Identifying Ions and Gases
Tests for Gases
Hydrogen
Test: Hold a burning splint at the open mouth of a test tube containing the gas.
Positive result: The gas burns with a loud “squeaky pop”. (The hydrogen rapidly combusts with oxygen in the air to produce water.)
Oxygen
Test: Place a glowing (not burning) splint inside a test tube containing the gas.
Positive result: The glowing splint relights.
Exam Tip
A helpful memory aid: a lighted splint for Hydrogen; a glowing splint for oxygen (G for glowing = O).
Carbon Dioxide
Test: Bubble the gas through limewater (calcium hydroxide solution).
Positive result: The limewater turns milky white (a white precipitate of calcium carbonate forms).
Equation: CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l)
Ammonia
Test: Hold damp red litmus paper near the mouth of a test tube containing the gas. Do not let the paper touch the sides of the tube.
Positive result: The damp red litmus paper turns blue. Ammonia also has a characteristic sharp, choking smell.
Chlorine
Test: Hold damp blue litmus paper near the mouth of a test tube containing the gas.
Positive result: The damp blue litmus paper turns red (because chlorine dissolves in water to form an acid) and then is bleached white. Chlorine also has a characteristic sharp, choking smell.
Chlorine must always be handled in a fume cupboard due to its toxicity.
Summary Table
| Gas | Test | Positive result |
|---|---|---|
| Hydrogen | Burning splint held at mouth of tube | Squeaky pop |
| Oxygen | Glowing splint inserted into tube | Splint relights |
| Carbon dioxide | Bubble gas through limewater | Limewater turns milky white |
| Ammonia | Damp red litmus paper held near tube | Turns blue |
| Chlorine | Damp blue litmus paper held near tube | Turns red, then bleached white |
Flame Tests
Flame tests are used to identify metal cations by the colour of the flame they produce when heated.
Method
- Dip a nichrome or platinum wire loop in dilute hydrochloric acid.
- Hold the wire in the blue Bunsen flame until no colour is seen (cleaning the wire to prevent contamination).
- Dip the clean wire into the solid sample or solution.
- Hold the wire at the edge of the blue Bunsen flame and observe the colour.
Flame Colours
| Cation | Flame colour |
|---|---|
| Li+ | Red |
| Na+ | Yellow |
| K+ | Lilac |
| Ca2+ | Orange-red |
| Cu2+ | Blue-green |
Exam Tip
Sodium (Na+) gives an intense yellow flame that can mask other colours. If you suspect sodium contamination, clean the wire thoroughly between tests.
Tests for Cations
Metal Cations Using Sodium Hydroxide Solution
Adding sodium hydroxide (NaOH) solution to a solution of a metal salt causes the metal hydroxide to precipitate if the hydroxide is insoluble:
| Cation | Effect of adding NaOH solution | Colour of precipitate |
|---|---|---|
| Cu2+ | Light blue precipitate forms | Light blue |
| Fe2+ | Green precipitate forms | Green |
| Fe3+ | Brown precipitate forms | Brown |
Ammonium Ions (NH4+)
Test: Add sodium hydroxide solution to the sample and gently warm.
Positive result: Ammonia gas is given off, which turns damp red litmus paper blue.
Exam Tip
Be careful to distinguish between ammonium (the ion, NH4+, in solution) and ammonia (the gas, NH3). The test for ammonium ions detects the ammonia gas produced when NaOH is added.
Tests for Anions
Chloride, Bromide and Iodide Ions (Halide Ions)
Test:
- Acidify the sample with dilute nitric acid (not hydrochloric acid, as Cl- ions in HCl would interfere).
- Add silver nitrate solution (AgNO3).
Results:
| Halide ion | Precipitate | Colour |
|---|---|---|
| Cl- | Silver chloride (AgCl) | White |
| Br- | Silver bromide (AgBr) | Cream |
| I- | Silver iodide (AgI) | Yellow |
Example equations:
- KCl(aq) + AgNO3(aq) → KNO3(aq) + AgCl(s)
- KBr(aq) + AgNO3(aq) → KNO3(aq) + AgBr(s)
- KI(aq) + AgNO3(aq) → KNO3(aq) + AgI(s)
Exam Tip
Nitric acid (not hydrochloric acid) is used to acidify the sample, because hydrochloric acid contains chloride ions (Cl-) which would produce a false positive white precipitate with silver nitrate.
Sulfate Ions (SO42-)
Test:
- Acidify the sample with dilute hydrochloric acid (to remove any carbonates, which would also produce a precipitate and give a false positive).
- Add barium chloride solution (BaCl2) (or barium nitrate solution).
Positive result: A white precipitate of barium sulfate forms.
Ba2+(aq) + SO42-(aq) → BaSO4(s)
Carbonate Ions (CO32-)
Test:
- Add dilute acid (e.g. dilute hydrochloric acid) to the sample.
- Bubble the gas produced through limewater.
Positive result: Effervescence (fizzing) as carbon dioxide is produced; the limewater turns milky white.
CO32-(aq) + 2H+(aq) → CO2(g) + H2O(l)
CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l)
Tests for Water
Chemical Test
Test: Add anhydrous copper(II) sulfate (white powder) to the sample.
Positive result: The white powder turns blue, indicating water is present.
CuSO4(s) + 5H2O(l) → CuSO4.5H2O(s) (white) (blue)
Physical Test for Pure Water
Test: Measure the boiling point of the liquid.
Positive result for pure water: The liquid boils at exactly 100 degrees Celsius at standard atmospheric pressure.
If the sample is impure, dissolved substances will raise the boiling point above 100 degrees Celsius. A pure sample of water also melts at exactly 0 degrees Celsius; impurities depress the melting point.
Exam Questions
Q: Describe the test for hydrogen gas and state the result.
Hold a burning splint at the open mouth of the test tube. If hydrogen is present, the gas burns with a loud squeaky pop.
Q: Describe the test for oxygen gas and state the result.
Insert a glowing splint into the test tube. If oxygen is present, the glowing splint relights.
Q: Explain why nitric acid, rather than hydrochloric acid, is used to acidify the sample in the test for halide ions.
Hydrochloric acid contains chloride ions (Cl-) which would react with silver nitrate to produce a white precipitate of silver chloride, giving a false positive result regardless of which halide ion is present. Nitric acid does not contain chloride ions, so it does not interfere.
Q: Describe a flame test and state the colours for Na+, K+ and Ca2+.
Dip a clean nichrome wire in dilute acid, then hold it in the Bunsen flame until no colour is seen. Dip the wire in the sample and hold it at the edge of the blue flame. Na+ gives a yellow flame; K+ gives a lilac flame; Ca2+ gives an orange-red flame.
Q: Describe how to test for sulfate ions.
Acidify the sample with dilute hydrochloric acid (to remove interfering carbonates), then add barium chloride solution. A white precipitate of barium sulfate indicates sulfate ions are present.
Q: Describe the chemical test for water and state the positive result.
Add anhydrous copper(II) sulfate (white powder) to the sample. If water is present, the white powder turns blue, forming hydrated copper(II) sulfate.
Exam Tip
For tests, you must describe the test (what you add / how you carry it out) as well as state the positive result. Just stating the result without describing the test will not earn full marks.