidified KMnO4 → purple to colourless

Common Misconceptions

Key Concepts from Past Papers

Recurring Mark Scheme Answers

• “Flame test / put potassium compound in non-luminous flame → lilac colour”
• “Glowing splint → relights” (oxygen)
• “Add nitric acid → add aqueous silver nitrate → white precipitate” (chloride)
• “Add (hydrochloric) acid → test gas given off with limewater → turns milky/cloudy/white precipitate” (carbonate)
• “Add HCl, then BaCl2 → white precipitate” (sulfate)
• “NaOH + Al + warm → gas turns damp red litmus blue” (nitrate)

Common Question Types

• Short recall (state/give/name): “State the colour of the precipitate formed when…”
• Extended writing (describe/explain/suggest): “Describe how you would test for…”
• Table completion: “Complete the table to show the test, observation, and conclusion”
• Multiple choice / tick-box: “Which reagent is used to test for…”
• Flowchart/sequence: “Devise a sequence of tests to identify compound X”

Exam Tips

• This topic appears in 35 papers in the database
• Total Q+A entries: 92
• Average marks per question: 1.1
• Always specify “dilute” acid and “aqueous” reagent where appropriate
• For the nitrate test, you MUST mention aluminium powder/foil AND warming — both are essential marking points
• Learn the exact colour descriptions: “red-brown” not “brown” (Fe3+); “blue-green” not “green” (Cu2+ flame)
• Don’t confuse the test for NH4+ (NaOH + warm → NH3) with the test for NO3- (NaOH + Al + warm → NH3)

Keywords from Past Papers

acid, add, aqueous, precipitate, colour, indicator, carbon, dioxide, white, sodium, hydroxide, nitrogen, limewater, flame, test

Related Notes

• Chemical Tests — Detailed tests for gases, cations, anions, and water
• Indicators and pH — pH scale and indicators
• Solubility Rules — Solubility of common salts
• Making Salts — Preparation methods for soluble and insoluble salts
• IGCSE-Chem-Index — Full IGCSE Chemistry index

Sources

• OpenStax Chemistry 2e — [Chapter 15: Equilibria of Other Reaction Classes (Qualitative Analysis)], Rice University (free, CC BY 4.0)
• BBC Bitesize GCSE Chemistry — [Qualitative Analysis and Ion Tests], BBC (free educational resource)
• Cambridge IGCSE Chemistry 0620 — Syllabus Section 8: Acids, Bases and Salts (Qualitative Analysis), Cambridge Assessment International Education
• CK-12 Chemistry for High School — [Chapter 23: Qualitative Chemical Analysis], CK-12 Foundation (free, CC BY-NC 3.0)

Past Paper Sources

• 0620/31 May/June 2015: Q11(c)(i) (0m), Q66(a)(i) (0m), Q66(a)(ii) (0m)
• 0620/32 Feb/March 2015: Q66(e)(ii) (2m), Q66(b)(ii) (1m), Q77(d)(i) (0m)
• 0620/32 Feb/March 2017: Q22(f)(ii) (1m), Q44(d)(i) (1m), Q55(f)(i) (1m) (+1 more)
• 0620/32 Feb/March 2018: Q88(b)(ii) (3m), Q22(a)(iv) (2m), Q77(b)(i) (1m) (+1 more)
• 0620/32 Feb/March 2019: Q44(b)(ii) (1m)
• 0620/32 Feb/March 2020: Q55(d)(iii) (2m), Q66(a)(ii) (1m)
• 0620/32 Feb/March 2021: Q66(a)(ii) (1m)
• 0620/32 Feb/March 2022: Q88(d)(ii) (2m), Q44(a)(ii) (0m)
• 0620/32 Feb/March 2023: Q33(b)(ii) (1m), Q88(c)(iii) (1m)
• 0620/32 May/June 2018: Q33(b)(i) (1m), Q33(b)(ii) (1m), Q44(c)(i) (1m)
• 0620/32 May/June 2020: Q88(e)(i) (1m)
• 0620/33 May/June 2016: Q88(c)(ii) (1m), Q88(c)(iii) (0m)