OCR GCSE Chemistry A: Identifying the products of chemical reactions (J248) – Revision Notes

For full explanations and six worked examples, read the C4.2 study guide first. These notes condense it for the final weeks before the exam.

They cover sub-topic C4.2 Identifying the products of chemical reactions (C4.2a to C4.2g, with CM4.2i and the PAG C5 practical suggestions) as set out by OCR in J248, Chemistry A (Gateway Science) at GCSE (9-1), specification Version 4.0 of August 2026, first assessed in 2018. Only C4.2b, the cation and anion tests, is Higher Tier only; the rest is for both tiers. Topic C4 sits in Paper 2 (Foundation) and Paper 4 (Higher).

Then try the C4.2 practice questions, mark your progress against the J248 checklist, open the J248 course page or take a free diagnostic. Reactivity and Group trends come just before this in C4.1 Predicting chemical reactions.

Key words

• Precipitate: an insoluble solid that forms when two solutions are mixed.
• Cation / anion: a positive ion / a negative ion.
• Excess: more of a reagent than is needed to react with everything present.
• Instrumental method: analysis done by a machine rather than by a test-tube reaction.
• m/z: mass-to-charge ratio, the horizontal axis of a mass spectrum.
• Molecular ion peak: the peak at the highest m/z; its value is the Mr of the compound.
• Base peak: the tallest peak in a mass spectrum.

Gas tests (C4.2a, both tiers)

Gas	Test	Result
O₂	Glowing splint	Relights
H₂	Lighted splint at mouth of tube	Squeaky pop
CO₂	Bubble through limewater	Limewater turns milky
Cl₂	Damp blue litmus paper (fume cupboard)	Turns red, then bleached white

Memory hook: Glowing for Oxygen (it needs oxygen to burst back into flame); Lighted for hydrogen (the hydrogen burns with a pop).

Flame tests (C4.2c and C4.2e, both tiers)

Method in steps

1. Clean a nichrome loop: dip in hydrochloric acid, heat in a blue flame, repeat until no colour shows.
2. Moisten the loop with acid and pick up a little solid.
3. Hold it at the edge of a blue Bunsen flame (air hole open).
4. Note the colour. Clean the loop before the next sample.

Ion	Flame colour	Metal
Li⁺	Crimson	lithium
Na⁺	Yellow	sodium
K⁺	Lilac	potassium
Ca²⁺	Orange-red	calcium
Cu²⁺	Green	copper

Why a blue flame? The yellow flame would add its own colour. Why clean the loop? Traces of an old sample, sodium above all, give a false colour.

Cation tests with sodium hydroxide (C4.2b, Higher Tier only)

Add NaOH(aq) a few drops at a time, then in excess.

Ion	Precipitate	In excess NaOH
Ca²⁺	White	Does not dissolve
Zn²⁺	White	Dissolves, colourless solution
Cu²⁺	Blue	Does not dissolve
Fe²⁺	Green	Does not dissolve
Fe³⁺	Orange-brown	Does not dissolve

General ionic equation: Mⁿ⁺(aq) + nOH⁻(aq) → M(OH)ₙ(s). Worked reminder: copper(II) gives Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s); iron(III) gives Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s).

Anion tests (C4.2b, Higher Tier only)

Ion	Add	Positive result
CO₃²⁻	Dilute acid; gas into limewater	Fizzing; limewater milky
Cl⁻	Acidify with dilute nitric acid; add silver nitrate solution	AgCl, white
Br⁻	Same as Cl⁻	AgBr, cream
I⁻	Same as Cl⁻	AgI, yellow
SO₄²⁻	Acidify with dilute hydrochloric acid; add barium chloride solution (or nitric acid with barium nitrate)	BaSO₄, white

Acid goes in first because carbonate ions also precipitate with silver and barium ions; the acid destroys them as carbon dioxide.

Identifying an unknown (C4.2d, PAG C5)

Method in steps for one unknown salt

1. Flame test the solid. Note the colour, or that none of the five colours appears.
2. Make a solution. Sodium hydroxide test on one portion, adding excess if the precipitate is white.
3. Fresh portion: dilute acid. Fizzing plus milky limewater means carbonate.
4. Fresh portions: acidified barium chloride (sulfate) and acidified silver nitrate (halides).
5. Name the salt and balance the charges for the formula.

Small worked reminder (Higher Tier only, as it uses the C4.2b tests): green flame + blue precipitate with NaOH + cream precipitate with acidified AgNO₃ = Cu²⁺ and Br⁻, so copper(II) bromide, CuBr₂.

Instrumental methods (C4.2f, both tiers)

Three advantages, as the specification names them:

Advantage	What it means
Sensitivity	Detects very small amounts
Accuracy	Gives a reliable, correct identification or measurement
Speed	Fast results; many samples can be run, often automatically

Reading a chart against a reference set (C4.2g, CM4.2i, both tiers)

Method in steps for a mass spectrum

1. Read the axes: m/z across, relative abundance up.
2. Find the peak with the highest m/z. That value is Mr.
3. Rule out any reference compound with a different Mr.
4. Compare the other peaks and their heights with each remaining reference.
5. Choose the reference that matches every main peak.

Worked reminder: an unknown shows peaks at m/z 29, 31 (tallest) and 32, with nothing higher. Mr = 32, not 31: the tallest peak is not always the molecular ion. Methanol, CH₃OH, has Mr = 12 + (4 × 1) + 16 = 32.

The same matching idea works for any chart or table supplied with reference data in the same form: line up each feature of the unknown with the references.

Tip for charts: read peak positions off the axis carefully, using a ruler if needed. Two references with the same Mr usually differ in at least one fragment peak or in the relative heights of the peaks, so check heights as well as positions. If the unknown has a peak that no reference has, say so: the sample may be impure or may not be in the reference set.

Why the tests work (one line each)

• Limewater: carbon dioxide reacts with calcium hydroxide solution to form insoluble calcium carbonate, which makes the liquid milky.
• Squeaky pop: hydrogen mixed with air burns rapidly, making a small explosion.
• Glowing splint: the extra oxygen lets the smouldering wood burst back into flame.
• Hydroxide precipitates: most of the listed metal hydroxides are insoluble in water, so they come out of solution as solids with a colour typical of the metal ion.
• Silver halides and barium sulfate: these salts are insoluble, so the ions pair up and form a precipitate as soon as the solutions mix.

Mass during tests: the misconception to avoid

The specification lists this misconception for C4.2. In a closed container, the total mass never changes during a test. A precipitate is not “heavier” than the dissolved ions it came from. If a test is done in an open container and a gas escapes (for example, carbon dioxide from a carbonate), the balance reading falls, but only because the gas has left. Mass is conserved in every case.

Must-know distinctions

• Glowing splint (oxygen) vs lighted splint (hydrogen).
• Crimson (lithium) vs orange-red (calcium).
• White hydroxide that dissolves in excess (zinc) vs white hydroxide that stays (calcium).
• Green hydroxide (iron(II)) vs orange-brown hydroxide (iron(III)).
• White silver halide (chloride) vs cream (bromide) vs yellow (iodide).
• White from silver nitrate (chloride) vs white from barium chloride (sulfate). Same colour, different reagent.
• Nitric acid before silver nitrate, never hydrochloric acid.
• Base peak (tallest) vs molecular ion peak (highest m/z).

Quick self-test

1. State the test and result for oxygen.
2. Why must litmus be damp in the chlorine test?
3. A solid gives a lilac flame. Which ion is present?
4. Why is a blue flame used for a flame test?
5. (Higher Tier only) A solution gives a white precipitate with NaOH that stays in excess. Which ion is present?
6. (Higher Tier only) Write the ionic equation for zinc ions reacting with hydroxide ions.
7. (Higher Tier only) Why is hydrochloric acid not used before silver nitrate?
8. (Higher Tier only) Name the white precipitate in the sulfate test.
9. Give the three advantages of instrumental methods named in the specification.
10. A mass spectrum has peaks at m/z 43 (tallest), 57 and 72, and nothing higher. What is the Mr of the compound?
11. (Higher Tier only) A solid gives a lilac flame, and adding dilute nitric acid then silver nitrate to its solution forms a yellow solid. Name the compound.

Answers

1. Put a glowing splint in the gas; it relights.
2. Chlorine reacts with the water to form the substances that turn the litmus red and then bleach it.
3. Potassium, K⁺.
4. The yellow flame has its own colour, which would hide the test colour.
5. Calcium, Ca²⁺ (zinc hydroxide would dissolve).
6. Zn²⁺(aq) + 2OH⁻(aq) → Zn(OH)₂(s)
7. It contains chloride ions, which would give a white precipitate whatever the sample contained.
8. Barium sulfate, BaSO₄.
9. Sensitivity, accuracy and speed.
10. 72 (the highest m/z peak, not the tallest).
11. Potassium iodide, KI.

Where marks are usually lost

• Giving a test without a result, or a result without the test. Both halves are needed.
• “Lighted splint relights” for oxygen: the splint must be glowing.
• “Limewater turns cloudy” with no mention of bubbling the gas through it.
• Dry litmus, or blue litmus “turning red” with no mention of bleaching.
• Confusing lithium (crimson) and calcium (orange-red) flames.
• Concluding “calcium” from a white hydroxide without the excess step.
• Reversing the iron colours: green is iron(II), orange-brown is iron(III).
• Acidifying with hydrochloric or sulfuric acid in the wrong test.
• Reading Mr from the base peak instead of the highest m/z peak.
• Naming an instrumental advantage such as “it is better” or “more reliable” without using the terms sensitivity, accuracy or speed and saying what each means.

Official syllabus

Based on sub-topic C4.2 Identifying the products of chemical reactions. Document: OCR J248, the Chemistry A (Gateway Science) specification for GCSE (9-1); Version 4.0 from August 2026; first assessment 2018.