2.4Unit 2 · Inorganic Chemistry
Chemical tests — Edexcel IGCSE Chemistry Modular - UNIT 2 Revision Notes & Topical Questions
10 min read 7 parts Lesson 4 of 4 1 quiz
Lesson summary
Describe tests for hydrogen, oxygen, carbon dioxide, ammonia and chlorine.Hydrogenlighted splint gives a squeaky pop.Oxygen relights a glowing splint.Ammonia turns damp red litmus blue; chlorine bleaches damp litmus paper.Anhydrous copper(II) sulfate turns from white to blue with…
- Tests for gases
- Flame test
- Cations with sodium hydroxide
- Halide ions
- Sulfate ions
- Carbonate ions
Section 1 of 7Chemical tests
Essential knowledge
Tests for gases
H2 – squeaky pop with a lighted splint; O2 – relights a glowing splint; CO2 – turns limewater milky (white CaCO3 precipitate); NH3 – turns damp red litmus blue; Cl2 – bleaches damp litmus paper.
Flame test
Flame test the loop must be clean so no other colours contaminate the result. Colours: lithium red, sodium yellow, potassium lilac, calcium orange-red, copper blue-green. Sodium's strong yellow can mask other colours in mixtures.
Cations with sodium hydroxide
Cations with sodium hydroxide add a few drops of NaOH(aq). Cu2+ gives a blue precipitate, Fe2+ green, Fe3+ orange-brown. NH4+ gives no precipitate but releases ammonia gas on warming, tested with damp red litmus.
Halide ions
Halide ions add dilute nitric acid, then silver nitrate solution. Chloride gives a white precipitate (AgCl), bromide cream (AgBr), iodide yellow (AgI). Nitric acid is used, not hydrochloric, because HCl would add chloride ions.
Sulfate ions
Sulfate ions add dilute hydrochloric acid, then barium chloride solution; a white precipitate of barium sulfate forms.
Carbonate ions
Carbonate ions add dilute acid; effervescence occurs and the gas turns limewater milky: CO32− + 2H+ → CO2 + H2O.
Water
Water the presence of water turns white anhydrous copper(II) sulfate blue (or blue cobalt chloride paper pink). To show water is pure, check it boils at exactly 100 °C (or melts at 0 °C).
Ammonia vs ammonium
Ammonia vs ammonium ammonia, NH3, is the gas; ammonium, NH4+, is the ion. When testing gas from an ammonium compound warmed with NaOH, hold damp red litmus at the mouth without touching the tube, or traces of NaOH will turn it blue.
Gas test technique
Gas test technique for hydrogen, hold the lighted splint at the mouth of the tube, not inside it; the pop is hydrogen burning in air to make water. A burning splint going out is not a test for CO2, because other gases such as nitrogen also put it out.
Limewater equation
CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l). The insoluble calcium carbonate makes limewater cloudy. When describing the carbonate test, say the gas is bubbled through limewater, not just that limewater goes milky.
Halide test equations
Halide test equations the silver halide forms as a precipitate, e.g. NaBr(aq) + AgNO3(aq) → NaNO3(aq) + AgBr(s), cream. In the same way NaI gives yellow AgI(s) and NaCl gives white AgCl(s).
Sulfate test alternative
Sulfate test alternative barium nitrate solution can be used instead of barium chloride. The sample is acidified first, then Ba2+(aq) + SO42−(aq) → BaSO4(s) gives a white precipitate.
Water test equation
CuSO4(s) + 5H2O(l) → CuSO4·5H2O(s); white anhydrous powder becomes blue hydrated crystals. This shows water is present but not that it is pure.
Effect of impurities
Effect of impurities impurities usually raise the boiling point and lower the melting point of a substance. So water that boils above 100 °C or freezes below 0 °C is not pure. Heat it gently in a boiling tube and read the temperature with a thermometer.
Flame test wire
Flame test wire use an unreactive nichrome or platinum loop cleaned in acid, and heat the sample in the hot blue (roaring) part of the flame. Traces of another ion can mask or mix colours.
Section 2 of 7Process & sequence
1
Clean a nichrome wire loop by dipping it in concentrated hydrochloric acid and holding it in a blue Bunsen flame until no colour shows.
2
Dip the clean loop in the acid and then into the solid sample.
3
Hold the loop at the edge of a roaring blue flame.
4
Observe and record the flame colour.
5
Clean the loop again before testing the next sample.
Identifying an unknown saltwrite these steps in this order; each step is a marking point.
1
Flame test for metal ion
2
Add NaOH for precipitate
3
Warm: test for ammonia
4
Add acid: test CO2
5
AgNO3 or BaCl2 for anion
6
Combine to name salt
Section 3 of 7Compare
| Ion | Flame colour | With NaOH(aq) |
|---|---|---|
| Li+ | Red | – |
| Na+ | Yellow | – |
| K+ | Lilac | – |
| Ca2+ | Orange-red | White precipitate |
| Cu2+ | Blue-green | Blue precipitate |
| Fe2+ / Fe3+ / NH4+ | – | Green ppt / brown ppt / ammonia gas on warming |
| Substance | Test | Positive result |
|---|---|---|
| Hydrogen | Lighted splint | Squeaky pop |
| Oxygen | Glowing splint | Relights |
| Carbon dioxide | Bubble through limewater | Limewater turns milky |
| Ammonia / Chlorine | Damp red litmus / damp blue litmus | Turns blue / bleached white (may turn red first) |
| Halides (Cl−, Br−, I−) | Dilute HNO3 then AgNO3 | White / cream / yellow ppt |
| SO42− / CO32− | Dilute HCl + BaCl2 / add dilute acid | White ppt / fizzing, CO2 turns limewater milky |
Section 4 of 7Worked examples
1Potassium bromide, KBr
A white solid gives a lilac flame and a cream precipitate with dilute nitric acid and silver nitrate. Identify it.
- Lilac flame → potassium ion, K+. Cream precipitate → bromide ion, Br−. The solid is potassium bromide, KBr.
2Explain why dilute nitric acid is added before silver nitrate in a halide test.
- It reacts with and removes carbonate ions, which would otherwise form a precipitate with silver ions and give a false positive.
- Hydrochloric acid is not used because it contains chloride ions.
Section 5 of 7Key terms
Section 6 of 7Command terms
Command terms
Give / State / Name
Recall a fact; one idea per mark.
Define
Give the exact meaning using the specification keywords.
Describe
Give an account of what happens / the pattern in data, quoting values; no reasons needed.
Explain
Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.
Suggest
Use knowledge in a new context; more than one answer may be accepted.
Compare
Similarities and differences; each point must refer to both items.
Calculate
Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.
Discuss
Explore the issue from different angles and link the points.
Evaluate / Assess
Weigh strengths and weaknesses and reach a supported judgement.
Deduce
Draw a conclusion from the information given.
Justify
Support a statement/choice with evidence.
Section 7 of 7Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Flame test · Precipitate · Cation · Anion · Limewater · Damp litmus paper · Anhydrous copper(II) sulfate · Acidified silver nitrate · Nichrome wire loop
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
How to get full marks (examiner guidance)
Use specification keywords exactly (e.g. electronegativity, nucleophile, dynamic equilibrium, activation energy).
Link ideas
Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.
Use the information in the question
“discuss/evaluate” answers must use the data provided and match the number of marks.
Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.
Avoid vague words
“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.
Do not extrapolate a graph line beyond the data or to zero unless asked.
Six-mark extended responses are marked on content AND a logical, linked line of reasoning.
Marked practiceQuizzes for this lesson
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