Pearson Edexcel International GCSE Chemistry 4CH1: Chemical tests – Revision Notes

These notes condense sub-topic (h) Chemical tests of Topic 2, Inorganic chemistry, in the Pearson Edexcel International GCSE Chemistry (4CH1) specification, Issue 3 (September 2024). They cover points 2.44–2.50. The qualification is untiered, and none of these points carries a C reference, so everything here can appear on Paper 1 and Paper 2. For full explanations and worked identifications, use the Chemical tests study guide.

Links: 4CH1 Chemistry course hub, printable 4CH1 checklist, free 4CH1 all-topics diagnostic. The wider topic is in the Reactivity and group trends revision notes. For exam-style questions, use the Groups 1 and 7, acids, salts and tests practice set and the Reactivity and group trends practice set.

The answer pattern for every test

Reagent (named in full) → method (warm, damp paper, bubble through) → observation (with a colour or a sound).

“Precipitate” with no colour scores nothing. “Sulfate present” is a conclusion, not an observation.

2.44 Gas tests

Gas	Do this	See this
H₂	Lighted splint at mouth of tube	Squeaky pop
O₂	Glowing splint into gas	Relights
CO₂	Bubble through limewater	Limewater turns milky
NH₃	Damp red litmus paper	Turns blue
Cl₂	Damp litmus paper	Bleached white

Limewater equation:

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
                         (the white solid that makes it milky)

Memory hooks:

• Glowing splint → Oxygen relights it. Lighted splint → hydrogen pops.
• Ammonia is the only common alkaline gas, so red litmus goes blue.
• Chlorine is a bleach. Blue litmus may go red first; the result you write is “bleached”.
• Paper must be damp for both ammonia and chlorine.

2.45 Flame test method in four steps

1. Clean a nichrome/platinum wire: dip in conc. HCl, heat in flame until no colour.
2. Dip the clean wire in conc. HCl, then into the solid sample.
3. Hold it in a blue (non-luminous) Bunsen flame.
4. Record the flame colour.

Why: a dirty wire gives a colour from the last sample; a yellow flame hides the result.

2.46 Flame colours

Ion	Colour
Li⁺	red
Na⁺	yellow
K⁺	lilac
Ca²⁺	orange-red
Cu²⁺	blue-green

• Lithium is red; calcium is orange-red. Keep the words exact.
• Sodium’s strong yellow can mask a weaker colour such as lilac.
• A flame test tells you the metal ion only.

2.47 Cation tests with sodium hydroxide solution

Ion	Add NaOH(aq)	Result
Cu²⁺	a few drops	blue precipitate
Fe²⁺	a few drops	green precipitate
Fe³⁺	a few drops	brown precipitate
NH₄⁺	then warm	no precipitate; gas turns damp red litmus blue

Ionic equations (the hydroxide count equals the metal ion’s charge):

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)

Full equation for an ammonium salt you may not have met:

(NH₄)₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 2NH₃(g) + 2H₂O(l)

Green iron(II) hydroxide darkens to brown on standing in air. Record the colour you see at once.

2.48 Anion tests

Ion	Add	Result
Cl⁻	dilute HNO₃, then AgNO₃(aq)	white precipitate (AgCl)
Br⁻	dilute HNO₃, then AgNO₃(aq)	cream precipitate (AgBr)
I⁻	dilute HNO₃, then AgNO₃(aq)	yellow precipitate (AgI)
SO₄²⁻	dilute HCl, then BaCl₂(aq)	white precipitate (BaSO₄)
CO₃²⁻	dilute HCl	fizzing; gas turns limewater milky (CO₂)

Ionic equations:

Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

A full carbonate equation, for a coloured carbonate:

CuCO₃(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l) + CO₂(g)

Which acid goes with which test

Test	Acid to use	Acid to avoid, and why
Silver nitrate (halides)	nitric	hydrochloric – adds Cl⁻, always gives white precipitate
Barium chloride (sulfate)	hydrochloric	sulfuric – adds SO₄²⁻, always gives white precipitate

The acid is added first to remove carbonate ions, which would otherwise form a precipitate too.

Halide colours in order

Going down Group 7 the precipitate darkens: white → cream → yellow (Cl → Br → I).

2.49 Test for water

Anhydrous copper(II) sulfate:  white → blue
CuSO₄(s) + 5H₂O(l) → CuSO₄·5H₂O(s)

This shows water is present, not that it is pure.

2.50 Physical test for pure water

Property	Pure water (normal pressure)	Impure water
Boiling point	exactly 100 °C	above 100 °C, often over a range
Freezing point	exactly 0 °C	below 0 °C

Method: heat the sample and read the boiling temperature with a thermometer.

Method in steps: identifying an unknown salt

1. Flame test on the solid            → metal ion (Li⁺, Na⁺, K⁺, Ca²⁺, Cu²⁺)
2. Dissolve; add NaOH(aq)             → Cu²⁺, Fe²⁺, Fe³⁺ by precipitate colour
   No precipitate? Warm, damp red litmus → NH₄⁺ if it turns blue
3. New sample: add dilute HCl         → fizzing + milky limewater = CO₃²⁻
4. New sample: HNO₃ then AgNO₃(aq)    → Cl⁻ / Br⁻ / I⁻ by precipitate colour
5. New sample: HCl then BaCl₂(aq)     → white precipitate = SO₄²⁻
6. Combine cation and anion; balance the charges for the formula

Use a fresh sample for each test. Reagents from one test would interfere with the next.

Small worked reminders

Reminder 1. A pale green solution gives a green precipitate with sodium hydroxide. With dilute nitric acid and silver nitrate it gives a yellow precipitate.

• Green hydroxide → Fe²⁺. Yellow silver halide → I⁻.
• Charges: Fe²⁺ needs two I⁻. The salt is iron(II) iodide, FeI₂.

Reminder 2. A white solid gives no flame colour. Its solution gives no precipitate with sodium hydroxide, but on warming a gas turns damp red litmus blue. With dilute nitric acid and silver nitrate it gives a white precipitate.

• Gas is ammonia → NH₄⁺. White silver halide → Cl⁻.
• The salt is ammonium chloride, NH₄Cl.

Reminder 3. A white solid gives a lilac flame. With dilute hydrochloric acid and barium chloride it gives a white precipitate.

• Lilac → K⁺. White precipitate with acidified BaCl₂ → SO₄²⁻.
• Charges: two K⁺ balance one SO₄²⁻. The salt is potassium sulfate, K₂SO₄.

Must-know distinctions

• Glowing vs lighted splint. Glowing tests for oxygen; lighted tests for hydrogen.
• Red vs blue litmus. Damp red litmus for ammonia (goes blue). Any damp litmus for chlorine (bleached).
• Fe²⁺ vs Fe³⁺. Green vs brown hydroxide. Always write the charge or the Roman numeral.
• NH₄⁺ vs metal ions. Ammonium gives no precipitate; you must warm and test the gas.
• Bromide vs iodide. Cream vs yellow.
• Sulfate vs carbonate. Sulfate gives a precipitate with acidified BaCl₂; carbonate fizzes with acid and gives no precipitate.
• Present vs pure. Copper(II) sulfate shows water is present; the boiling point shows if it is pure.
• Flame test vs NaOH test. Cu²⁺ is the only ion in both lists (blue-green flame; blue precipitate).

Quick self-test

1. Which gas gives a squeaky pop with a lighted splint?
2. Describe the test for ammonia, with its result.
3. Give the flame colour of Cu²⁺.
4. A sample gives a red flame. Which ion from the specification list is present?
5. Why is the wire cleaned in concentrated hydrochloric acid before a flame test?
6. Sodium hydroxide solution gives a brown precipitate. Name the ion present.
7. Write the ionic equation for the reaction of iron(II) ions with hydroxide ions.
8. Dilute nitric acid and silver nitrate give a cream precipitate. Name the ion present.
9. Name the two reagents used to test for sulfate ions, and the positive result.
10. A white powder gives a yellow flame. It fizzes with dilute hydrochloric acid and the gas turns limewater milky. Name the compound.
11. Give the colour change when water is added to anhydrous copper(II) sulfate.
12. A clear liquid freezes at −2 °C. Is it pure water? Explain.

Answers

1. Hydrogen.
2. Hold damp red litmus paper in the gas; it turns blue.
3. Blue-green.
4. Lithium ion, Li⁺.
5. To remove traces of other substances, which would colour the flame.
6. Iron(III), Fe³⁺.
7. Fe²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(s)
8. Bromide, Br⁻.
9. Dilute hydrochloric acid, then barium chloride solution; white precipitate.
10. Sodium carbonate, Na₂CO₃ (yellow flame = Na⁺; fizzing plus milky limewater = CO₃²⁻).
11. White to blue.
12. No. Pure water freezes at 0 °C; dissolved impurities lower the freezing point.

Where marks are usually lost

• Writing “the gas is tested with a splint” without saying glowing or lighted.
• Holding dry litmus paper in ammonia or chlorine.
• Giving “turns red” as the chlorine result instead of “bleached”.
• Stating “precipitate forms” with no colour.
• Writing “iron” for a green precipitate; the answer needs iron(II) or Fe²⁺.
• Leaving out “warm” in the ammonium test, or saying the litmus is dipped into the solution.
• Naming “acidified silver nitrate” correctly but then adding hydrochloric acid in the method.
• Swapping the cream and yellow halide precipitates.
• Saying “bubbles” for carbonate without testing the gas with limewater.
• Claiming that a blue colour with anhydrous copper(II) sulfate proves the water is pure.

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.