Pearson Edexcel International GCSE Chemistry 4CH1: Chemical tests – Study Guide

This guide teaches sub-topic (h) Chemical tests of Topic 2, Inorganic chemistry, in the Pearson Edexcel International GCSE Chemistry (4CH1) specification, Issue 3 (September 2024). It covers points 2.44–2.50 in depth. The qualification is untiered. None of points 2.44–2.50 carries a C reference, so all of this content can be assessed on both Paper 1 and Paper 2.

Links: 4CH1 course hub, printable checklist, Chemical tests revision notes, the wider Reactivity and group trends guide and its revision notes, and the free 4CH1 all-topics diagnostic. For exam-style questions on these tests, use the Groups 1 and 7, acids, salts and tests practice set and the Reactivity and group trends practice set.

What this sub-topic covers

Spec point	What you must be able to do	Paper
2.44	Describe tests for hydrogen, oxygen, carbon dioxide, ammonia and chlorine	1 and 2
2.45	Describe how to carry out a flame test	1 and 2
2.46	Know the flame colours of Li⁺, Na⁺, K⁺, Ca²⁺ and Cu²⁺	1 and 2
2.47	Describe tests for NH₄⁺ (sodium hydroxide, then identify the gas) and for Cu²⁺, Fe²⁺ and Fe³⁺ (sodium hydroxide)	1 and 2
2.48	Describe tests for Cl⁻, Br⁻ and I⁻ (acidified silver nitrate), SO₄²⁻ (acidified barium chloride) and CO₃²⁻ (hydrochloric acid, then identify the gas)	1 and 2
2.49	Describe a test for the presence of water using anhydrous copper(II) sulfate	1 and 2
2.50	Describe a physical test to show whether a sample of water is pure	1 and 2

How a test answer earns its marks

Every “describe a test” answer has three parts. Leave one out and you lose a mark.

1. What you add or do – the reagent, named in full (for example “dilute nitric acid, then silver nitrate solution”).
2. How you do it – warm it, hold damp paper at the mouth of the tube, bubble the gas through.
3. What you see – a colour, a sound, a change. “White precipitate”, not just “precipitate”.

A result is something you observe. “Chloride ions are present” is a conclusion, not a result.

2.44 Tests for gases

Gas	Test	Positive result
Hydrogen, H₂	Hold a lighted splint at the mouth of the tube	Squeaky pop
Oxygen, O₂	Put a glowing splint into the gas	Splint relights
Carbon dioxide, CO₂	Bubble the gas through limewater (calcium hydroxide solution)	Limewater turns milky (cloudy)
Ammonia, NH₃	Hold damp red litmus paper in the gas	Litmus turns blue
Chlorine, Cl₂	Hold damp litmus paper in the gas	Litmus is bleached (turns white)

Why each test works:

• Hydrogen burns rapidly with oxygen from the air. The pop is a small explosion: 2H₂(g) + O₂(g) → 2H₂O(l).
• Oxygen supports combustion, so the glowing wood bursts back into flame.
• Carbon dioxide reacts with limewater to form insoluble calcium carbonate, which makes the liquid cloudy:

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

• Ammonia is the only common alkaline gas. It dissolves in the water on the paper to give an alkaline solution.
• Chlorine is a bleach. Damp blue litmus may turn red first, then white. The final white colour is the result to give.

The paper must be damp for ammonia and chlorine. A gas has to dissolve in water before it can change the indicator.

Worked example 1 – three unlabelled gas jars

Three gas jars contain hydrogen, oxygen and carbon dioxide. Describe how to identify each, using as few tests as possible.

Step 1. Put a glowing splint into each jar. The jar where it relights is oxygen.

Step 2. Hold a lighted splint at the mouth of one remaining jar. A squeaky pop means hydrogen.

Step 3. Confirm the last jar by shaking it with limewater. Milky limewater means carbon dioxide.

“The splint goes out” is not a test for carbon dioxide. Many gases put out a flame; only limewater confirms CO₂.

2.45 How to carry out a flame test

1. Clean a nichrome (or platinum) wire loop: dip it in concentrated hydrochloric acid and hold it in a hot Bunsen flame until it gives no colour.
2. Dip the clean loop in the acid again, then into the solid sample so some sticks to it.
3. Hold the loop at the edge of a blue (non-luminous) Bunsen flame.
4. Note the colour of the flame.

Why the details matter:

• The wire must be clean, or traces of an earlier sample colour the flame.
• Use a blue flame. A yellow (luminous) flame would hide the colour you are looking for.

2.46 Flame colours

Cation	Flame colour
Li⁺	red
Na⁺	yellow
K⁺	lilac
Ca²⁺	orange-red
Cu²⁺	blue-green

Learn these exact words. Lithium is red and calcium is orange-red; they are close, so read the result carefully.

Sodium’s yellow is very strong. A small amount of sodium impurity can hide a fainter colour such as potassium’s lilac.

Worked example 2 – reading a flame test

A white solid gives an orange-red flame. A second white solid gives a lilac flame. Identify the cation in each.

Answer. The first contains calcium ions, Ca²⁺. The second contains potassium ions, K⁺.

A flame test identifies only the metal ion. It tells you nothing about the anion, so you need a separate test for that.

2.47 Tests for cations using sodium hydroxide

Add a few drops of sodium hydroxide solution to a solution of the substance.

Cation	What you see
Cu²⁺	Blue precipitate
Fe²⁺	Green precipitate
Fe³⁺	Brown precipitate
NH₄⁺	No precipitate. Warm the mixture: a gas is given off that turns damp red litmus paper blue (ammonia)

The precipitates are insoluble metal hydroxides. Full equations, with state symbols:

CuSO₄(aq) + 2NaOH(aq) → Cu(OH)₂(s) + Na₂SO₄(aq)      blue precipitate
FeSO₄(aq) + 2NaOH(aq) → Fe(OH)₂(s) + Na₂SO₄(aq)      green precipitate
FeCl₃(aq) + 3NaOH(aq) → Fe(OH)₃(s) + 3NaCl(aq)       brown precipitate
NH₄Cl(aq) + NaOH(aq) → NaCl(aq) + NH₃(g) + H₂O(l)    ammonia gas on warming

Written with only the ions that react:

Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s)
Fe²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(s)
Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s)
NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l)

The number of hydroxide ions matches the charge on the metal ion. Fe²⁺ needs two; Fe³⁺ needs three.

The green iron(II) hydroxide slowly turns brown at the surface if left in air, as it is oxidised to iron(III). Report the colour you see straight away.

For the ammonium test, warming matters. Ammonia is very soluble, so at room temperature little gas escapes. The litmus must be damp and held at the mouth of the tube, not dipped in the liquid (the sodium hydroxide itself would turn it blue).

Worked example 3 – iron(II) or iron(III)?

A student has two pale solutions labelled “iron sulfate”. Describe how to show which contains iron(II) and which contains iron(III).

Answer. Add a few drops of sodium hydroxide solution to a sample of each. A green precipitate shows iron(II), Fe²⁺. A brown precipitate shows iron(III), Fe³⁺.

The word “iron” on its own scores nothing here. The whole point of the test is to tell the two ions apart.

2.48 Tests for anions

Halide ions: acidified silver nitrate

Add dilute nitric acid, then silver nitrate solution.

Halide ion	Precipitate
Cl⁻	white (silver chloride)
Br⁻	cream (silver bromide)
I⁻	yellow (silver iodide)

NaCl(aq) + AgNO₃(aq) → AgCl(s) + NaNO₃(aq)
KBr(aq) + AgNO₃(aq) → AgBr(s) + KNO₃(aq)
NaI(aq) + AgNO₃(aq) → AgI(s) + NaNO₃(aq)
ionic, for chloride:  Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

Sulfate ions: acidified barium chloride

Add dilute hydrochloric acid, then barium chloride solution. A white precipitate of barium sulfate shows sulfate ions.

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)
ionic:  Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

Why acidify, and with which acid?

The acid removes any carbonate ions first. Carbonate ions would also form a precipitate with silver ions or barium ions, giving a false positive.

The choice of acid matters:

• For the silver nitrate test use nitric acid. Hydrochloric acid would add chloride ions, so you would always see a white precipitate.
• For the barium chloride test use hydrochloric acid. Sulfuric acid would add sulfate ions, so you would always see a white precipitate.

Carbonate ions: hydrochloric acid

Add dilute hydrochloric acid. Look for effervescence (fizzing). Bubble the gas through limewater: it turns milky, so the gas is carbon dioxide and the substance contains carbonate ions.

Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)

Fizzing alone is not enough. Metals such as magnesium fizz with acid too, giving hydrogen. You must identify the gas.

Worked example 4 – identifying a salt from two tests

A blue solid, X, is dissolved in water.

• Flame test: blue-green flame.
• Sodium hydroxide solution: blue precipitate.
• Dilute hydrochloric acid, then barium chloride solution: white precipitate.

Identify X.

Step 1. Blue-green flame suggests Cu²⁺. The blue hydroxide precipitate confirms copper(II).

Step 2. White precipitate with acidified barium chloride shows sulfate.

Step 3. Combine the ions. Cu²⁺ and SO₄²⁻ have equal and opposite charges, so X is copper(II) sulfate, CuSO₄. (The blue solid is the hydrated salt, CuSO₄·5H₂O; anhydrous CuSO₄ is white.)

Worked example 5 – spotting the faulty method

A student adds dilute hydrochloric acid and then silver nitrate solution to an unknown solution. A white precipitate forms. The student concludes the solution contains chloride ions. Evaluate this conclusion.

Answer. The conclusion is not valid. The hydrochloric acid itself supplies chloride ions, so a white precipitate would form whatever the unknown contained. Repeat the test using dilute nitric acid in place of hydrochloric acid.

2.49 Test for the presence of water

Add the liquid to anhydrous copper(II) sulfate. It turns from white to blue if water is present.

CuSO₄(s) + 5H₂O(l) → CuSO₄·5H₂O(s)
white                  blue

This shows that water is present. It does not show the water is pure. A salt solution also turns it blue.

2.50 Physical test for pure water

• Pure water boils at exactly 100 °C (at normal atmospheric pressure).
• Pure water freezes (or melts) at exactly 0 °C.

Heat the sample and measure the temperature at which it boils with a thermometer. If it boils at 100 °C, the water is pure. Dissolved impurities raise the boiling point and lower the freezing point. An impure sample also tends to boil over a range of temperatures, not at one fixed value.

Worked example 6 – present versus pure

A sample of seawater turns anhydrous copper(II) sulfate blue and boils at 101 °C. What do the two results show?

Answer. The colour change shows water is present. The boiling point is above 100 °C, so the water is not pure: it contains dissolved substances (salts).

Common errors

• Using a lighted splint for oxygen or a glowing splint for hydrogen.
• Using dry litmus paper for ammonia or chlorine.
• Writing “chlorine turns litmus red” as the result. The test result is bleaching.
• Giving “precipitate” with no colour, or giving “iron” without (II) or (III).
• Forgetting to warm in the ammonium test, or putting the litmus into the liquid.
• Acidifying with hydrochloric acid before silver nitrate, or with sulfuric acid before barium chloride.
• Writing “cream” for iodide or “yellow” for bromide. Bromide is cream; iodide is yellow.
• Using anhydrous copper(II) sulfate to prove water is pure.

Where to go next

Learn the tables with the Chemical tests revision notes, then try the identification questions in the Groups 1 and 7, acids, salts and tests practice set. Check the whole course with the free 4CH1 all-topics diagnostic.

Official syllabus

Pearson Edexcel International GCSE in Chemistry (4CH1) specification, Issue 3, September 2024, published by Pearson Education Limited – Topic 2 Inorganic chemistry, sub-topic (h) Chemical tests, points 2.44–2.50.