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2.8Inorganic Chemistry

Chemical Tests — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

5 min read 8 parts Lesson 15 of 23 Unit quiz

📚 Edexcel Linear - OL (Chemistry) Hub / Topical Revision
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Lesson summary

Chemical Tests for Gases, Cations and Anions — EDEXCEL LINEAR – OL (CHEMISTRY) notes: Carry out and interpret the standard tests for common gases, metal cations, ammonium ions and anions.GasTestPositive result HydrogenLit splintSqueaky pop OxygenGlowing splintRelights Carbon diox…

  • flame test
  • sodium hydroxide solution
  • Carbonate (CO32-)
  • Chloride, bromide, iodide
  • Sulfate (SO42-)
  • Nitrate (NO3-)

Part 1 of 6Tests for gases

GasTestPositive result
HydrogenLit splintSqueaky pop
OxygenGlowing splintRelights
Carbon dioxideBubble through limewaterTurns milky/cloudy (calcium carbonate)
ChlorineDamp blue litmus paperTurns red then bleached white
AmmoniaDamp red litmus paperTurns blue; pungent smell
WaterAnhydrous copper(II) sulfate or cobalt chloride paperWhite to blue; blue to pink

Pure water is confirmed by its boiling point (100 °C) and freezing point (0 °C), not by the chemical tests, which only show that water is present.

Part 2 of 6Flame tests and sodium hydroxide tests for cations

In a flame test a clean nichrome or platinum wire, dipped in dilute hydrochloric acid, is dipped into the solid and held in a blue Bunsen flame. Colours: lithium crimson red, sodium yellow, potassium lilac, calcium orange-red, copper(II) blue-green.

Adding sodium hydroxide solution drop by drop to a solution of the metal ions gives precipitates of the metal hydroxides:

IonPrecipitateIn excess NaOH
Cu2+Pale blueInsoluble
Fe2+GreenInsoluble
Fe3+Red-brownInsoluble
Al3+, Zn2+, Ca2+WhiteAl and Zn dissolve to give a colourless solution; Ca stays

The ionic equation for copper is Cu2+(aq) + 2OH-(aq) → Cu(OH)2(s). Ammonium ions: warm with sodium hydroxide; ammonia gas is given off and turns damp red litmus blue. Ammonia solution gives the same hydroxide precipitates, with copper(II) hydroxide dissolving in excess to give a deep blue solution.

Part 3 of 6Tests for anions

1Carbonate (CO32−)
Carbonate (CO32−) add dilute hydrochloric acid; fizzing and the gas turns limewater milky.
2Chloride, bromide, iodide
Chloride, bromide, iodide add dilute nitric acid, then silver nitrate solution; white (Cl-), cream (Br-) or yellow (I-) precipitate. Ag+ + Cl- → AgCl(s).
3Sulfate (SO42−)
Sulfate (SO42−) add dilute hydrochloric acid, then barium chloride solution; white precipitate of barium sulfate.
4Nitrate (NO3−)
Nitrate (NO3−) warm with sodium hydroxide and aluminium foil; ammonia gas evolves.

The acid in tests 2 and 3 removes carbonate ions, which would also give a precipitate and give false positives. Hydrochloric acid must not be used in the chloride test and nitric acid is not used in the sulfate test for the same reason (introducing the ion being tested).

Part 4 of 6Planning an identification

To identify an unknown salt

  • carry out the flame test or sodium hydroxide test for the cation, then one anion test.
  • Record observations (not conclusions) such as "white precipitate" and then state the ion.
  • Always give the reagent, the observation and the identity, and use one sample for each test.

Part 5 of 6Practical details and safety for test work

Good technique makes each test reliable.

1
For a flame test the wire is cleaned by dipping in concentrated hydrochloric acid and heating until it gives no colour; a dirty wire gives the persistent yellow of sodium.
2
For precipitation tests add the reagent drop by drop, shaking, and then in excess, because the behaviour in excess separates aluminium and zinc from calcium.
3
Use about 1 cm depth of solution in a test tube, a fresh sample for every test, and never taste or directly smell the contents; waft gently.
4
Wear eye protection because acids, alkalis and silver nitrate are hazardous; silver nitrate stains skin.

Write results in a table with three columns: test, observation, conclusion. This layout earns marks because each box is marked independently, and it helps you spot when a result is ambiguous and a second test is needed, for example lilac flame (potassium) confirmed by no precipitate with sodium hydroxide.

Part 6 of 6Worked examples

  1. A solid gives a lilac flame and, on adding dilute HCl then barium chloride to its solution, a white precipitate. Identify the compound. [3]3 marks

  2. Describe the test for carbon dioxide and the result. [2]2 marks

  3. Sodium hydroxide gives a green precipitate which turns brown on standing. What ion is present? [2]2 marks

  4. Why is nitric acid added before silver nitrate when testing for halides? [2]2 marks

Key terms

TermMeaning
CationPositive ion
AnionNegative ion
Flame testIdentifying metal ions by flame colour
PrecipitateInsoluble solid formed in a solution
LimewaterCalcium hydroxide solution used to test for CO2
ObservationWhat is seen, heard, smelt or measured

Avoid these mistakes

1
Writing conclusions in the observation column (e.g. "it is copper" instead of "blue precipitate").
2
Using hydrochloric acid in the silver nitrate test.
3
Confusing the oxygen and hydrogen tests.
4
Saying limewater turns white — it turns cloudy/milky.

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