Group 1: the alkali metals

Lithium, sodium and potassium are a family because they react with water in the same way, each forming a hydroxide (an alkali) and hydrogen gas:

The resulting solution is alkaline (turns universal indicator purple) because a metal hydroxide forms. We can use this family trend to predict the properties of metals further down, such as rubidium (even more violently reactive).

Why more reactive down the group (2.4C): each atom down the group has an extra shell, so the outer electron is further from the nucleus and more shielded. It is lost more easily, so the metal reacts faster.

Rubidium below potassium

Why are Group 1 metals not equally reactive?

Group 7: the halogens

Chlorine, bromine and iodine are non-metals that get darker and change state as you go down the group:

Reactivity decreases down Group 7 — the opposite of Group 1. A more reactive halogen displaces a less reactive one from a solution of its salt (a displacement reaction):

Why less reactive down the group (2.8C): a halogen atom gains one electron to react. Down the group the outer shell is further out and more shielded, so it attracts an electron less strongly — reactivity falls.

Halogen displacement

What's in clean, dry air?

You need the approximate composition of the four most abundant gases in dry air:

• % oxygen experiment (2.10): pass air over heated copper, leave damp iron wool to rust in trapped air, or burn phosphorus — the oxygen is used up and the trapped gas volume falls by about a fifth.
• Combustion in oxygen (2.11): magnesium burns with a bright white flame → MgO; hydrogen → H₂O; sulfur burns with a blue flame → SO₂.
• CO₂ from carbonates (2.12): heating copper(II) carbonate decomposes it: CuCO₃ → CuO + CO₂.
• Greenhouse gas (2.13): carbon dioxide is a greenhouse gas; rising amounts may contribute to climate change.

Spec watch: for 4CH1 Section 2 the greenhouse gas you must name is CO₂. (Acid rain and SO₂/NOₓ belong to Section 4, not here.)

Burning in air

Why does carbon work for iron but not aluminium?

The reactivity series

Metals are ranked by how readily they react with water and dilute acid, and by their displacement reactions. Carbon and hydrogen (non-metals) are slotted in as reference points for extraction:

• With water: only the most reactive metals (K, Na, Li, Ca) react readily with cold water.
• With dilute acid: metals above hydrogen fizz, giving a salt + hydrogen; copper, silver and gold do not.
• Displacement: a more reactive metal displaces a less reactive one from its compound (e.g. Mg + CuSO₄ → MgSO₄ + Cu).

Metal displacement

Rusting of iron & its prevention

Iron rusts only when both water and oxygen are present (salt speeds it up). Rusting is an oxidation — the iron gains oxygen. We prevent it by keeping water and oxygen out, or by using a more reactive metal:

Redox in a line: OIL RIG — Oxidation Is Loss, Reduction Is Gain (of electrons). The thing oxidised is the reducing agent; the thing reduced is the oxidising agent.

How position decides the method

Most metals are dug up as ores (compounds). Unreactive metals like gold are found native (uncombined). The method of extraction depends on reactivity (2.23C):

The 4CH1 spec uses iron (carbon reduction) and aluminium (electrolysis) to illustrate this link — detailed process steps are not required, but you should comment on a process when given information.

Uses (2.25C): aluminium — low density, corrosion-resistant; copper — good conductor; iron/steel — strong and cheap. An alloy (e.g. steel) is harder than a pure metal because different-sized atoms disrupt the layers so they can't slide over each other as easily.

How would you extract it?

Why does carbon work for iron but not aluminium?

Why do only some acid reactions fizz?

The pH scale & indicators

Acids release H⁺ ions in water; alkalis release OH⁻ ions. The pH scale (0–14) classifies how acidic or alkaline a solution is:

Universal indicator gives an approximate pH from its colour (red→purple). When an alkali neutralises an acid, H⁺ + OH⁻ → H₂O and the pH moves toward 7.

Titration (2.33C): add acid from a burette to a measured volume of alkali containing an indicator, swirling until the colour just changes — that is the volume needed to neutralise it exactly.

Reading an indicator

Acids make salts — and the acid names them

Acids react with metals, bases (metal oxides/hydroxides) and carbonates to make salts. The acid decides the salt's surname:

• acid + metal → salt + hydrogen  (e.g. Mg + H₂SO₄ → MgSO₄ + H₂)
• acid + base → salt + water  (e.g. CuO + H₂SO₄ → CuSO₄ + H₂O)
• acid + carbonate → salt + water + carbon dioxide  (e.g. CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂)

Watch out: the metal/base supplies the first word of the salt; the acid supplies the second. Reactions of nitric acid with metals are excluded from this spec (they do not simply give salt + hydrogen) — use a base or carbonate.

What salt forms?

Why do only some acid reactions fizz?

Making salts: soluble vs insoluble

Which method you use depends on whether the salt is soluble or insoluble — so you need the solubility rules:

• Common sodium, potassium & ammonium salts are soluble; all nitrates are soluble.
• Common chlorides soluble — except silver and lead(II).
• Common sulfates soluble — except barium, calcium and lead(II).
• Common carbonates & hydroxides insoluble — except Na/K/(NH₄), and Na/K/Ca hydroxide.

Practical (2.42): make hydrated copper(II) sulfate crystals from copper(II) oxide + warm sulfuric acid (add CuO until excess, filter off the unreacted excess, evaporate, leave to crystallise).

Soluble or insoluble?

How can a lab name a mystery powder?

Flame tests & gas tests

Dip a clean wire in the sample and hold it in a blue Bunsen flame — the cation colours you must know:

Gas tests (2.44):

• Hydrogen: a lit splint gives a squeaky pop.
• Oxygen: relights a glowing splint.
• Carbon dioxide: turns limewater milky/cloudy.
• Ammonia: turns damp red litmus blue (it's alkaline).
• Chlorine: bleaches damp litmus paper (white).

Match the test to the result

Tap a test on the left, then its correct result on the right.

Precipitates & anion tests

Add sodium hydroxide solution to identify these cations by the colour of the hydroxide precipitate:

Anion tests (2.48):

• Carbonate (CO₃²⁻): add dilute acid → fizzes, CO₂ turns limewater milky.
• Sulfate (SO₄²⁻): add dilute HCl then barium chloride → white precipitate.
• Halides (Cl⁻/Br⁻/I⁻): add dilute nitric acid then silver nitrate → Cl⁻ white, Br⁻ cream, I⁻ yellow precipitate.

Watch out: the colours are easy to mix up. Hydroxide precipitates: Cu blue, Fe(II) green, Fe(III) brown. Silver halides: chloride white, bromide cream, iodide yellow — they get darker just like the elements do.

Which ion is it?

How can a lab name a mystery powder?

More reactive than carbon?

Tap a metal, then tap the box for whether it is above or below carbon — this decides its extraction method.

Testing for an anion

Section 2 at a glance

You've covered the key ideas across the eight sub-topics of Edexcel iGCSE 4CH1 Section 2. Press Finish to see your score.