12.5Experimental Techniques and Chemical Analysis
Identification of ions and gases — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 12 parts Lesson 5 of 5 1 quiz
Lesson summary
12.5 Identification of ions and gases — Notes — IGCSE Chemistry notes: Describe the tests for anions, cations and gases and record observations correctly.Qualitative analysis identifies the ions or gases in a substance using chemical tests.Record the test, the observation and the…
- Ionic equations
- Colour clues
- Carbon dioxide
- Hydrogen and oxygen
- Conclusions
- Colourless and clear
Section 1 of 12Qualitative analysis
1
Qualitative analysis identifies the ions or gases in a substance using chemical tests.
2
Record the test, the observation and the conclusion.
3
Many cation tests use sodium hydroxide or aqueous ammonia to form coloured insoluble metal hydroxides.
4
A sample is analysed with one test for the cation and another for the anion.
Section 2 of 12Tests for anions
| Anion | Test | Positive result |
|---|---|---|
| Carbonate, CO32− | Add dilute acid and bubble the gas through limewater | Effervescence; limewater turns cloudy |
| Chloride, Cl− | Acidify with dilute nitric acid, then add aqueous silver nitrate | White precipitate of AgCl |
| Bromide, Br− | As for chloride | Cream precipitate of AgBr |
| Iodide, I− | As for chloride | Yellow precipitate of AgI |
| Nitrate, NO3− | Add aluminium foil and aqueous sodium hydroxide, warm gently | Ammonia given off; turns damp red litmus blue |
| Sulfate, SO42− | Acidify with dilute nitric acid, then add aqueous barium nitrate or chloride | White precipitate of BaSO4 |
| Sulfite, SO32− | Add dilute acid, warm gently and bubble the gas through acidified potassium manganate(VII) | Purple to colourless |
Section 3 of 12Anion test details and equations
Anion test details and equations
Carbonate
- CO32−(aq) + 2H+(aq) → CO2(g) + H2O(l).
CO2 + Ca(OH)2 → CaCO3 + H2O, and the insoluble calcium carbonate makes limewater cloudy.
Describe the test by bubbling the gas through limewater, not only the result.
Halides
- KBr(aq) + AgNO3(aq) → KNO3(aq) + AgBr(s).
Acidify with nitric acid, not hydrochloric acid, because HCl contains chloride ions.
Sulfate
- Ba2+(aq) + SO42−(aq) → BaSO4(s).
Nitric acid is added first to remove carbonates, which would also give a precipitate.
The ammonia from nitrate has a sharp choking smell.
Chloride: Ag+(aq) + Cl−(aq) → AgCl(s), a white precipitate.
Section 4 of 12Tests for cations with sodium hydroxide and ammonia
Tests for cations with sodium hydroxide and ammonia
Method: add a few drops of the solution slowly, record the precipitate, then add excess and record any change.
Adding too quickly can dissolve a precipitate as fast as it forms, so it is missed.
| Cation | NaOH few drops | NaOH excess | NH3 few drops | NH3 excess |
|---|---|---|---|---|
| Al3+ | White precipitate | Dissolves, colourless solution | White precipitate | Insoluble |
| Ca2+ | White precipitate | Insoluble | No precipitate or very slight | No change |
| Cr3+ | Green precipitate | Dissolves | Green precipitate | Insoluble |
| Cu2+ | Light blue precipitate | Insoluble | Light blue precipitate | Dissolves, dark blue solution |
| Fe2+ | Green precipitate | Insoluble; surface turns brown on standing | Green precipitate | Insoluble; surface turns brown on standing |
| Fe3+ | Red-brown precipitate | Insoluble | Red-brown precipitate | Insoluble |
| Zn2+ | White precipitate | Dissolves, colourless solution | White precipitate | Dissolves, colourless solution |
Ionic equations
- Cu2+(aq) + 2OH−(aq) → Cu(OH)2(s) is a light blue precipitate, Fe2+(aq) + 2OH−(aq) → Fe(OH)2(s) is green and Fe3+(aq) + 3OH−(aq) → Fe(OH)3(s) is red-brown.
Colour clues
- Colour clues a blue solution suggests copper(II), a pale green solution iron(II) and a yellow-brown solution iron(III).
Section 5 of 12Test for ammonium ions
- Method
- Warm the solution gently with sodium hydroxide solution, which supplies hydroxide ions.
- Ammonia gas is released.
- Hold damp red litmus paper near the mouth of the tube without touching the sides.
- The litmus turns blue.
- Ammonium NH4+ is the aqueous ion, and ammonia NH3 is the gas.
Section 6 of 12Distinguishing white precipitates
Al3+, Ca2+ and Zn2+ all give white precipitates with sodium hydroxide.
The calcium precipitate does not dissolve in excess sodium hydroxide.
The aluminium and zinc precipitates dissolve in excess sodium hydroxide.
Zinc hydroxide dissolves in excess ammonia, but aluminium hydroxide does not.
Wording
- Wording colourless means no colour, whereas clear means transparent, so a clear solution can be coloured.
Section 7 of 12Flame tests
- Method
- Clean a nichrome or platinum wire loop by dipping it in concentrated acid and holding it in a blue Bunsen flame until there is no colour.
- Dip the loop in the solid or solution.
- Hold it at the edge of the blue flame without letting the wire glow red.
- - Observe the flame colour.
| Cation | Flame colour |
|---|---|
| Li+ | Red |
| Na+ | Yellow |
| K+ | Lilac |
| Ca2+ | Orange-red |
| Ba2+ | Light green |
| Cu2+ | Blue-green |
- Only one type of ion must be present, because two ions would mix their colours.
Section 8 of 12Tests for gases
| Gas | Test | Positive result |
|---|---|---|
| Ammonia, NH3 | Damp red litmus paper | Turns blue; sharp choking smell |
| Carbon dioxide, CO2 | Bubble through limewater | Turns cloudy white |
| Chlorine, Cl2 | Damp blue litmus paper | Turns red then bleached white |
| Hydrogen, H2 | Lighted splint at the mouth of the tube | Burns with a squeaky pop |
| Oxygen, O2 | Glowing splint in the gas | Relights |
| Sulfur dioxide, SO2 | Bubble through acidified potassium manganate(VII) | Purple to colourless |
Ammonia
- Ammonia hold the litmus near the tube mouth without touching the sides, so traces of sodium hydroxide do not turn it blue.
Carbon dioxide
- Carbon dioxide a splint going out is not a test because nitrogen does the same.
Chlorine is toxic, so it is handled in a fume cupboard, and its smell is a property, not a test.
Hydrogen and oxygen
- Hydrogen and oxygen a lighted splint is for hydrogen and a glowing splint is for oxygen.
Sulfur dioxide has a sharp choking smell, and its test is similar to the test for sulfite ions.
Section 9 of 12Strategy for an unknown salt and recording observations
1
Note the appearance of the solid and of its solution.
2
Do a flame test if a Group 1 metal, calcium or barium is suspected.
3
Dissolve the solid and test with sodium hydroxide, then ammonia, to find the cation.
4
Test portions for the anion: dilute acid for carbonate, acidified silver nitrate for halides, acidified barium nitrate for sulfate,
- and aluminium foil with sodium hydroxide for nitrate.
5
Combine the results and state the formula with the evidence for each ion.
Conclusions
- Conclusions each one must be supported by an observation.
Wording
- Wording write that a precipitate forms, and whether it dissolves in excess, not that it turns green.
Colourless and clear
- Colourless and clear colourless means no colour, whereas clear means see-through, so a solution can be clear and blue.
Safety
- Safety wear eye protection and use dilute reagents.
Fresh apparatus
- Fresh apparatus use clean apparatus for each test.
Section 10 of 12Purpose of each reagent
| Step | Purpose |
|---|---|
| Acidify with nitric acid before silver nitrate | Removes carbonate, which would also give a precipitate |
| Acidify before barium nitrate or chloride | Prevents white barium carbonate giving a false result |
| Add sodium hydroxide in excess | Shows whether the precipitate redissolves |
| Warm with sodium hydroxide for NH4+ | Releases ammonia gas |
| Aluminium foil for nitrate | Reduces nitrate to ammonia with sodium hydroxide |
Section 11 of 12Key terms
| Term | Meaning |
|---|---|
| Qualitative analysis | identifying the ions or gases in a substance using chemical tests. |
| Precipitate | an insoluble solid formed when solutions react. |
| Flame test | a test that identifies metal ions by the colour they give a flame. |
| Limewater | a solution of calcium hydroxide used to test for carbon dioxide. |
| Effervescence | fizzing caused by a gas being given off. |
| Colourless | having no colour, unlike clear which means transparent. |
| Ionic equation | an equation showing only the ions that react. |
| Cation | a positive ion, tested with sodium hydroxide and ammonia. |
| Anion | a negative ion, tested with specific reagents. |
Section 12 of 12Exam technique
1
Skipping the acidification with nitric acid before the halide or sulfate test, or using hydrochloric acid which adds chloride ions
2
Describing the result only (limewater turns cloudy) instead of the test, or giving a conclusion without the observation that supports it
3
Adding sodium hydroxide too fast, so a precipitate that redissolves is missed
4
Confusing the lighted splint (hydrogen) with the glowing splint (oxygen)
5
Writing clear for colourless, or confusing ammonium (aqueous ion) with ammonia (gas)
Marked practiceQuizzes for this lesson
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