2.8 Chemical TestsEdexcel IGCSE Chemistry: Revision notes
Section 1
How do you test for common gases?
| Gas | Test | Positive result |
|---|---|---|
| Hydrogen | Hold a lit splint at the mouth of the test tube | A squeaky pop |
| Oxygen | Insert a glowing splint into the test tube | The splint relights |
| Carbon dioxide | Bubble the gas through limewater | The limewater turns milky/cloudy |
| Ammonia | Hold damp red litmus paper at the mouth of the test tube | The litmus paper turns blue |
| Chlorine | Hold damp litmus paper at the mouth of the test tube | The litmus paper turns white (bleached) |
Chlorine bleaches litmus paper white — it does not simply turn it a colour like ammonia does; students often confuse the two.
Section 2
How do you carry out and interpret a flame test?
A flame test identifies certain metal ions from the colour they produce when heated strongly in a flame.
Method: dip a clean nichrome (or platinum) wire into concentrated hydrochloric acid, then into the solid sample, then hold it in the edge of a blue Bunsen flame; observe the flame colour.
| Metal ion | Flame colour |
|---|---|
| Li+ | Red |
| Na+ | Yellow |
| K+ | Lilac |
| Ca2+ | Orange-red |
| Cu2+ | Blue-green |
Wear these five colours as a fixed list — Li red, Na yellow, K lilac, Ca orange-red, Cu blue-green — examiners expect exact recall.
Section 3
How do you test for cations using sodium hydroxide?
Adding sodium hydroxide solution produces different coloured precipitates (or a gas) with certain cations:
| Cation | Observation with NaOH(aq) |
|---|---|
| NH4+ (ammonium) | No precipitate; ammonia gas released on warming (turns damp red litmus blue) |
| Cu2+ | Blue precipitate forms |
| Fe2+ | Green precipitate forms |
| Fe3+ | Orange/red-brown precipitate forms |
Ammonium ions (NH4+) do NOT form a precipitate with sodium hydroxide — heating releases ammonia gas instead, which is confirmed with damp red litmus paper turning blue.
Section 4
How do you test for common anions?
| Anion | Test | Positive result |
|---|---|---|
| Cl- | Add dilute nitric acid, then acidified silver nitrate solution | White precipitate |
| Br- | Add dilute nitric acid, then acidified silver nitrate solution | Cream precipitate |
| I- | Add dilute nitric acid, then acidified silver nitrate solution | Yellow precipitate |
| SO4^2- | Add dilute hydrochloric acid, then acidified barium chloride solution | White precipitate |
| CO3^2- | Add dilute hydrochloric acid | Fizzing; gas turns limewater milky (carbon dioxide) |
To distinguish sodium chloride, bromide and iodide solutions, add dilute nitric acid then silver nitrate solution to each: chloride gives a white precipitate, bromide gives cream, iodide gives yellow.
Section 5
How do you test for water?
- Chemical test: Add the liquid to white anhydrous copper(II) sulfate. If water is present, the white solid turns blue as it forms hydrated copper(II) sulfate. This test shows water is present but does not prove the water is pure.
- Physical test for purity: measure the boiling point of the sample. Pure water boils at exactly 100°C (at standard atmospheric pressure); impure water boils at a different temperature (usually higher).
Must Know
- Hydrogen: squeaky pop with lit splint. Oxygen: relights a glowing splint. Carbon dioxide: turns limewater milky. Ammonia: turns damp red litmus blue. Chlorine: bleaches damp litmus white.
- Flame test colours: Li red, Na yellow, K lilac, Ca orange-red, Cu blue-green.
- NaOH(aq) with cations: Cu2+ blue precipitate, Fe2+ green precipitate, Fe3+ orange/red-brown precipitate; NH4+ releases ammonia gas on warming (no precipitate).
- Halide test: acidified silver nitrate — Cl- white, Br- cream, I- yellow precipitate.
- SO4^2- test: acidified barium chloride gives a white precipitate. CO3^2- test: dilute HCl gives fizzing/CO2.
- Water test: turns white anhydrous copper(II) sulfate blue; pure water's purity is confirmed by boiling point of exactly 100°C.
That's the notes covered.
Carry on to the next subtopic.