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

This guide teaches sub-topic C4.2 Identifying the products of chemical reactions (Topic C4) using the OCR specification J248, Chemistry A (Gateway Science) at GCSE (9-1), in its Version 4.0 dated August 2026; it was first assessed in 2018. It works through statements C4.2a to C4.2g, the mathematical learning outcome CM4.2i and the practical suggestions linked to PAG C5. One statement, C4.2b (tests for aqueous cations and anions), is tested only in the Higher Tier papers, so it is labelled Higher Tier only wherever it appears; the rest applies to both tiers. The specification places Topic C4 in Paper 2 (Foundation Tier) and Paper 4 (Higher Tier).

Pair this guide with the C4.2 revision notes and the C4.2 practice questions. The J248 course page lists every topic, the J248 checklist records which statements you have mastered, and a free diagnostic points to your weakest areas.

What C4.2 asks you to do

Ref	What you must be able to do	Tier
C4.2a	Describe tests for oxygen, hydrogen, carbon dioxide and chlorine	Both
C4.2b	Describe tests for aqueous Ca²⁺, Cu²⁺, Fe²⁺, Fe³⁺ and Zn²⁺ (sodium hydroxide) and for carbonate, chloride, bromide, iodide and sulfate ions	Higher Tier only
C4.2c	Describe how to carry out a flame test	Both
C4.2d	Identify species from test results	Both
C4.2e	Interpret flame tests for lithium, sodium, potassium, calcium and copper ions	Both
C4.2f	Describe the advantages of instrumental methods: sensitivity, accuracy and speed	Both
C4.2g	Interpret an instrumental result in chart or table form against a reference set in the same form, including a mass spectroscopy chart	Both
CM4.2i	Interpret charts, particularly in spectroscopy	Both

The specification assumes you know cations and anions from electrolysis.

A point about mass

The specification flags a common misconception. A precipitate does not make the products heavier than the reactants; the atoms are just rearranged into a solid. A gas-forming test in an open tube does not destroy mass either: the gas escapes. Count the gas and the total is unchanged.

Tests for gases (C4.2a)

Gas	What you do	Positive result
Oxygen	Put a glowing splint into the gas	The splint relights
Hydrogen	Bring a lighted splint to the open end of the tube	A squeaky pop
Carbon dioxide	Pass the gas into limewater (calcium hydroxide solution)	The limewater goes milky (cloudy)
Chlorine	Hold damp blue litmus paper in the gas	Paper turns red, then is bleached white

Chlorine is toxic, so test it in a fume cupboard. The litmus must be damp: chlorine reacts with the water to form the substances that redden and bleach the dye. Limewater turns milky because insoluble calcium carbonate forms.

Worked example 1: a gas that fails two tests

A student collects a colourless gas from an unknown reaction and records:

• a glowing splint goes out
• the gas does not change limewater
• a lighted splint brought to the open end of the tube makes a squeaky pop

Which gas is it?

Step 1. The glowing splint did not relight, so the gas is not oxygen. Step 2. Limewater stayed clear, so it is not carbon dioxide. Step 3. The squeaky pop is the positive test for hydrogen. The gas is hydrogen.

Negative results rule gases out, but always finish with the positive test.

Flame tests (C4.2c and C4.2e)

Some metal ions colour a flame, which identifies the metal ion in a sample.

How to carry out a flame test (C4.2c)

1. Clean a nichrome wire loop: dip it in hydrochloric acid, then heat it in a hot, blue Bunsen flame. Repeat until the flame stays uncoloured.
2. Wet the cleaned loop with acid and touch it on the solid sample to pick up a few grains.
3. Place the grains in the edge of a blue (air hole open) Bunsen flame.
4. Record the colour of the flame.
5. Clean the loop again before the next sample, or use a fresh loop.

Wear eye protection, because the acid is corrosive. The yellow safety flame has its own colour, which would hide the result. Cleaning matters because a trace of an earlier sample, especially a sodium compound, adds its own colour.

Interpreting flame colours (C4.2e)

Metal ion	Flame colour
Lithium, Li⁺	Crimson (red)
Sodium, Na⁺	Yellow (orange-yellow)
Potassium, K⁺	Lilac
Calcium, Ca²⁺	Orange-red (brick red)
Copper, Cu²⁺	Green (blue-green)

Worked example 2: two red flames

Two white solids are labelled “lithium chloride” and “calcium chloride”, but the labels may have been swapped. Both give a reddish flame. Which is which?

Step 1. Compare the flames side by side: lithium is crimson, calcium is orange-red. Step 2. To confirm (Higher Tier only), add sodium hydroxide solution to a solution of each. Calcium ions give a white precipitate; lithium ions give none, because lithium hydroxide is soluble. Conclusion. Crimson flame, no precipitate: lithium chloride. Orange-red flame, white precipitate: calcium chloride.

Tests for aqueous cations (C4.2b, Higher Tier only)

Add a few drops of dilute sodium hydroxide solution to the test solution, then add more until it is in excess. Many metal ions form an insoluble hydroxide precipitate.

Cation	Precipitate with a few drops of NaOH(aq)	With excess NaOH(aq)
Calcium, Ca²⁺	White	Stays
Zinc, Zn²⁺	White	Dissolves to give a colourless solution
Copper, Cu²⁺	Blue	Stays
Iron(II), Fe²⁺	Green (the surface slowly turns orange-brown in air)	Stays
Iron(III), Fe³⁺	Orange-brown	Stays

Calcium and zinc both give white precipitates, so the excess step is how you tell them apart.

Worked example 3: writing the ionic equation

Sodium hydroxide solution is added to a pale green solution, and a green solid forms. Identify the cation and write an ionic equation with state symbols.

Step 1. A green precipitate means iron(II) ions, Fe²⁺. Step 2. Each Fe²⁺ ion needs two OH⁻ ions to make a neutral solid, Fe(OH)₂. Step 3. Fe²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(s)

Sodium ions and the original anion are spectators. For iron(III) the solid is Fe(OH)₃.

Tests for aqueous anions (C4.2b, Higher Tier only)

Anion	Test	Positive result
Carbonate, CO₃²⁻	Add dilute acid; pass the gas into limewater	Bubbles of carbon dioxide, which turns limewater milky
Chloride, Cl⁻	Acidify with dilute nitric acid; add dilute silver nitrate solution	A white solid forms (silver chloride)
Bromide, Br⁻	Same method as chloride	A cream solid forms (silver bromide)
Iodide, I⁻	Same method as chloride	A yellow solid forms (silver iodide)
Sulfate, SO₄²⁻	Add dilute hydrochloric acid, then barium chloride solution (or dilute nitric acid, then barium nitrate solution)	White precipitate (barium sulfate)

Why acidify first? Carbonate ions also give precipitates with silver and barium ions. The acid removes carbonate as carbon dioxide, so any precipitate comes from the ion you are testing for.

Why that acid? Hydrochloric acid would add chloride ions to the halide test, so use nitric acid. Sulfuric acid would add sulfate ions to the sulfate test, so use hydrochloric or nitric acid.

The precipitates form by simple ion pairing, for example Ag⁺(aq) + Br⁻(aq) → AgBr(s) and Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s). The carbonate reaction is CO₃²⁻ + 2H⁺ → CO₂ + H₂O.

Identifying species from results (C4.2d)

PAG C5 suggests identifying an unknown compound using cation, anion and flame tests. For a single unknown salt:

1. Flame test on the solid.
2. Dissolve some in water; add sodium hydroxide to one portion.
3. Add dilute acid to a fresh portion to check for carbonate.
4. Use fresh portions for the sulfate and halide tests.
5. Combine cation and anion into a name and formula.

Worked example 4: an unknown white solid (uses Higher Tier only tests)

Solid Z dissolves in water to give a colourless solution. Results:

• Flame test: none of the five colours in the flame test table.
• A few drops of sodium hydroxide give a white precipitate, which dissolves when more is added.
• Dilute nitric acid then silver nitrate solution give a yellow precipitate.

Step 1. A white precipitate that dissolves in excess sodium hydroxide shows zinc ions, Zn²⁺. (Calcium would also be white, but it would not dissolve, and it would give an orange-red flame.) Step 2. A yellow precipitate with acidified silver nitrate shows iodide ions, I⁻. Step 3. Zn²⁺ needs two I⁻ to balance the charges. Z is zinc iodide, ZnI₂.

Instrumental methods (C4.2f)

Instrumental methods use machines, rather than test-tube reactions, to detect and identify substances. The specification names three advantages:

• Sensitivity: they detect very small amounts of a substance, far below what a colour change or precipitate would show.
• Accuracy: they identify substances reliably and measure amounts close to the true value.
• Speed: they give results quickly, and many samples can be run one after another, often automatically.

Worked example 5: choosing the method

A food laboratory must check hundreds of drinks a day for a contaminant present only in tiny traces. Explain why it uses an instrumental method rather than precipitation tests.

Answer. Only tiny traces are present, so high sensitivity is needed to detect them at all. A positive result must be right, which needs accuracy. With hundreds of samples a day, the speed of an automated instrument matters.

Reading instrumental results (C4.2g and CM4.2i)

Statement C4.2g pairs the result with a reference set in the same form. You match the unknown to the references; you do not memorise spectra.

Features of a mass spectroscopy chart

• The horizontal axis is m/z, the mass-to-charge ratio. For 1+ ions, m/z is the ion’s mass.
• The vertical axis is relative abundance. The tallest peak, the base peak, is often set to 100.
• The molecular ion peak is the peak at the highest m/z value. It gives the relative molecular mass, Mr.
• The other peaks are fragments of the molecule. Their pattern acts like a fingerprint, so compounds with the same Mr can still be told apart.

Worked example 6: matching a spectrum

An unknown liquid gives the chart below, shown as bar heights. Simplified reference data for three compounds are supplied (use these values).

m/z	Unknown (relative abundance)	Bar
15	25	██▌
43	100	██████████
58	60	██████

Reference compound	Mr	Main peaks, m/z (relative abundance)
Ethanol, C₂H₅OH	46	31 (100), 45 (50), 46 (20)
Butane, C₄H₁₀	58	43 (100), 29 (45), 58 (10)
Propanone, CH₃COCH₃	58	43 (100), 58 (60), 15 (25)

Step 1. The highest m/z peak is at 58, so Mr = 58. This rules out ethanol (Mr 46). Step 2. Butane and propanone both have Mr 58 and a base peak at 43, so compare the other peaks. Step 3. The unknown has a peak at 15 and no peak at 29. Its peak at 58 is 60% of the base peak, not 10%. Step 4. Every peak and relative height matches propanone. The unknown is propanone.

Check your Mr from the formula: propanone, C₃H₆O, is (3 × 12) + (6 × 1) + 16 = 58.

Common errors

• Testing chlorine with dry litmus paper.
• Calling the lithium flame “orange-red”. Crimson is lithium; orange-red is calcium.
• Identifying calcium from a white hydroxide without adding excess sodium hydroxide to rule out zinc.
• Acidifying the silver nitrate test with hydrochloric acid.
• Taking the tallest peak in a mass spectrum as the molecular ion. The molecular ion is the highest m/z peak.
• Saying a precipitate makes the mixture heavier, or that a gas-forming reaction “loses mass” overall.

Where next

Next, use the C4.2 revision notes and then the question set. Ionic equations and state symbols are explained in the guide to formulae and equations; chromatography and purity are in the guide to purity and chromatography. Run a free diagnostic to find your next topic.

Official syllabus

Source: OCR’s J248 document, Chemistry A (Gateway Science) for GCSE (9-1); Version 4.0 dated August 2026; first assessment 2018. This page follows its sub-topic C4.2 Identifying the products of chemical reactions.