Chemical Tests
Summary: Qualitative chemical tests for identifying gases, cations, anions, and water using specific reagents and observing characteristic results. These tests are essential for the practical component of the IGCSE Chemistry 0620 exam. Tags: igcse chemistry practical-skills Created: 2026-07-14 Last Updated: 2026-07-16
Learning Objectives
By the end of this topic, you should be able to:
- Describe tests for common gases: H2, O2, CO2, Cl2, NH3, SO2
- Describe the chemical test for water using anhydrous copper(II) sulfate and anhydrous cobalt(II) chloride
- Describe flame tests to identify metal cations: Li+, Na+, K+, Ca2+, Cu2+
- Describe tests for cations using aqueous sodium hydroxide and aqueous ammonia
- Describe tests for anions: Cl-, Br-, I- (with AgNO3 + HNO3), SO4 2- (with BaCl2 + HCl), CO3 2- (with acid + limewater), NO3- (with Al/NaOH + litmus)
Content
1. Gas Tests
This is the most frequently examined section. Learn ALL six gas tests exactly.
| Gas | Test | Positive Result |
|---|---|---|
| Hydrogen (H2) | Hold a lighted splint near the mouth of the test tube | Gas burns with a squeaky pop (a small explosion) |
| Oxygen (O2) | Insert a glowing splint into the test tube | The glowing splint relights |
| Carbon dioxide (CO2) | Bubble gas through limewater (calcium hydroxide solution) | Limewater turns milky/cloudy (white precipitate of CaCO3 forms) |
| Chlorine (Cl2) | Hold damp blue litmus paper or damp universal indicator paper in the gas | Litmus paper turns red then bleaches white (Cl2 is acidic and a bleaching agent) |
| Ammonia (NH3) | Hold damp red litmus paper in the gas | Red litmus paper turns blue (NH3 is the only common alkaline gas) |
| Sulfur dioxide (SO2) | Bubble gas through acidified potassium dichromate(VI) or acidified potassium manganate(VII) solution | Orange → green (dichromate); purple → colourless (manganate) |
Equations for the tests:
- CO2 + Ca(OH)2 → CaCO3 (s, white) + H2O
- CaCO3 + H2O + CO2 (excess) → Ca(HCO3)2 (aq, colourless) — precipitate dissolves with excess CO2
2. Test for Water
There are two chemical tests for water:
| Test | Observation | Equation |
|---|---|---|
| Add anhydrous copper(II) sulfate (white) to the sample | Turns blue | CuSO4 (s, white) + 5H2O (l) → CuSO4.5H2O (s, blue) |
| Add anhydrous cobalt(II) chloride (blue) to the sample | Turns pink | CoCl2 (s, blue) + 6H2O (l) → CoCl2.6H2O (s, pink) |
Physical test for pure water: Pure water boils at 100°C and freezes at 0°C. Impurities lower the melting point and raise the boiling point.
3. Flame Tests for Metal Cations
Clean the nichrome/platinum wire by dipping in concentrated HCl and heating in a hot Bunsen flame until no colour is seen. Repeat between tests.
| Cation | Flame Colour | Memory Aid |
|---|---|---|
| Li+ (lithium) | Red (crimson) | Li-red (sounds like “lyre”) |
| Na+ (sodium) | Yellow (intense, persistent) | Na in sodium street lamps — yellow |
| K+ (potassium) | Lilac (pale purple) | K for “Kool-aid” lilac |
| Ca2+ (calcium) | Orange-red (brick red) | Ca — bricks contain calcium compounds |
| Cu2+ (copper) | Blue-green | Cu — copper compounds are often blue/green |
Note: The intense yellow of sodium can mask other flame colours. Cobalt blue glass filters out yellow, allowing other colours to be seen.
4. Testing Cations with NaOH and NH3 (aq)
Adding aqueous sodium hydroxide (NaOH) or aqueous ammonia (NH3) to a solution of a metal salt often produces a precipitate of the metal hydroxide.
| Cation | With NaOH (aq) | With NH3 (aq) — small amount | With excess NH3 (aq) |
|---|---|---|---|
| Al3+ | White precipitate, dissolves in excess NaOH | White precipitate | Precipitate does NOT dissolve |
| Ca2+ | White precipitate, does NOT dissolve in excess | No precipitate / very faint white | — |
| Cu2+ | Blue precipitate, does NOT dissolve in excess | Blue precipitate | Blue precipitate dissolves, forms deep blue solution |
| Fe2+ | Green precipitate, does NOT dissolve in excess | Green precipitate | Precipitate does NOT dissolve |
| Fe3+ | Red-brown precipitate, does NOT dissolve in excess | Red-brown precipitate | Precipitate does NOT dissolve |
| Zn2+ | White precipitate, dissolves in excess NaOH | White precipitate | White precipitate dissolves |
| NH4+ | Ammonia gas produced on warming (test with damp red litmus → turns blue) | — | — |
Equations (for completeness — IGCSE may not require all but good to know):
- Cu2+ + 2OH- → Cu(OH)2 (s, blue)
- Fe2+ + 2OH- → Fe(OH)2 (s, green)
- Fe3+ + 3OH- → Fe(OH)3 (s, red-brown)
- Zn(OH)2 + 2OH- → [Zn(OH)4]2- (dissolves in excess NaOH — amphoteric)
- Al(OH)3 + OH- → [Al(OH)4]- (dissolves in excess NaOH — amphoteric)
5. Anion Tests
| Anion | Test | Positive Result | Notes |
|---|---|---|---|
| Cl- (chloride) | Add nitric acid (HNO3), then aqueous silver nitrate (AgNO3) | White precipitate of AgCl | Add HNO3 first to remove carbonate ions that would also form a precipitate (Ag2CO3) |
| Br- (bromide) | Add HNO3, then AgNO3 | Cream precipitate of AgBr | — |
| I- (iodide) | Add HNO3, then AgNO3 | Yellow precipitate of AgI | — |
| SO4 2- (sulfate) | Add hydrochloric acid (HCl), then aqueous barium chloride (BaCl2) or barium nitrate | White precipitate of BaSO4 | Add HCl first to remove carbonate/sulfite ions |
| CO3 2- (carbonate) | Add any dilute acid (HCl/HNO3/H2SO4) | Effervescence (fizzing) — gas turns limewater milky | The gas is CO2 |
| NO3- (nitrate) | Add sodium hydroxide solution and aluminium powder/foil, warm gently | Ammonia gas produced — turns damp red litmus blue | The Al reduces NO3- to NH3 in alkaline conditions |
Key equations:
- Ag+ (aq) + Cl- (aq) → AgCl (s, white)
- Ag+ (aq) + Br- (aq) → AgBr (s, cream)
- Ag+ (aq) + I- (aq) → AgI (s, yellow)
- Ba2+ (aq) + SO4 2- (aq) → BaSO4 (s, white)
- CO3 2- (aq) + 2H+ (aq) → CO2 (g) + H2O (l)
6. Why Add Acid Before Precipitating Agents
When testing for chloride/bromide/iodide, add nitric acid first. When testing for sulfate, add hydrochloric acid first.
Reason: Carbonates and sulfites also form precipitates with Ag+ and Ba2+ (e.g., Ag2CO3, BaCO3, BaSO3). Adding acid removes these interfering ions by decomposing them:
- CO3 2- + 2H+ → CO2 + H2O
- SO3 2- + 2H+ → SO2 + H2O
The acid must be added first, and effervescence confirms the presence of carbonate/sulfite that has now been removed.
Worked Examples (Exam-Style)
Example 1: Identifying an Unknown Salt
Question: A white solid X produces a lilac flame. It dissolves in water. Adding NaOH and warming produces a gas that turns damp red litmus blue. Identify X. [3 marks]
Solution (model answer):
- Lilac flame → K+ (potassium ion) ✓
- Gas turns damp red litmus blue → NH3 gas → NH4+ ion present ✓
- The white solid X is potassium nitrate (KNO3) ✓
Example 2: Distinguishing Between Two Salts
Question: Describe a test to distinguish between sodium chloride and sodium iodide. Include the expected observations. [3 marks]
Solution (model answer):
- Dissolve each solid in water and add nitric acid, then aqueous silver nitrate ✓
- Sodium chloride gives a white precipitate ✓
- Sodium iodide gives a yellow precipitate ✓
Example 3: Sulfate Test
Question: Describe the test for sulfate ions in an aqueous solution. [3 marks]
Solution (model answer):
- Add dilute hydrochloric acid to the solution ✓
- Then add aqueous barium chloride (or barium nitrate) solution ✓
- A white precipitate of barium sulfate confirms sulfate ions ✓
Practice Questions
-
Describe the test for each of the following gases and state the positive result: (a) hydrogen, (b) oxygen, (c) carbon dioxide. [6 marks]
-
A student adds aqueous sodium hydroxide to a blue solution. A blue precipitate forms. (a) Identify the cation present. (b) State what happens when excess aqueous ammonia is added. [3 marks]
-
Describe the chemical test for water using anhydrous copper(II) sulfate and state the colour change. [2 marks]
-
Explain why dilute nitric acid is added before silver nitrate when testing for halide ions. [2 marks]
-
A solution gives a red-brown precipitate with sodium hydroxide and a white precipitate with acidified barium chloride. Identify the two ions present and name one compound that gives these results. [3 marks]
Key Facts to Memorise
Gas Tests:
- H2: lighted splint → squeaky pop
- O2: glowing splint → relights
- CO2: limewater → milky/cloudy
- Cl2: damp blue litmus → red then bleaches white
- NH3: damp red litmus → blue
- SO2: acidified KMnO4 → purple to colourless / acidified K2Cr2O7 → orange to green
Water Tests:
- White anhydrous CuSO4 → blue (with water)
- Blue anhydrous CoCl2 → pink (with water)
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 on warming
Anion Tests:
- Cl-/Br-/I-: HNO3 + AgNO3 → white / cream / yellow precipitate
- SO4 2-: HCl + BaCl2 → white precipitate
- CO3 2-: acid → effervescence; gas turns limewater milky
- NO3-: NaOH + Al, warm → NH3 (turns damp red litmus blue)
Common Misconceptions
| Students often think… | But the correct understanding is… |
|---|---|
| ”You test for hydrogen with a glowing splint” | Hydrogen is tested with a lighted splint (squeaky pop). A glowing splint is for oxygen (relights) |
| “Limewater goes clear with CO2” | Limewater goes milky/cloudy. Only with EXCESS CO2 does it go clear again |
| ”Chlorine bleaches litmus immediately” | Chlorine first turns damp blue litmus red (acidic), then bleaches it white |
| ”All white precipitates are the same” | White precipitates include Al(OH)3, Ca(OH)2, Zn(OH)2, AgCl, BaSO4 — they are different compounds with different solubilities |
| ”You don’t need to add acid before the halide test” | Nitric acid must be added first to remove carbonate ions that would also form a white precipitate with Ag+ |
Key Concepts from Past Papers
Recurring Mark Scheme Answers
- Flame test: put potassium/sodium compound in (non-luminous) flame → lilac / yellow colour
- Oxygen: glowing splint → relights
- Carbon dioxide: add acid → test gas with limewater → turns milky/cloudy/white precipitate
- Chloride test: add nitric acid → add (aqueous) silver nitrate → white precipitate
- “Add (hydrochloric) acid; test gas given off with limewater” (for carbonates)
- “1 mark if silver nitrate alone” (must add nitric acid first for full marks)
Common Question Types
- Extended writing (describe/explain/suggest): “Describe a chemical test for…”
- Short recall (state/give/name): “State the colour of the precipitate formed when…”
- Table completion: “Complete the table showing the test, observation, and conclusion”
- Sequence/flowchart: “Describe the sequence of tests to identify…”
- Diagram: flame test setup, gas collection apparatus
Exam Tips
- This topic appears in 13 papers in the database
- Total Q+A entries: 14
- Average marks per question: 1.6
- Learn the exact wording of gas tests — “squeaky pop” and “glowing splint relights” must be precise
- ALWAYS mention adding acid first in anion tests — missing this costs a mark
- For flame test, you must mention using a non-luminous flame (not the yellow safety flame)
- The aluminium + NaOH test for nitrates is a specific detail that is easy to forget — revise it
Keywords from Past Papers
aqueous, precipitate, add, white, flame, test, sodium, nitrate, hydroxide, ammonia, acid, silver, limewater, solid, barium
Related Notes
- Experimental Techniques — Separation and purification methods
- Qualitative Analysis — Systematic identification of ions
- Indicators and pH — pH scale and indicators
- Solubility Rules — Solubility of common 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 — [Chemical Tests], BBC (free educational resource)
- Cambridge IGCSE Chemistry 0620 — Syllabus Section 8: Acids, Bases and Salts (Chemical Tests), Cambridge Assessment International Education
- CK-12 Chemistry for High School — [Chapter 23: Chemical Analysis], CK-12 Foundation (free, CC BY-NC 3.0)
Past Paper Sources
- 0620/32 Feb/March 2018: Q22(a)(iv) (2m), Q88(b)(ii) (3m)
- 0620/32 Feb/March 2022: Q88(d)(ii) (2m)
- 0620/32 May/June 2018: Q44(c)(i) (1m)
- 0620/33 May/June 2021: Q77(c)(ii) (0m)
- 0620/33 May/June 2023: Q22(b)(ii) (0m)
- 0620/33 October/November 2016: Q55(d)(i) (1m)
- 0620/33 October/November 2017: Q33(b)(ii) (1m)
- 0620/33 October/November 2020: Q22(e)(i) (2m)
- 0620/33 October/November 2023: Q33(d)(iii) (2m)
- 0971/31 May/June 2018: Q44(e)(i) (2m)
- 0971/31 May/June 2021: Q55(a)(ii) (4m)
- 0971/32 May/June 2023: Q33(c)(ii) (1m)