IGCSE Chemistry: Chemical Tests
Complete revision page covering every specification point in the chemical tests checklist
Specification Checklist from the Table
The table below shows each specification point from the image and where it is covered on this page.
| Specification Point | Status | Covered In |
|---|---|---|
| Describe tests for these gases: hydrogen, oxygen, carbon dioxide, ammonia and chlorine | Included | Section 1: Tests for Gases |
| Describe a test for the presence of water using anhydrous copper(II) sulfate | Included | Section 2: Test for Water |
| Describe a physical test to show whether a sample of water is pure | Included | Section 3: Physical Test for Pure Water |
| Describe how to carry out a flame test | Included | Section 4: How to Carry Out a Flame Test |
| Know the colours formed in flame tests for lithium, sodium, potassium, calcium and copper(II) ions | Included | Section 4: Flame Test Colours |
| Know the test and results for ammonium, copper(II), iron(II) and iron(III) ions using sodium hydroxide solution | Included | Section 5: Tests Using Sodium Hydroxide |
| Know the test and results for chloride, bromide and iodide ions using acidified silver nitrate solution | Included | Section 6: Tests for Halide Ions |
| Know the test and results for sulfate ions using acidified barium chloride solution | Included | Section 7: Test for Sulfate Ions |
| Know the test and results for carbonate ions using hydrochloric acid and identifying the gas produced | Included | Section 8: Test for Carbonate Ions |
1. Tests for Gases
You must be able to describe the test and positive result for hydrogen, oxygen, carbon dioxide, ammonia and chlorine.
| Gas | Formula | Test | Positive Result | Extra Detail |
|---|---|---|---|---|
| Hydrogen | H2 | Place a lighted splint near the gas. | The gas burns with a squeaky pop. | Hydrogen is flammable and reacts quickly with oxygen in the air. |
| Oxygen | O2 | Place a glowing splint into the gas. | The glowing splint relights. | Oxygen supports combustion. |
| Carbon dioxide | CO2 | Bubble the gas through limewater. | Limewater turns milky or cloudy. | The milky colour is caused by a white precipitate of calcium carbonate. |
| Ammonia | NH3 | Hold damp red litmus paper near the gas. | Damp red litmus paper turns blue. | Ammonia is an alkaline gas. |
| Chlorine | Cl2 | Hold damp blue litmus paper near the gas. | Damp blue litmus paper turns red, then is bleached white. | Chlorine is acidic in damp conditions and is a bleaching agent. |
Exam Tip: In exams, always state both the test and the observation.
2. Test for the Presence of Water
The required chemical test in the specification uses anhydrous copper(II) sulfate.
| Substance Tested | Reagent Used | Test Method | Positive Result |
|---|---|---|---|
| Water | Anhydrous copper(II) sulfate | Add the liquid sample to white anhydrous copper(II) sulfate. | The solid changes from white to blue. |
Key Point: Anhydrous copper(II) sulfate is white. When water is present,
it becomes hydrated copper(II) sulfate, which is blue.
3. Physical Test to Show Whether Water Is Pure
A physical test checks a physical property of water, such as boiling point or freezing point.
| Physical Test | Result for Pure Water | What This Shows |
|---|---|---|
| Measure the boiling point of the sample. | Pure water boils at 100°C at normal atmospheric pressure. | If the boiling point is not 100°C, the sample may be impure. |
| Measure the freezing point or melting point. | Pure water freezes or melts at 0°C. | If the freezing point is lower than 0°C, dissolved impurities may be present. |
Exam Tip: Pure substances have fixed melting and boiling points.
Impure substances usually melt over a range of temperatures.
4. Flame Tests
How to Carry Out a Flame Test
Flame tests are used to identify some metal ions by the colour they produce in a flame.
- Use a clean nichrome or platinum wire loop.
- Dip the wire loop into dilute hydrochloric acid to clean it.
- Place the wire loop in a hot blue Bunsen flame until no colour is seen.
- Dip the clean loop into the sample.
- Place the sample in the hottest part of the blue flame.
- Observe and record the flame colour.
Safety: Wear eye protection, tie back long hair and take care when using a Bunsen burner.
Flame Test Colours
| Ion | Formula | Flame Colour |
|---|---|---|
| Lithium ion | Li+ | Red / crimson |
| Sodium ion | Na+ | Yellow / orange-yellow |
| Potassium ion | K+ | Lilac |
| Calcium ion | Ca2+ | Orange-red / brick red |
| Copper(II) ion | Cu2+ | Blue-green |
5. Tests for Ammonium, Copper(II), Iron(II) and Iron(III) Ions Using Sodium Hydroxide
Sodium hydroxide solution is used to test for some positive ions. Metal ions often form coloured precipitates with hydroxide ions.
| Ion | Formula | Test Using Sodium Hydroxide | Result / Observation | Conclusion |
|---|---|---|---|---|
| Ammonium ion | NH4+ | Add sodium hydroxide solution and warm gently. | Ammonia gas is produced. It turns damp red litmus paper blue. | Ammonium ions are present. |
| Copper(II) ion | Cu2+ | Add sodium hydroxide solution. | Blue precipitate forms. | Copper(II) ions are present. |
| Iron(II) ion | Fe2+ | Add sodium hydroxide solution. | Green precipitate forms. | Iron(II) ions are present. |
| Iron(III) ion | Fe3+ | Add sodium hydroxide solution. | Red-brown precipitate forms. | Iron(III) ions are present. |
Precipitate Meaning: A precipitate is an insoluble solid that forms when two solutions react.
6. Tests for Chloride, Bromide and Iodide Ions Using Acidified Silver Nitrate
Chloride, bromide and iodide ions are halide ions. They are tested using acidified silver nitrate solution.
| Halide Ion | Formula | Test | Result / Observation | Precipitate Formed |
|---|---|---|---|---|
| Chloride ion | Cl- | Add dilute nitric acid, then add silver nitrate solution. | White precipitate forms. | Silver chloride, AgCl |
| Bromide ion | Br- | Add dilute nitric acid, then add silver nitrate solution. | Cream precipitate forms. | Silver bromide, AgBr |
| Iodide ion | I- | Add dilute nitric acid, then add silver nitrate solution. | Yellow precipitate forms. | Silver iodide, AgI |
Important: The silver nitrate solution is acidified using dilute nitric acid.
7. Test for Sulfate Ions Using Acidified Barium Chloride Solution
| Ion | Formula | Test | Result / Observation | Precipitate Formed |
|---|---|---|---|---|
| Sulfate ion | SO42- | Add dilute hydrochloric acid, then add barium chloride solution. | White precipitate forms. | Barium sulfate, BaSO4 |
Exam Tip: Barium sulfate is insoluble, so it appears as a white precipitate.
8. Test for Carbonate Ions Using Hydrochloric Acid and Identifying the Gas Produced
Carbonate ions react with acids to produce carbon dioxide gas.
| Ion | Formula | Test | Result / Observation | Gas Produced | How to Identify the Gas |
|---|---|---|---|---|---|
| Carbonate ion | CO32- | Add dilute hydrochloric acid to the sample. | Fizzing or effervescence occurs. | Carbon dioxide, CO2 | Bubble the gas through limewater. Limewater turns milky. |
General reaction: carbonate + acid → salt + water + carbon dioxide
9. Extra Revision: Solubility
Solubility is not shown as a separate item in the checklist image, but it is useful because many chemical tests depend on the formation of insoluble precipitates.
Key Solubility Rules
| Compound Type | Solubility in Water | Examples |
|---|---|---|
| All sodium, potassium and ammonium salts | Soluble | NaCl, KNO3, NH4Cl |
| All nitrates | Soluble | NaNO3, Cu(NO3)2, AgNO3 |
| Most chlorides | Soluble | NaCl, KCl, CuCl2 |
| Silver chloride and lead(II) chloride | Insoluble or slightly soluble | AgCl, PbCl2 |
| Most sulfates | Soluble | Na2SO4, CuSO4, MgSO4 |
| Barium sulfate, lead(II) sulfate and calcium sulfate | Insoluble or slightly soluble | BaSO4, PbSO4, CaSO4 |
| Most carbonates | Insoluble | CaCO3, CuCO3, ZnCO3 |
| Sodium, potassium and ammonium carbonates | Soluble | Na2CO3, K2CO3, (NH4)2CO3 |
| Most hydroxides | Insoluble | Cu(OH)2, Fe(OH)2, Fe(OH)3 |
| Sodium hydroxide, potassium hydroxide and ammonium hydroxide | Soluble | NaOH, KOH, NH4OH |
Why Solubility Matters in Chemical Tests
- Silver chloride is insoluble, so chloride ions give a white precipitate with silver nitrate.
- Silver bromide is insoluble, so bromide ions give a cream precipitate.
- Silver iodide is insoluble, so iodide ions give a yellow precipitate.
- Barium sulfate is insoluble, so sulfate ions give a white precipitate with barium chloride.
- Metal hydroxides such as copper(II) hydroxide and iron hydroxides are insoluble, so they appear as coloured precipitates with sodium hydroxide.
10. Final Quick Summary
| Specification Item | Must-Know Result |
|---|---|
| Hydrogen gas | Lighted splint gives a squeaky pop. |
| Oxygen gas | Glowing splint relights. |
| Carbon dioxide gas | Limewater turns milky. |
| Ammonia gas | Damp red litmus paper turns blue. |
| Chlorine gas | Damp blue litmus paper turns red, then white. |
| Water with anhydrous copper(II) sulfate | White solid turns blue. |
| Pure water | Boils at 100°C and freezes/melts at 0°C. |
| Flame test method | Clean wire loop, dip in sample, place in blue flame, observe colour. |
| Lithium ion | Red / crimson flame. |
| Sodium ion | Yellow flame. |
| Potassium ion | Lilac flame. |
| Calcium ion | Orange-red / brick-red flame. |
| Copper(II) ion flame test | Blue-green flame. |
| Ammonium ion using sodium hydroxide | On warming, ammonia is produced; damp red litmus turns blue. |
| Copper(II) ion using sodium hydroxide | Blue precipitate. |
| Iron(II) ion using sodium hydroxide | Green precipitate. |
| Iron(III) ion using sodium hydroxide | Red-brown precipitate. |
| Chloride ion using acidified silver nitrate | White precipitate. |
| Bromide ion using acidified silver nitrate | Cream precipitate. |
| Iodide ion using acidified silver nitrate | Yellow precipitate. |
| Sulfate ion using acidified barium chloride | White precipitate. |
| Carbonate ion using hydrochloric acid | Fizzing occurs; carbon dioxide is produced. |
| Gas from carbonate test | Carbon dioxide turns limewater milky. |