OCR GCSE Chemistry: Predicting and Identifying Reactions Guide (C4)
OCR GCSE Chemistry: Predicting and Identifying Reactions Guide (C4)
Topic C4 — Predicting and identifying reactions and products is one of the most satisfying parts of the OCR Gateway Science A GCSE Chemistry specification (J248), because it is the most practical. It has two halves that fit neatly together. The first half is about predicting: once you know how reactive metals are, you can predict whether a reaction will happen, which metal will displace which, and how each metal must be extracted from its ore. The second half is about identifying: a toolkit of tests — for gases, for metal ions and for negative ions — that lets you work out what an unknown substance is. Students often find C4 reassuring to revise because so much of it is concrete, observable and rule-based: a colour change, a precipitate, a gas with a characteristic test.
This guide walks through every major idea in C4 at GCSE depth: the reactivity series, reactions of metals with water and dilute acid, displacement reactions and redox, the extraction of metals, predicting products, then the identification half — tests for hydrogen, oxygen, carbon dioxide and chlorine, flame tests, metal hydroxide precipitate tests, anion tests for carbonates, sulfates and halides, and the instrumental method of flame emission spectroscopy. Higher-tier-only material is flagged with [H] throughout. For structured practice alongside this guide, work through the LearningBro OCR GCSE Chemistry: Predicting and Identifying Reactions course, which covers every section below with exam-style questions in the OCR format.
How C4 Fits the J248 Specification
OCR Gateway Science A GCSE Chemistry (J248) is assessed by two papers. Paper 1 covers topics C1–C3, and Paper 2 covers topics C4–C6, so C4 sits firmly on Paper 2. Each paper is worth 90 marks, lasts 1 hour 45 minutes, and counts for 50% of the qualification. The same content is examined on Foundation and Higher tiers, but Higher papers reach further into the more demanding ideas — the electron-transfer view of redox and the ionic half equations in metal extraction are largely Higher territory, which is why the [H] flags below matter for targeting your revision. C4 is rich in required-practical content, so expect questions on methods, observations and evaluation as well as recall.
Chemistry questions use the OCR command words precisely. "Predict" asks you to use a rule (usually the reactivity series) to say what will happen; "Describe what you would observe" asks for the colour, gas or precipitate you would actually see; "Identify" asks you to name the substance from the test results; and "Explain" asks for the reason why. Knowing exactly what each word wants — especially the difference between describing an observation and naming a product — will protect a lot of marks in this topic.
The Reactivity Series
The reactivity series ranks metals in order of how readily they react — how easily they lose electrons to form positive ions. The more reactive a metal, the more vigorously it reacts and the more strongly it holds on to oxygen in its compounds.
| Reactivity | Metals (most → least reactive) |
|---|---|
| Most reactive | potassium, sodium, lithium, calcium, magnesium |
| (carbon sits here) | carbon |
| Middle | (zinc, iron) |
| (hydrogen sits here) | hydrogen |
| Least reactive | copper, silver, gold |
Two non-metals, carbon and hydrogen, are placed in the series as reference points, because a metal's position relative to carbon and hydrogen decides how it reacts with acids and how it is extracted. A handy mnemonic for the metals from potassium down to gold is "Please Stop Lying, Cats May Zip In Cars, Silly Gits" — but the rules below matter far more than the rhyme.
Reactions of Metals with Water and Acid
How vigorously a metal reacts with water and with dilute acid is direct evidence of its position in the series.
Metals with Water
The most reactive metals react with cold water to give a metal hydroxide and hydrogen gas. For example, with sodium:
2Na+2H2O→2NaOH+H2
Potassium reacts most violently (it ignites the hydrogen with a lilac flame), sodium fizzes rapidly, and calcium reacts more steadily. Less reactive metals such as magnesium react only very slowly with cold water, while metals below hydrogen — copper, silver and gold — do not react with water at all.
Metals with Dilute Acid
A metal above hydrogen in the reactivity series reacts with dilute acid to give a salt and hydrogen gas. The general pattern is acid + metal → salt + hydrogen, and the rate of fizzing (hydrogen bubbling off) shows how reactive the metal is. For example:
Mg+2HCl→MgCl2+H2
Magnesium fizzes vigorously, zinc more slowly, iron slowly still. Metals below hydrogen (copper, silver, gold) do not react with dilute acid, because they are less reactive than hydrogen and cannot displace it. Comparing the rate of reaction of different metals with the same acid is a standard required practical for ordering them by reactivity.
Common misconception: "no fizzing" does not mean "no reaction is possible ever" — it means the metal is less reactive than hydrogen, so it cannot displace hydrogen from the acid. That is itself a useful piece of evidence about the metal's position.
Displacement Reactions and Redox
A displacement reaction is one where a more reactive metal takes the place of a less reactive metal in a compound. For example, iron is more reactive than copper, so iron displaces copper from copper sulfate solution:
Fe+CuSO4→FeSO4+Cu
You would observe the blue copper sulfate solution fading and a pink-brown coating of copper forming on the iron. If you tried the reverse — copper in iron sulfate — nothing would happen, because copper is less reactive than iron. Displacement reactions are therefore a neat way to confirm the order of the reactivity series.
Displacement is a redox reaction [H]. In the example above, each iron atom loses two electrons to become Fe2+ (it is oxidised), while each Cu2+ ion gains two electrons to become copper metal (it is reduced). Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain of electrons. The more reactive metal is always the one oxidised, because it loses its electrons more readily.
Extraction of Metals
How a metal is found in nature, and how it is extracted, both depend on its reactivity. Unreactive metals such as gold are found native (as the metal itself). Most metals, though, occur as ores — compounds, usually oxides — and must be extracted by a chemical reaction that removes the oxygen (a reduction).
The method depends on the metal's position relative to carbon:
- Metals less reactive than carbon (such as iron, zinc, copper) can be extracted by reduction with carbon. The carbon is more reactive than the metal, so it takes the oxygen away. Iron is extracted this way in the blast furnace, where carbon reduces iron oxide:
2Fe2O3+3C→4Fe+3CO2
- Metals more reactive than carbon (such as aluminium, magnesium, sodium) cannot be extracted with carbon, because carbon is not reactive enough to displace them. These metals are extracted by electrolysis of the molten compound — a more expensive process because it needs a lot of electrical energy and high temperatures to melt the ore. This is why aluminium, though abundant, was historically costly to produce.
This is the key predictive rule of metal extraction: carbon reduction for metals below carbon, electrolysis for metals above carbon. Reduction with carbon is the cheaper option, which is why it is used wherever a metal's low reactivity allows it.
Predicting Reactions
Pulling the first half of C4 together, the reactivity series lets you predict the outcome of many reactions before doing them:
- Will a metal react with water or acid, and how vigorously? Compare its position with hydrogen — above hydrogen reacts, below does not.
- Will a displacement happen? A more reactive metal displaces a less reactive one from its compound; the reverse does not occur.
- How should a metal be extracted? Below carbon → reduce with carbon; above carbon → electrolysis.
A typical exam task gives you an unfamiliar metal, some observations (say it reacts slowly with acid but is displaced by magnesium), and asks you to place it in the series and predict another reaction. Treat the reactivity series as a ruler: everything in the predicting half of C4 is read off it.
Identifying Substances: Tests for Gases
The identifying half of C4 is a toolkit of tests. Start with the four gas tests, which are pure recall and appear on almost every paper — learn the test and the positive result exactly.
| Gas | Test | Positive result |
|---|---|---|
| Hydrogen (H2) | Hold a lit splint at the mouth of the tube | A squeaky pop |
| Oxygen (O2) | Insert a glowing splint | The splint relights |
| Carbon dioxide (CO2) | Bubble the gas through limewater | The limewater turns milky (cloudy) |
| Chlorine (Cl2) | Hold damp litmus paper in the gas | The litmus is bleached white (turns red first, then white) |
Common misconception: the oxygen test uses a glowing splint that relights, while the hydrogen test uses a lit splint that gives a pop. Mixing these up is one of the most frequent errors — keep them firmly apart.
Identifying Metal Ions: Flame Tests
Some metal ions give a characteristic colour when held in a Bunsen flame on a clean wire loop. This is a quick way to identify the metal in a compound.
| Metal ion | Flame colour |
|---|---|
| Lithium (Li+) | Crimson (red) |
| Sodium (Na+) | Yellow |
| Potassium (K+) | Lilac |
| Calcium (Ca2+) | Orange-red |
| Copper (Cu2+) | Green |
The wire loop must be clean (dipped in acid and held in the flame until it gives no colour) before each test, otherwise traces of a previous sample — especially sodium, whose strong yellow masks fainter colours — give a false result. A limitation of flame tests is that if a mixture of ions is present, the brightest colour can hide the others.
Identifying Metal Ions: Metal Hydroxide Precipitate Tests
A more discriminating test adds sodium hydroxide solution to a solution of the unknown. Many metal ions form an insoluble metal hydroxide that appears as a coloured precipitate, and the colour identifies the metal ion.
| Metal ion | Precipitate colour with NaOH |
|---|---|
| Copper(II) (Cu2+) | Blue |
| Iron(II) (Fe2+) | Green |
| Iron(III) (Fe3+) | Brown |
| Aluminium (Al3+) | White (then dissolves in excess NaOH) |
| Calcium (Ca2+) | White |
| Magnesium (Mg2+) | White |
The general reaction is the metal ion combining with hydroxide ions, for example:
Cu2++2OH−→Cu(OH)2
The three coloured precipitates — copper(II) blue, iron(II) green, iron(III) brown — are unambiguous. The three white precipitates (aluminium, calcium, magnesium) look alike, so they must be told apart by other means: aluminium hydroxide redissolves in excess sodium hydroxide (the others do not), while calcium gives an orange-red flame test and magnesium gives none. Being able to separate the three whites is a favourite Higher-tier discriminator.
Identifying Negative Ions (Anions)
The third set of tests identifies the negative ion in a compound.
Carbonates
To test for a carbonate (CO32−), add dilute acid. Carbonates fizz, releasing carbon dioxide, which you confirm by bubbling the gas through limewater — it turns milky:
CaCO3+2HCl→CaCl2+H2O+CO2
Sulfates
To test for a sulfate (SO42−), add a few drops of dilute hydrochloric acid followed by barium chloride solution. A white precipitate of barium sulfate confirms a sulfate:
Ba2++SO42−→BaSO4
The acid is added first to remove any carbonate ions, which would otherwise also give a precipitate and a false positive.
Halides
To test for a halide (chloride, bromide or iodide), add dilute nitric acid followed by silver nitrate solution. A coloured precipitate of the silver halide forms, and its colour identifies which halide:
| Halide | Precipitate with acidified silver nitrate |
|---|---|
| Chloride (Cl−) | White (AgCl) |
| Bromide (Br−) | Cream (AgBr) |
| Iodide (I−) | Yellow (AgI) |
For example, Ag++Cl−→AgCl. Again the nitric acid is added first to remove carbonate ions that would interfere. The white-cream-yellow sequence runs in the same order as the halides down Group 7.
Common misconception: the order of adding reagents matters. For both the sulfate and halide tests, the acid goes in first to destroy any carbonate ions present, which would otherwise produce a misleading precipitate.
Instrumental Methods: Flame Emission Spectroscopy
Chemical tests are useful but have limits — they can be slow, need fairly large samples, and struggle with mixtures. Modern laboratories use instrumental methods instead, and the one OCR examines is flame emission spectroscopy.
In flame emission spectroscopy, a sample of a metal-ion solution is put into a flame, and the light it gives out is passed into a spectroscope that produces a line spectrum. The position of the lines identifies which metal ions are present, and the intensity of the lines shows how much of each is there — so the method is both qualitative and quantitative.
The advantages over the traditional tests are worth knowing for the exam: instrumental methods are fast, very sensitive (they detect tiny amounts), accurate, and able to analyse very small samples and complex mixtures that would defeat flame tests and precipitate tests. The trade-off is that the equipment is expensive and requires training to use and interpret. The development of instrumental methods is a good example of how advances in technology improve chemical analysis.
Common Mistakes in C4
The same slips recur every year. Knowing them is half the battle.
- Mixing up the splint tests. Hydrogen = lit splint, squeaky pop; oxygen = glowing splint that relights. Keep them apart.
- Forgetting carbon and hydrogen in the reactivity series. Their positions are what determine extraction method and reaction with acid.
- Getting the extraction rule backwards. Below carbon → reduce with carbon (cheaper); above carbon → electrolysis. Reactive metals need electrolysis.
- Not telling the three white precipitates apart. Aluminium hydroxide redissolves in excess NaOH; calcium and magnesium are separated by flame test.
- Adding reagents in the wrong order. For sulfate and halide tests, add the acid first to remove interfering carbonate ions.
- Describing the product instead of the observation. "Describe what you observe" wants the colour, fizz or precipitate you would see, not the name of the salt.
- Saying an unreactive metal "doesn't react" is a non-answer. It is positive evidence that the metal is below hydrogen or below carbon.
Exam Technique for C4 on J248
- Read the command word. "Predict" wants a reactivity-series deduction; "describe what you observe" wants a visible change; "identify" wants the named substance. Answer the one asked.
- Quote the actual observation. For tests, state both the test and the positive result precisely — "limewater turns milky", "white precipitate", "cream precipitate" — because vague answers lose the mark.
- Use the reactivity series as a tool. Almost every "predict" question is solved by comparing positions: metal vs hydrogen, metal vs another metal, metal vs carbon.
- Justify the order of reagents. In sulfate and halide tests, explain that acid is added first to remove carbonate ions, which earns the explanation mark.
- Balance redox half equations for charge [H]. When asked, show the electrons lost in oxidation and gained in reduction, and check the charges balance.
- Compare instrumental methods fairly. State a concrete advantage (fast, sensitive, small samples, mixtures) rather than a vague "they are better".
Prepare with LearningBro
The LearningBro OCR GCSE Chemistry: Predicting and Identifying Reactions course covers every part of C4 — the reactivity series, reactions with water and acid, displacement and redox, metal extraction, and the full identification toolkit of gas tests, flame tests, precipitate tests, anion tests and instrumental methods — with worked examples and practice questions that match the OCR J248 format, plus immediate feedback on your answers.
For broader preparation across the whole specification and both papers, the OCR GCSE Chemistry Exam Prep course walks you through the paper structure, command words and answering technique. And for the wider picture of the entire subject, start with our OCR GCSE Chemistry complete revision guide.
C4 rewards practice above almost any other topic, because so much of it is recall of tests and rules that become automatic with repetition. The more tests you learn cold and the more reactivity-series predictions you work through, the more reliably C4 turns into marks on the page.
Good luck with your revision.