OCR Gateway GCSE Chemistry A (J248) · C4 — Predicting and identifying reactions and products
Mini-Lesson
Predicting & Identifying Reactions and Products
This mini-lesson covers the whole of OCR Gateway Topic C4: how the periodic table lets you predict reactions (Groups 1, 7 and 0, the reactivity series, displacement and redox), and how the chemical tests let you identify the cations, anions and gases that come out.
Work through each screen, answer the questions as you go (group trends, displacement, ion and gas tests) and collect ⭐ stars. Press Start when you're ready.
Group 1 — the alkali metals
Group 1 gets MORE reactive down
The alkali metals (Li, Na, K…) each have one electron in their outer shell. They react by losing that electron to form a 1+ ion. With water they fizz, giving an alkaline hydroxide solution plus hydrogen gas:
Down the group the outer electron is in a shell further from the nucleus, so it is lost more easily — reactivity increases.
Watch out: Group 1 and Group 7 trends run opposite ways. Metals get more reactive when they lose electrons more easily → so reactivity increases DOWN Group 1.
Quick check
Predict a Group 1 trend
?Rubidium sits below potassium in Group 1. Compared with potassium, how should rubidium react with water?
Group 7 — the halogens
Group 7 gets LESS reactive down
The halogens (F, Cl, Br, I) each have seven outer electrons. They react by gaining one electron to form a 1− ion. They are non-metals that get darker and denser down the group:
Down the group the outer shell is further from the nucleus, so the incoming electron is harder to attract — reactivity decreases down Group 7.
Watch out: halogens are oxidising agents — they take electrons. The most reactive (top of the group) is the strongest oxidiser. Fluorine > chlorine > bromine > iodine.
Halogen displacement & redox
A more reactive halogen displaces a less reactive one
Add chlorine to a colourless solution of potassium bromide. Chlorine is more reactive than bromine, so it displaces it — the solution turns orange as bromine is set free:
Redox: chlorine is reduced (gains electrons → Cl⁻); bromide is oxidised (loses electrons → Br₂). Iodine cannot displace bromine or chlorine — it is lower in the group.
Watch out — the displacement rule: a reaction only happens when the element you add is more reactive than the one already in the compound. A less reactive halogen (or metal) added to a more reactive one's salt gives no reaction.
Quick check
Will it displace?
?Bromine water is added to a colourless solution of potassium iodide (KI). What happens?
Group 0 — the noble gases
Group 0 barely reacts at all
The noble gases (He, Ne, Ar…) are unreactive (inert). Their outer shells are full — helium has 2, the rest have 8 — so they have no tendency to gain or lose electrons. They exist as single atoms.
Boiling points increase down Group 0 (heavier atoms), but reactivity stays essentially zero.
Watch out: noble gases do not form ions or molecules under normal conditions. Their lack of reaction is the whole point — it is why argon fills light bulbs and helium fills balloons.
The reactivity series of metals
Ranking metals by how readily they form ions
The reactivity of a metal depends on its tendency to lose electrons and form a positive ion. The more easily it does this, the more reactive it is. We test this with water, dilute acid and displacement:
A metal higher up will displace a metal lower down from its salt solution — e.g. magnesium displaces copper from copper sulfate.
Deducing the order: if metal A displaces metal B from B's salt, A is more reactive than B. The bigger the fizz with acid, the higher up the series.
Quick check
Deduce the order
?An iron nail placed in blue copper(II) sulfate solution becomes coated with copper and the blue fades. What does this tell you?
Oxidation & reduction (redox)
Redox is electron transfer
Whenever a metal or halogen reacts, electrons move. Remember OIL RIG:
OIL RIGOxidation Is Loss of electrons · Reduction Is Gain of electrons
Magnesium loses electrons (oxidised); the copper ion gains them (reduced). Both happen together — that's a redox reaction.
Watch out: the more reactive species is the one that gets oxidised (it loses electrons most readily). In halogen displacement the halide ion is oxidised and the added halogen is reduced.
Quick check
Oxidised or reduced?
?In the reaction Zn + Cu²⁺ → Zn²⁺ + Cu, what happens to the zinc atom?
C4.2 — tests for gases
Four gas tests to learn cold
These four turn up constantly. Memorise the test and the positive result:
Watch out: a glowing splint relights for oxygen; a lighted splint pops for hydrogen — don't swap them. Chlorine's bleaching is what distinguishes it from an ordinary acid gas.
Identify the gas
Name the gas from its test
Tap the gas that gives each result.
C4.2 — flame tests for metal cations
Five flame colours
Dip a clean, wet nichrome wire in the sample, then hold it in a roaring Bunsen flame. The metal ion gives a characteristic colour:
Li = crimson · Na = yellow · K = lilac · Ca = orange-red · Cu = green-blue.
Watch out: the sodium yellow is very strong and can mask others — that's a known limitation. Use a clean wire each time so colours don't carry over.
Match it
Flame colour → metal ion
Tap a flame colour, then tap the metal ion that causes it.
C4.2 — cation tests with sodium hydroxide
Coloured hydroxide precipitates
Add a few drops of sodium hydroxide solution to an unknown solution. Many metal ions form an insoluble metal hydroxide with a tell-tale colour:
Ca²⁺ & Zn²⁺ = white · Fe²⁺ = green · Fe³⁺ = orange-brown · Cu²⁺ = blue. Zinc hydroxide redissolves in excess NaOH.
Watch out: don't confuse the two irons — Fe²⁺ is green, Fe³⁺ is orange-brown. Calcium and zinc are both white, so use a flame test (Ca = orange-red) or excess NaOH (Zn redissolves) to tell them apart.
Identify the cation
Read the precipitate
?A few drops of sodium hydroxide are added to an unknown solution and an orange-brown precipitate forms. Which ion is present?
C4.2 — tests for anions
Carbonate, halide and sulfate
Three negative ions, three reagents. The order of adding reagents matters:
Watch out: always add the acid first — nitric acid before silver nitrate, hydrochloric acid before barium chloride. The acid removes carbonate ions that would otherwise give a false white precipitate.
Identify the anion
Which anion is it?
?A solution is acidified with dilute hydrochloric acid, then barium chloride is added and a white precipitate forms. Which ion is present?
Test sort
Cation test or anion test?
Tap a reagent/test, then drop it in the right box.
➕ Tests a CATION
➖ Tests an ANION
C4.2 — instrumental analysis
Flame emission spectroscopy
Chemists increasingly use machines instead of test-tube tests. Flame emission spectroscopy is an instrumental method for identifying and measuring metal ions in a solution:
The sample is put into a flame; the light given off is passed to a spectrometer.
It produces a line spectrum — each metal ion has its own pattern of lines (its "fingerprint").
The position of the lines identifies the ion; the intensity shows the concentration.
Each metal ion has a unique line spectrum — instruments can detect even a tiny mixture.
Advantages of instrumental methods: they are faster, more accurate and more sensitive (detect tiny amounts) than test-tube tests — and they can analyse very small samples or complex mixtures.
Quick check
Why use a machine?
?Give the main advantage of using flame emission spectroscopy instead of a simple flame test to find which metal ions are in a sample.
Recap
The C4 facts to know
Group 1: 1+ ions, react with water → alkali + H₂; reactivity increases down.
Group 7: 1− ions; reactivity decreases down; a more reactive halogen displaces a less reactive one.
Group 0: full shells → inert.
Reactivity series: tendency to form a positive ion; more reactive metal displaces less reactive from its salt.
Redox: OIL RIG — oxidation is loss, reduction is gain of electrons.