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You have now worked through the whole of Topic C4 of OCR Gateway Science A — the reactivity series, reactions of metals with water and acid, displacement and redox, the extraction of metals, predicting products, and the full set of tests for gases, cations and anions including instrumental methods. This final lesson pulls it all together. It shows how the two halves of C4 — predicting reactions and identifying products — connect, gathers the qualitative-analysis test scheme in one place, drills the exam technique for writing test methods and predictions, collects the misconceptions that catch students out, and finishes with a synoptic model answer. Treat it as a revision and exam-technique session rather than new content.
By the end of this lesson you should be able to recall the C4 reasoning for predicting reactions and extraction, carry out a complete qualitative-analysis scheme to identify an unknown salt, write test methods in the correct form, and structure a top-band C4 answer.
Topic C4 is one connected story built on a single tool — the reactivity series — and a single skill — identifying ions by their reactions. One half lets you predict what will react and what is made; the other half lets you identify what is there from observations.
flowchart TD
A["Topic C4"] --> B["PREDICTING reactions"]
A --> C["IDENTIFYING products"]
B --> D["Reactivity series<br/>(metals + water / acid)"]
D --> E["Displacement<br/>(= redox)"]
D --> F["Extraction<br/>(carbon vs electrolysis)"]
B --> G["Predicting products<br/>+ balanced equations"]
C --> H["Gas tests<br/>(H₂, O₂, CO₂, Cl₂)"]
C --> I["Cation tests<br/>(flame + hydroxide)"]
C --> J["Anion tests<br/>(carbonate, sulfate, halide)"]
C --> K["Instrumental methods<br/>(flame emission spectroscopy)"]
H -.->|"identify a fully unknown salt"| I
I -.-> J
Notice the link at the bottom: identifying an unknown salt means combining a gas/cation test with an anion test. And the reactivity series underpins the whole predicting half — it decides displacement, extraction and whether a reaction happens at all. Seeing these connections is exactly the synoptic thinking that lifts an answer.
The heart of the "identifying" half of C4 is a set of tests. Here they are all in one place — the scheme you use to identify an unknown ionic compound.
| Gas | Test | Positive result |
|---|---|---|
| Hydrogen | Lighted splint | Squeaky pop |
| Oxygen | Glowing splint | Relights |
| Carbon dioxide | Bubble through limewater | Milky |
| Chlorine | Damp blue litmus | Bleached white |
| Ion | Flame colour | Hydroxide (NaOH) precipitate |
|---|---|---|
| Lithium, Li⁺ | Crimson | — |
| Sodium, Na⁺ | Yellow | — |
| Potassium, K⁺ | Lilac | — |
| Calcium, Ca²⁺ | Orange-red | White (does not redissolve) |
| Copper(II), Cu²⁺ | Green | Blue |
| Iron(II), Fe²⁺ | — | Green |
| Iron(III), Fe³⁺ | — | Brown |
| Aluminium, Al³⁺ | — | White (redissolves in excess) |
| Magnesium, Mg²⁺ | — | White (does not redissolve) |
| Anion | Test | Positive result |
|---|---|---|
| Carbonate, CO₃²⁻ | Add dilute acid | Fizzes (CO₂, limewater milky) |
| Sulfate, SO₄²⁻ | HCl then barium chloride | White precipitate |
| Chloride, Cl⁻ | Nitric acid then silver nitrate | White precipitate |
| Bromide, Br⁻ | Nitric acid then silver nitrate | Cream precipitate |
| Iodide, I⁻ | Nitric acid then silver nitrate | Yellow precipitate |
Exam Tip: To identify a salt, always do both a cation test and an anion test. Add the acid first in the sulfate and halide tests to remove carbonate (use hydrochloric for sulfate, nitric for halides).
Several reasoning patterns recur in C4. Here is each one again, with a fresh worked example so you can check your method.
A metal reacts with dilute acid only if it is above hydrogen; it displaces another metal only if it is more reactive.
Worked example: Will silver react with dilute hydrochloric acid? Silver is below hydrogen, so no reaction.
Identify the reaction family, name the salt from the acid, then balance.
Worked example: Predict the products of magnesium + sulfuric acid and balance. Metal + acid → salt + hydrogen; sulfuric → sulfate: Mg+H2SO4→MgSO4+H2.
Compare the metal with carbon: below carbon → reduce with carbon; above carbon → electrolysis.
Worked example: How is calcium extracted? Calcium is above carbon, so by electrolysis of its molten compound (carbon cannot reduce it).
Match the observation to the test.
Worked example: A solution gives a green precipitate with NaOH. Which ion? Green hydroxide precipitate = iron(II), Fe²⁺.
Exam Tip: For predicting, the reactivity series answers nearly everything — above hydrogen (acids), more reactive (displacement), relative to carbon (extraction). For identifying, match the observation to its test.
Examiners often ask you to describe a test. A reliable structure earns full marks every time: add the reagent → state the observation → give the conclusion.
| Step | What to write | Example (testing for a sulfate) |
|---|---|---|
| Add reagent | Name the reagent(s) and any acid added first | "Add dilute hydrochloric acid, then barium chloride solution" |
| Observe result | State the specific positive observation | "A white precipitate forms" |
| Conclude | Say what the result means | "This shows a sulfate is present" |
Applying this to a gas test: "Hold a lighted splint at the mouth of the tube (reagent/method); a squeaky pop is heard (observation); this shows the gas is hydrogen (conclusion)."
Exam Tip: Structure every "describe a test" answer as reagent → observation → conclusion. A vague "test it with chemicals" scores nothing; naming the reagent, the exact result and the conclusion scores full marks.
OCR uses specific command words that tell you exactly what kind of answer to give.
| Command word | What it asks for |
|---|---|
| Predict | Use the rules (reactivity series) to say what will happen or be made |
| Describe | Say what happens or what you do (e.g. describe a test or an observation) |
| Explain | Give reasons why — use "because", "so that" (e.g. why a metal does not react) |
| Identify / Name / State | A short answer — name the ion, gas or product |
| Compare | Give similarities and/or differences (e.g. chemical tests vs instrumental methods) |
Exam Tip: The difference between describe and explain decides many marks. "A white precipitate forms" describes; "...because insoluble barium sulfate is produced" explains. If the command word is explain, give the reason.
Many C4 questions give a table of test results for one or more unknowns. A reliable routine is: (1) read each row as test → result; (2) match each result to the ion it indicates; (3) for a single unknown, combine the cation and anion clues to name the compound; (4) watch for the white-precipitate trap — use the excess-NaOH behaviour and the silver-halide colours to separate similar ions. Working row by row turns a daunting table into a simple identification.
Use this as a final recall list. Cover the right-hand column and test yourself.
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