Identification of Ions and Gases (Qualitative Analysis)
Chemistry· 12.5· 25 min read
1. Testing for Common Cations (Core)★★☆☆☆⏱ 8 min
Cation test
A test using aqueous sodium hydroxide or aqueous ammonia to identify positive metal or ammonium ions via characteristic precipitate colour or solubility.
All core cation tests use two reagents: aqueous sodium hydroxide (NaOH) and aqueous ammonia (NH₃). You add the reagent dropwise, then continue adding until it is in excess, recording the precipitate colour and whether it dissolves in excess. Three cations — aluminium, calcium and zinc — all give a white precipitate, so you must use their behaviour in excess reagent to tell them apart. The exception to the precipitate pattern is the ammonium ion, which releases ammonia gas when warmed with NaOH.
Cation | Observation with NaOH (excess) | Observation with aqueous ammonia (excess) |
|---|---|---|
Al³+ (aluminium) | White precipitate, soluble in excess to give a colourless solution | White precipitate, insoluble in excess |
NH₄+ (ammonium) | Ammonia gas released on warming (pungent odour, turns damp red litmus blue) | No reaction |
Ca²+ (calcium) | White precipitate, insoluble in excess | No precipitate, or only a very slight white precipitate |
Cr³+ (chromium(III)) | Green precipitate, soluble in excess | Green precipitate, insoluble in excess |
Cu²+ (copper(II)) | Light blue precipitate, insoluble in excess | Light blue precipitate, dissolves in excess to form dark blue solution |
Fe²+ (iron(II)) | Green precipitate, insoluble in excess (turns brown at the surface on standing) | Green precipitate, insoluble in excess |
Fe³+ (iron(III)) | Reddish-brown precipitate, insoluble in excess | Reddish-brown precipitate, insoluble in excess |
Zn²+ (zinc) | White precipitate, soluble in excess to give a colourless solution | White precipitate, soluble in excess to give a colourless solution |
A student adds excess aqueous sodium hydroxide to an unknown solution, and observes a green precipitate that does not dissolve. Identify the cation present.
- 1
Recall the observations with excess NaOH. A green precipitate could be iron(II) or chromium(III), so its behaviour in excess is the deciding factor:
- 2
Cu²+ forms a light blue precipitate, insoluble in excess
- 3
Fe²+ forms a green precipitate that is insoluble in excess
- 4
Cr³+ also forms a green precipitate, but it dissolves in excess NaOH
- 5
Fe³+ forms a reddish-brown precipitate, insoluble in excess
- 6
Al³+, Ca²+ and Zn²+ all form white precipitates, not green
- 7
The precipitate here is green AND does not dissolve in excess, which matches iron(II), not chromium(III)
- 8
Final answer: Iron(II) ions (Fe²+)
Exam tip:
Always state the full name of the ion as well as its charge if asked, e.g. write 'iron(II) ion' not just 'iron ion' to avoid losing marks.
2. Testing for Common Anions (Core)★★☆☆☆⏱ 7 min
Anion test
A test using specific reagents to identify negative ions via precipitate formation or gas release.
Core anion tests require you to follow specific steps to avoid false positive results. Always add dilute acid before testing for sulfate or halide ions to remove carbonate impurities that would form unwanted precipitates.
Anion | Test method | Positive observation |
|---|---|---|
CO₃²⁻ (carbonate) | Add dilute hydrochloric acid; pass any gas produced through limewater | Effervescence; limewater turns milky (cloudy white) |
Cl⁻ (chloride) | Acidify with dilute nitric acid, then add aqueous silver nitrate | White precipitate forms |
Br⁻ (bromide) | Acidify with dilute nitric acid, then add aqueous silver nitrate | Cream precipitate forms |
I⁻ (iodide) | Acidify with dilute nitric acid, then add aqueous silver nitrate | Yellow precipitate forms |
NO₃⁻ (nitrate) | Add aqueous sodium hydroxide, then a little aluminium foil, and warm carefully | Ammonia gas produced (turns damp red litmus blue) |
SO₄²⁻ (sulfate) | Acidify with dilute nitric acid, then add aqueous barium nitrate | White precipitate forms |
SO₃²⁻ (sulfite) | Add a small volume of acidified aqueous potassium manganate(VII) | Purple manganate(VII) turns colourless |
An unknown solution gives a white precipitate when it is acidified with dilute nitric acid and aqueous barium nitrate is then added. Identify the anion present.
- 1
Recall the anion test observations:
- 2
Carbonate ions produce effervescence with dilute acid, and give no precipitate with barium nitrate
- 3
Halide ions form precipitates with silver nitrate after nitric acid is added: chloride white, bromide cream, iodide yellow
- 4
Sulfate ions form a white precipitate with aqueous barium nitrate after acidifying with dilute nitric acid
- 5
Match the observation to the correct anion: white precipitate with acidified barium nitrate matches sulfate ions
- 6
Final answer: Sulfate ions (SO₄²⁻)
Exam tip:
Never forget to add dilute acid before testing for sulfate or halide ions – this is a common mark point in exam questions.
3. Testing for Common Gases (Core)★★☆☆☆⏱ 6 min
Gas tests are used to identify the gas released during a chemical reaction, often as part of ion tests or practical reaction observations. You will need to recall the test method and positive observation for each core gas.
Gas | Test method | Positive observation |
|---|---|---|
Hydrogen (H₂) | Hold a lighted splint near the mouth of the test tube | Squeaky 'pop' sound as the hydrogen burns |
Oxygen (O₂) | Hold a glowing splint near the mouth of the test tube | Glowing splint relights (catches fire again) |
Carbon dioxide (CO₂) | Bubble the gas through limewater (calcium hydroxide solution) | Limewater turns milky (cloudy white) |
Ammonia (NH₃) | Hold a piece of damp red litmus paper near the mouth of the test tube | Damp red litmus paper turns blue; pungent odour |
Chlorine (Cl₂) | Hold a piece of damp blue litmus paper near the mouth of the test tube | Damp blue litmus paper turns red, then is bleached white |
Sulfur dioxide (SO₂) | Bubble the gas through acidified potassium manganate(VII) solution | Purple potassium manganate(VII) solution turns colourless |
A reaction produces a gas that bleaches damp blue litmus paper white after turning it red. Identify the gas.
- 1
Recall the gas test observations:
- 2
Hydrogen gives a squeaky pop with a lighted splint
- 3
Oxygen relights a glowing splint
- 4
Carbon dioxide turns limewater milky
- 5
Ammonia turns damp red litmus blue
- 6
Chlorine turns damp blue litmus red then bleaches it white
- 7
Sulfur dioxide turns purple acidified manganate(VII) colourless
- 8
Match the observation to the correct gas: bleaching of damp blue litmus after turning red matches chlorine
- 9
Final answer: Chlorine gas (Cl₂)
Exam tip:
Always state that litmus paper is damp for ammonia, chlorine and sulfur dioxide tests – dry litmus paper will not change colour for these gases.
4. Interpreting Unknown Sample Results★★★☆☆⏱ 7 min
In exam questions, you will often be given a set of observations for an unknown sample, and asked to identify the ions or gases present. Work through each observation one by one to eliminate impossible options, then confirm the identity of the remaining substances.
A student performs tests on an unknown solid X, with the following results: 1. Adding dilute hydrochloric acid to X produces a gas that turns limewater milky. 2. Adding excess aqueous sodium hydroxide to a solution of X produces a light blue insoluble precipitate. Identify the ions present in X, and name X.
- 1
Analyse the first test result: gas that turns limewater milky is carbon dioxide, so the anion is carbonate (CO₃²⁻)
- 2
Analyse the second test result: light blue insoluble precipitate with excess NaOH is copper(II) ions (Cu²+)
- 3
Combine the two ions to form the neutral compound: Cu²+ and CO₃²⁻ form copper(II) carbonate
- 4
Final answer: Ions present are Cu²+ and CO₃²⁻; X is copper(II) carbonate
Exam tip:
Always cross-check your final answer against all observations to make sure it fits every test result, to avoid mistakes.
5. Flame Tests for Metal Ions (Core)★★☆☆☆⏱ 5 min
Flame test
A test that identifies certain metal cations from the characteristic colour they give to a hot Bunsen flame.
To carry out a flame test, clean a flame-test wire by dipping it in concentrated hydrochloric acid and holding it in a hot Bunsen flame until it gives no colour. Then dip the clean wire in the sample and hold it in the edge of a hot (blue) flame, observing the colour produced. In the 0620 syllabus a flame test identifies six cations.
Cation | Flame colour |
|---|---|
Lithium (Li⁺) | Red |
Sodium (Na⁺) | Yellow |
Potassium (K⁺) | Lilac |
Calcium (Ca²+) | Orange-red |
Barium (Ba²+) | Light green |
Copper(II) (Cu²+) | Blue-green |
A clean flame-test wire dipped in a solid sample gives a light green flame. Identify the metal ion present.
- 1
Recall the flame-test colours: lithium red, sodium yellow, potassium lilac, calcium orange-red, barium light green, copper(II) blue-green
- 2
A light green flame matches barium (a blue-green flame would instead indicate copper(II))
- 3
Final answer: Barium ions (Ba²+)
Exam tip:
Learn the six official flame colours exactly. Lithium 'red' and potassium 'lilac' are easily confused, and calcium is 'orange-red', not the plain yellow of sodium.
6. Common Pitfalls
Wrong move:
Writing 'copper ion' instead of 'copper(II) ion' for the cation that forms a blue precipitate with NaOH.
Why:
Copper can form two ions (Cu+ and Cu²+) with different test results, so you must specify the oxidation state for transition metal ions.
Correct move:
Always state the full name including the Roman numeral oxidation state for transition metal ions, e.g. iron(II), iron(III), copper(II).
Wrong move:
Forgetting to acidify with dilute nitric acid before testing for sulfate or halide ions.
Why:
Carbonate impurities present in the sample would react with barium nitrate or silver nitrate to form white precipitates, giving a false positive result.
Correct move:
Acidify with dilute nitric acid first (for both halides and sulfate) to remove carbonate ions, then add the test reagent (aqueous silver nitrate for halides, aqueous barium nitrate for sulfate).
Wrong move:
Stating that dry litmus paper is used to test for ammonia gas.
Why:
Ammonia only forms alkaline OH⁻ ions when dissolved in water, so dry litmus paper will not change colour.
Correct move:
Always specify that litmus paper is damp for tests involving ammonia, chlorine and sulfur dioxide gases.
Wrong move:
Writing 'limewater goes cloudy' as the only observation for carbonate test, without mentioning effervescence.
Why:
The first observation when acid is added to carbonate is bubbling (effervescence) of gas, which is a required mark point in most exam questions.
Correct move:
Write both observations: 'effervescence, gas turns limewater milky'.
Wrong move:
Stating the name of the product as an observation, e.g. 'silver chloride is formed' instead of 'white precipitate forms'.
Why:
Observations are what you see, not what you deduce is present. You will lose marks for stating product names as observations.
Correct move:
Only describe what you can see: colour changes, precipitates, bubbling, test strip colour changes, sounds (for hydrogen test).
7. Quick Reference Cheatsheet
Test type | Substance | Positive observation |
|---|---|---|
Cation (NaOH excess) | Al³+ | White precipitate, soluble in excess (colourless solution) |
Cation (NaOH excess) | Ca²+ | White precipitate, insoluble in excess |
Cation (NaOH excess) | Cr³+ | Green precipitate, soluble in excess |
Cation (NaOH excess) | Cu²+ | Light blue insoluble precipitate |
Cation (NaOH excess) | Fe²+ | Green insoluble precipitate |
Cation (NaOH excess) | Fe³+ | Reddish-brown insoluble precipitate |
Cation (NaOH excess) | Zn²+ | White precipitate, soluble in excess (colourless solution) |
Cation (NaOH warm) | NH₄+ | Ammonia gas released on warming |
Anion | CO₃²⁻ | Effervescence, gas turns limewater milky |
Anion | Cl⁻ | White precipitate with acidified silver nitrate |
Anion | Br⁻ | Cream precipitate with acidified silver nitrate |
Anion | I⁻ | Yellow precipitate with acidified silver nitrate |
Anion | NO₃⁻ | NaOH + aluminium foil, warm: ammonia gas produced |
Anion | SO₄²⁻ | White precipitate with barium nitrate (after dilute nitric acid) |
Anion | SO₃²⁻ | Acidified manganate(VII) turns purple to colourless |
Flame test | Li⁺ | Red flame |
Flame test | Na⁺ | Yellow flame |
Flame test | K⁺ | Lilac flame |
Flame test | Ca²+ | Orange-red flame |
Flame test | Ba²+ | Light green flame |
Flame test | Cu²+ | Blue-green flame |
Gas | H₂ | Squeaky pop with lighted splint |
Gas | O₂ | Relights glowing splint |
Gas | CO₂ | Turns limewater milky |
Gas | NH₃ | Turns damp red litmus blue |
Gas | Cl₂ | Bleaches damp blue litmus white |
Gas | SO₂ | Turns acidified purple manganate(VII) colourless |
8. Frequently Asked
Do I need to memorise all ion and gas tests for the core exam?
Yes, all core tests listed in the 0620 syllabus are examinable, and you will be expected to write full observations for positive test results. You will not be given a test reference sheet in the exam.
What counts as a valid observation for qualitative analysis?
Valid observations include colour changes, precipitate formation, effervescence (bubbling), gas odour, and changes to test strips (e.g. litmus paper colour change). Do not state the name of the substance formed as an observation.
Going deeper
What's Next
Now that you have mastered core qualitative analysis tests for CIE IGCSE Chemistry 0620, you can apply these skills to practical exam questions and core topics involving ionic compounds, acids and reactions of metals. Qualitative analysis is a common question in both Paper 2 (theory) and Paper 3 (practical alternative) core exams, so practice writing full, accurate observations for every test to maximise your marks. Next, you can move on to core quantitative analysis topics, including calculating concentration of solutions, to complete your study of Unit 12: Experimental Techniques and Chemical Analysis. You can also practice past paper questions focusing on qualitative analysis to familiarise yourself with the exam phrasing and required mark points.