OxfordAQA International GCSE Chemistry: Identification of ions (9202)

This study guide teaches the identification of ions for OxfordAQA International GCSE Chemistry (9202). It follows the OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards, and covers every statement in section 3.4.3 (statements a to f) plus the required practical on flame testing. There are no tiers in this qualification, so everything here is for every candidate. The specification says content from any part of the course may be assessed on Paper 1 or Paper 2.

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What this unit covers

Ref.	What you need to do
3.4.3 a	Recall the flame colours of lithium, sodium, potassium, calcium and barium compounds
Required practical	Identify the metal ion in an unknown compound using flame testing techniques
3.4.3 b	Use sodium hydroxide solution to tell aluminium ions apart from calcium and magnesium ions
3.4.3 c	Recall the precipitate colours for copper(II), iron(II) and iron(III) ions with sodium hydroxide
3.4.3 d	Test for carbonate ions with dilute acid and limewater
3.4.3 e	Test for chloride, bromide and iodide ions with silver nitrate and dilute nitric acid
3.4.3 f	Test for sulfate ions with barium chloride and dilute hydrochloric acid

You need ion charges and formulae to name the compounds you identify. Revise those in the atomic structure and periodic table guide if they are shaky.

Two words you will use all the time

• An ion test tells you which ion is present. Ionic compounds contain a positive ion (usually a metal ion) and a negative ion, so a full identification needs one test for each.
• A precipitate is a solid that cannot dissolve, produced when two solutions react. The mixture turns cloudy, and the solid slowly settles. Always give its colour.

Most tests here use a solution of the unknown. Dissolve a solid in distilled water first; tap water contains ions of its own.

3.4.3 a: flame tests

Some metal ions give a hot flame a colour typical of that metal. The colour depends on the metal ion, not the negative ion, so lithium chloride and lithium sulfate give the same result.

Flame colour	Metal ion in the compound
green	barium (Ba²⁺)
red	calcium (Ca²⁺)
crimson	lithium (Li⁺)
lilac	potassium (K⁺)
yellow	sodium (Na⁺)

Crimson (lithium) and red (calcium) are the pair most often confused. Crimson is a deep red; the specification simply calls calcium “red”. Learn the words exactly as in the table.

Required practical: identifying the metal ion in an unknown compound

The specification links this practical to designing a procedure, risk assessment, collecting and analysing data, making conclusions and evaluation. A sound procedure:

1. Light a Bunsen burner and open the air hole fully so the flame is blue and roaring. A yellow, smoky flame would hide the colours you are looking for.
2. Clean a loop of nichrome wire. Dip it in dilute hydrochloric acid, then hold it in the flame. Repeat until the wire gives no colour of its own.
3. Moisten the loop with acid once more and touch it on the solid so a few grains cling to it.
4. Hold the loop at the edge of the blue flame and note the colour straight away.
5. Repeat the cleaning between samples, or give each sample its own loop.

Risk assessment. The wire and the burner get very hot; hold the wire by its handle and let it cool on a heat-proof mat. Wear eye protection because acid can spit. Tie back long hair. Barium compounds are harmful if swallowed, so wash your hands afterwards.

Collecting data. Record the colour in words, in a table with one row per sample. A blank run with the clean wire on its own shows what “no colour” looks like.

Evaluation. The commonest problem is contamination. Sodium compounds are everywhere (fingers, dust, unwashed wire) and give a strong yellow colour that can swamp a weaker colour. Potassium’s lilac is pale and is easily masked this way. A flame test also only works well for a single metal ion; if a solid contains two of these metals, one colour may hide the other. Repeating each test with a freshly cleaned wire makes the conclusion more reliable.

Worked example 1: reading flame test results

A technician has five unlabelled bottles. Each holds a chloride of a different metal: lithium, sodium, potassium, calcium or barium. A student records these flame colours:

Bottle	V	W	X	Y	Z
Colour	green	crimson	yellow	lilac	red

Identify each metal ion and name each compound.

1. Match each colour to the table above. Green = barium; crimson = lithium; yellow = sodium; lilac = potassium; red = calcium.
2. Write the names, with the negative ion as chloride: V barium chloride, W lithium chloride, X sodium chloride, Y potassium chloride, Z calcium chloride.

3.4.3 b and c: sodium hydroxide solution

Add sodium hydroxide solution a few drops at a time to a solution of the unknown. Many metal ions form an insoluble metal hydroxide, which appears as a precipitate. Then keep adding sodium hydroxide until it is in excess and watch whether the precipitate disappears.

Colour of precipitate	Does it dissolve in excess NaOH?	Metal ion
white	yes, leaving a colourless solution	aluminium (Al³⁺)
white	no	calcium (Ca²⁺) or magnesium (Mg²⁺)
blue	no	copper(II) (Cu²⁺)
green	no	iron(II) (Fe²⁺)
brown	no	iron(III) (Fe³⁺)

Key points:

• Aluminium, calcium and magnesium all give white precipitates. Only aluminium hydroxide dissolves in excess sodium hydroxide. That is how you separate aluminium from the other two (statement b).
• Calcium and magnesium cannot be told apart with sodium hydroxide alone. Use a flame test: calcium gives red, while magnesium compounds give no characteristic colour.
• The coloured precipitates (statement c) identify copper(II), iron(II) and iron(III) at once, and tell you the charge on an iron ion.
• A green iron(II) hydroxide precipitate slowly darkens towards brown if left standing in air, as the iron(II) is oxidised to iron(III). Describe the colour you see as soon as it forms.

If you are asked for an equation, the metal ion and hydroxide ions form the hydroxide. For copper(II) chloride solution:

CuCl₂(aq) + 2NaOH(aq) → Cu(OH)₂(s) + 2NaCl(aq)

The (s) state symbol shows the precipitate.

Worked example 2: a white precipitate that stays

A colourless solution of a nitrate gives a white precipitate with a few drops of sodium hydroxide solution. The precipitate does not dissolve in excess. A flame test on the solid gives no characteristic colour. Identify the metal ion.

1. White precipitate: the ion is aluminium, calcium or magnesium.
2. It stays in excess, so it is not aluminium.
3. Calcium would give a red flame. No colour was seen, so it is not calcium.
4. The metal ion is magnesium, Mg²⁺, and the compound is magnesium nitrate.

Notice that two tests were needed. If one test leaves two possibilities, say which further test would decide between them.

3.4.3 d: carbonate ions

Carbonates react with dilute acids to form carbon dioxide gas, along with a salt and water.

1. Add a few cm³ of dilute hydrochloric acid to the solid or solution.
2. A carbonate fizzes (effervescence) as carbon dioxide forms.
3. Bubble the gas through limewater (calcium hydroxide solution) using a delivery tube, or draw it up with a teat pipette and bubble it through.
4. Carbon dioxide gives a white precipitate of calcium carbonate, so the limewater turns cloudy white.

Fizzing alone is not enough: a reactive metal in acid also fizzes, giving hydrogen. The limewater result confirms carbon dioxide.

A symbol equation, for copper(II) carbonate and dilute nitric acid:

CuCO₃(s) + 2HNO₃(aq) → Cu(NO₃)₂(aq) + H₂O(l) + CO₂(g)

3.4.3 e: halide ions (chloride, bromide, iodide)

1. To a solution of the unknown, add a few drops of dilute nitric acid.
2. Then add a few drops of silver nitrate solution.
3. A precipitate of the silver halide forms. Its colour tells you which halide is present.

Halide ion	Precipitate	Colour
Chloride, Cl⁻	silver chloride	white
Bromide, Br⁻	silver bromide	cream
Iodide, I⁻	silver iodide	yellow

Why add nitric acid first? The acid removes carbonate ions. Carbonate ions would also give a precipitate with silver ions, which could be mistaken for a halide result.

Why nitric acid and not hydrochloric acid? Hydrochloric acid contains chloride ions. They would give a white silver chloride precipitate every time, even if the unknown had no chloride in it.

Worked example 3: putting a flame test and a halide test together

A white crystalline solid gives a crimson flame. A solution of it, acidified with dilute nitric acid, gives a cream precipitate with silver nitrate solution. Name the solid.

1. Crimson flame: lithium ion, Li⁺.
2. Cream silver halide precipitate: bromide ion, Br⁻.
3. Li⁺ and Br⁻ have charges of +1 and −1, so they combine 1 : 1. The solid is lithium bromide, LiBr.

3.4.3 f: sulfate ions

1. To a solution of the unknown, add a few drops of dilute hydrochloric acid.
2. Then add a few drops of barium chloride solution.
3. If sulfate ions are there, white barium sulfate precipitates.

Without the acid, any carbonate ions in the sample would react with barium ions too, and the white barium carbonate formed would look like a positive result. Here hydrochloric acid is the right choice, because the test reagent is a chloride anyway. Do not use sulfuric acid: it adds sulfate ions and would always give a positive result.

Planning a full identification

When an exam question gives you an unknown ionic compound, plan one test for the positive ion and one for the negative ion. Use a fresh portion of the unknown for each test, so the reagents from one test cannot affect the next.

Worked example 4: planning the tests for an unknown sulfate

A student thinks a white powder is aluminium sulfate. Describe the tests that would confirm this, with the expected result of each.

1. Dissolve some powder in distilled water and split the solution into three test tubes.
2. Tube 1, the positive ion. Add sodium hydroxide solution drop by drop: a white precipitate forms. Keep adding until in excess: the precipitate dissolves. This shows aluminium ions.
3. Tube 2, the negative ion. Add a little dilute hydrochloric acid, followed by some barium chloride solution; a white precipitate appears. This shows sulfate ions.
4. Tube 3, ruling out a carbonate. Add dilute hydrochloric acid: no fizzing, so no carbonate is present.

Why separate tubes? If silver nitrate were added to the solution from tube 2, the chloride ions from the barium chloride and hydrochloric acid would give a white precipitate and suggest a chloride that was never in the powder.

Common errors

• Writing “orange” or “brick red” for lithium. The answer for lithium is crimson.
• Swapping the green flame (barium) with the green precipitate (iron(II) hydroxide). One is a flame colour, the other is a solid in a test tube.
• Saying all white hydroxide precipitates dissolve in excess. Only aluminium hydroxide does.
• Writing “limewater turns milky” with no mention of the gas test. Give both: fizzing, and the gas turns limewater cloudy white.
• Choosing hydrochloric acid for the halide test, or sulfuric acid for the sulfate test.
• Describing a precipitate without its colour. “A precipitate forms” will not identify the ion.

Where to go next

Condense this unit with the identification of ions revision notes, then try the practice questions. For calculations on precipitate masses, the quantitative chemistry guide covers moles and relative formula mass. Track the whole course on the printable checklist or start with one of the free 10-minute diagnostics.

Official syllabus

OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards, published by OxfordAQA. Scope of this guide: section 3.4.3 Identification of ions (statements a to f), with the flame test required practical from section 6.2.