Who this is for: Cambridge IGCSE Chemistry (0620) 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 table they only half-memorise.
What query it owns: how to understand and revise identification of ions and gases in Cambridge IGCSE Chemistry.
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.

Identification of ions and gases is tested throughout Cambridge IGCSE Chemistry (0620) — in structured theory, practical papers and Alternative to Practical. Examiners expect precise observations: lilac flame for potassium, white precipitate insoluble in excess for aluminium ions, limewater turning milky for carbon dioxide. This guide explains the core tests, how to present observations and conclusions, and where to practise each skill.

Key takeaways

• Flame tests identify metal cations by flame colour — use a clean wire loop and view through blue glass for sodium contamination.
• Aqueous sodium hydroxide precipitates metal hydroxides; note colour and whether precipitate dissolves in excess NaOH or NH₃.
• Anion tests include dilute acid for carbonates (CO₂), acidified silver nitrate for halides, acidified barium nitrate for sulfate.
• Gas tests must state reagent and positive result — e.g. damp red litmus turns blue for ammonia.
• Observations describe what you see; conclusions name the ion or gas.

What is identification of ions and gases in Cambridge IGCSE Chemistry?

Qualitative analysis identifies unknown ions in solution and gases evolved in reactions. Cambridge IGCSE Chemistry (0620) requires knowledge of flame tests, reactions with aqueous sodium hydroxide and ammonia, tests for carbonate, halide, sulfate and nitrate ions, and tests for hydrogen, oxygen, carbon dioxide, chlorine and ammonia. Questions often give a flow chart of tests and ask for observations or the identity of an ion.

You can read the full explanation, worked examples and notes on Tutopiya’s Identification of Ions and Gases subtopic page before you attempt questions.

The core ideas you must master

These five ideas appear again and again. Learn what each one means and the exam phrasing that signals it.

Idea	What it means	How the exam uses it
Flame test	Metal ion → flame colour	”State which ion gives a lilac flame.”
NaOH precipitate	Metal hydroxide forms	”Describe the precipitate with aqueous NaOH.”
Excess NaOH / NH₃	Soluble vs insoluble hydroxides	”State whether precipitate dissolves in excess.”
Halide test	Acidified AgNO₃ → coloured precipitate	”Identify the halide ion present.”
Gas test	Reagent + positive observation	”Describe the test for carbon dioxide.”

Cation tests with aqueous sodium hydroxide — quick reference

Ion	Precipitate with NaOH	In excess NaOH
Copper(II) Cu²⁺	Blue	No change
Iron(II) Fe²⁺	Green	No change
Iron(III) Fe³⁺	Red-brown	No change
Aluminium Al³⁺	White	Dissolves
Zinc Zn²⁺	White	Dissolves
Calcium Ca²⁺	White	No change (or slight dissolve — syllabus specific)
Ammonium NH₄⁺	No precipitate; ammonia on warming	—

Anion and gas tests — what examiners expect

Test	Positive result
Carbonate + dilute acid	Effervescence; gas turns limewater milky (CO₂)
Halide + acidified AgNO₃	White (Cl⁻), cream (Br⁻), yellow (I⁻) precipitate
Sulfate + acidified BaNO₃/BaCl₂	White precipitate
Hydrogen	Lit splint squeaky pop
Oxygen	Relights glowing splint
Carbon dioxide	Limewater turns milky
Ammonia	Damp red litmus turns blue
Chlorine	Bleaches damp litmus paper

How to answer identification questions — step by step

1. Read whether the question wants observation or conclusion — “Describe what you would see” vs “Identify the ion.”
2. Name the reagent first, then the result — “Add aqueous sodium hydroxide; a blue precipitate forms.”
3. For gases, bubble through or hold litmus near the gas; state the colour change.
4. Use precise colours — red-brown not just brown for Fe³⁺.
5. Link observations to ion name only when the command word asks you to identify or conclude.

Test yourself with the free Identification of Ions and Gases quiz once you have reviewed the tables.

Identification in past-paper wording: command words that matter

Command word / phrase	What the question wants	Typical identification stem
Describe the test	Reagent + method + positive result	”Describe a test for sulfate ions.”
State the observation	What you see only	”State the colour of the precipitate.”
Identify the ion	Name from given observations	”Identify ion X.”
Explain	Why a test works or what confirms identity	”Explain how the test shows carbonate ions.”
Suggest	Test for an unknown	”Suggest a test to show ammonia is present.”

Worked exam-style stems (how to answer the wording)

1. “Aqueous sodium hydroxide is added to a solution containing Al³⁺ ions. Describe what is seen when excess NaOH is added.” White precipitate forms; precipitate dissolves in excess NaOH to give a colourless solution. Reward: white ppt + dissolves in excess.
2. “Describe a test for carbon dioxide gas.” Bubble through limewater; limewater turns milky/cloudy. Reward: named reagent + correct colour change.
3. “Acidified silver nitrate is added to a solution. A cream precipitate forms. Identify the halide ion.” Bromide (Br⁻). Reward: correct ion linked to cream precipitate.

When you can recognise the wording instantly, work the full set on the Experimental Techniques topical past-paper questions and the Identification of Ions and Gases quiz.

How identification connects to the rest of Experimental Techniques

Ion tests follow Separation and Purification when analysing products. Chromatography complements qualitative work on mixtures. Alternative To Practical Skills frequently tests observation tables. The Cambridge IGCSE Chemistry resource hub links every Experimental Techniques subtopic.

Common mistakes students make

• Stating the ion name when only an observation is asked for.
• Forgetting acidified before silver nitrate or barium nitrate tests.
• Confusing cream (bromide) and yellow (iodide) silver halide precipitates.
• Saying litmus turns blue for ammonia without damp red litmus.
• Mixing up precipitates that dissolve in excess NaOH (Al³⁺, Zn²⁺) with those that do not.

When you need more support

If identification questions keep costing marks — especially flow-chart practical stems — work through the Experimental Techniques topical past-paper questions and the Identification of Ions and Gases quiz, then get focused help from a Cambridge IGCSE Chemistry tutor.

Frequently asked questions

Is identification of ions hard in Cambridge IGCSE Chemistry? The tests are fixed — marks are lost from vague observations and confusing similar precipitate colours.

Do I need to know nitrate tests? Yes — the syllabus includes tests for common anions; learn carbonate, halide and sulfate thoroughly first.

What is the difference between observation and conclusion? Observation is what you see (blue precipitate); conclusion names the species (copper(II) ions present).

How do I revise ion tests effectively? Build flashcards for cation NaOH table and gas tests, then take the Identification of Ions and Gases quiz under timed conditions.

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