Who this is for: Cambridge IGCSE Coordinated Science (0654) students who want identification of ions and gases — flame tests, precipitate reactions and gas tests — to become a reliable source of marks instead of a colour-guessing exercise.
What query it owns: how to understand and revise identification of ions and gases in Cambridge IGCSE Coordinated Science.
Why this is safe: this page owns the identification-of-ions-and-gases revision-guide angle, while Tutopiya’s Identification Of Ions And Gases subtopic page owns the learning resource and the free Identification Of Ions And Gases quiz owns the practice.

Chemical tests identify unknown ions in solution and gases evolved in reactions. Cambridge IGCSE Coordinated Science (0654) expects you to know flame test colours, precipitate tests for anions and cations, and gas tests with reagents and observations. This guide organises every test into tables examiners reward.

Key takeaways

• Flame tests identify metal cations by characteristic flame colours.
• Aqueous sodium hydroxide precipitates identify transition metal cations; excess NaOH may dissolve the precipitate.
• Acid + carbonate test produces CO₂ — turns limewater milky.
• Damp red litmus turns blue for ammonia (NH₃).
• Glowing splint relights for oxygen; pop sound for hydrogen.

What is identification of ions and gases in Cambridge IGCSE Coordinated Science?

Qualitative analysis uses specific reagents to confirm the presence of particular ions or gases. Each test has a characteristic positive result. You must state the reagent, the procedure and the expected observation — partial answers lose marks.

You can read the full test tables and notes on Tutopiya’s Identification Of Ions And Gases subtopic page before you attempt questions.

Flame tests — cation identification

Ion	Flame colour
Lithium (Li⁺)	Red
Sodium (Na⁺)	Yellow
Potassium (K⁺)	Lilac
Calcium (Ca²⁺)	Orange-red
Barium (Ba²⁺)	Light green
Copper(II) (Cu²⁺)	Blue-green

Anion tests

Ion	Test	Positive result
Carbonate (CO₃²⁻)	Add dilute acid	Effervescence; gas turns limewater milky
Sulfate (SO₄²⁻)	Add dilute HCl, then barium chloride	White precipitate
Chloride (Cl⁻)	Add dilute HNO₃, then silver nitrate	White precipitate
Iodide (I⁻)	Add dilute HNO₃, then silver nitrate	Yellow precipitate
Nitrate (NO₃⁻)	Add NaOH + aluminium foil, warm	Ammonia gas (damp red litmus → blue)

Cation tests with aqueous NaOH

Ion	Precipitate	With excess NaOH
Copper(II) Cu²⁺	Blue	Insoluble
Iron(II) Fe²⁺	Green	Insoluble
Iron(III) Fe³⁺	Red-brown	Insoluble
Aluminium Al³⁺	White	Dissolves
Zinc Zn²⁺	White	Dissolves
Calcium Ca²⁺	White	Insoluble

Gas tests

Gas	Test	Positive result
Hydrogen (H₂)	Lighted splint at mouth of tube	Squeaky pop
Oxygen (O₂)	Glowing splint	Relights
Carbon dioxide (CO₂)	Bubble through limewater	Turns milky / cloudy
Ammonia (NH₃)	Damp red litmus paper	Turns blue
Chlorine (Cl₂)	Damp litmus paper	Bleaches white

Identification tests in past-paper wording

Command word	What the question wants	Typical stem
Describe	Full test procedure	”Describe a test for sulfate ions.”
State	Observation or ion identified	”State the colour of the flame for potassium.”
Identify	Ion or gas from results	”Identify the gas that turns limewater milky.”
Name	Reagent used	”Name the reagent used to test for chloride ions.”

Worked exam-style stems

1. “Describe a test for carbonate ions.” Add dilute acid to the sample; effervescence occurs; collect gas and bubble through limewater, which turns milky. Reward: acid + effervescence + limewater milky.
2. “A white precipitate forms with NaOH and dissolves in excess NaOH. Identify the cation.” Aluminium (Al³⁺) or zinc (Zn²⁺) — both give white precipitates soluble in excess NaOH. Reward: Al³⁺ or Zn²⁺ with reasoning.
3. “State the test and result for ammonia gas.” Hold damp red litmus at the tube mouth; it turns blue. Reward: damp red litmus + turns blue.

Test yourself with the Identification Of Ions And Gases quiz once you can recall tests without a data booklet.

How ion and gas tests connect to the syllabus

Qualitative analysis applies acid-base chemistry, salt preparation products and redox. Gas tests appear in rate of reaction and carbonate reactions. The Cambridge IGCSE Coordinated Science resource hub links every Acids, Bases And Salts subtopic.

Common mistakes students make

• Using dilute HCl instead of dilute HNO₃ before silver nitrate (chloride interferes).
• Confusing lilac (potassium) with yellow (sodium) flame colours.
• Stating blue litmus for ammonia test (must be red litmus turning blue).
• Forgetting to add acid first before barium chloride for sulfate test.
• Saying hydrogen test uses a glowing splint (that is the oxygen test).

When you need more support

If ion and gas test questions keep costing marks, work through the Identification Of Ions And Gases quiz, then get focused help from a Cambridge IGCSE Coordinated Science tutor.

Frequently asked questions

Is identification of ions hard in Coordinated Science? The tests are fixed — organise them into flame, anion, cation and gas tables and drill observations.

Why is nitric acid added before silver nitrate for halide tests? To remove carbonate ions that would also form a precipitate and give a false positive.

How do you test for sulfate ions? Add dilute hydrochloric acid, then aqueous barium chloride; a white precipitate confirms sulfate ions.

How do I revise ion and gas tests effectively? Memorise the four test tables, practise describe questions with reagent + observation, then take the quiz.

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