Properties of Ionic Compounds and Small Molecules — AQA GCSE Chemistry

SPEC 4.2.2.3 & 4.2.2.4 • PAPER 1

Properties of Ionic Compounds & Small Molecules

Two structure types. Eight properties to memorise. And the most-failed precision question in Paper 1: why ions — not electrons — carry charge in molten sodium chloride.

Why This Lesson Matters

This lesson is one of the highest-yield in Paper 1 — explain-the-property questions appear nearly every year, and the wording must be exact.

EXAMINER VERDICT

P1 Jun24 Q08.1: "Only one in seven scored all three" marks on the propane low boiling point question — one of the most-failed three-markers in recent papers.

WHAT GOES WRONG

Students know the rough idea (weak forces, low boiling point) but lose marks on three precision points: (1) failing to name the structure type, (2) saying bonds break instead of intermolecular forces, and (3) claiming electrons (not ions) carry charge in molten ionic compounds.

By the end of this lesson, you will state every property with the exact mark-scheme phrasing.

Key Definitions

Four terms underpin every answer in this lesson. Learn them word for word.

GIANT IONIC LATTICE

A 3D regular arrangement of positive and negative ions held by strong electrostatic forces in all directions.

SIMPLE MOLECULAR SUBSTANCE

A substance made of small molecules held together by weak intermolecular forces.

INTERMOLECULAR FORCE

A weak attraction between neighbouring molecules — not within a molecule.

ELECTROSTATIC ATTRACTION

The force of attraction between oppositely charged particles (e.g. between positive and negative ions).

Ionic Compounds — 4 Key Properties

Every ionic compound (NaCl, MgO, CaCl₂, KBr…) has a giant ionic lattice structure. Its properties follow from two facts: forces between ions are strong, and the lattice is built from charged particles.

1High melting and boiling points

2Solid at room temperature

3Conduct when molten or dissolved

4Often soluble in water

Why High Melting Points?

This is the mark-scheme answer for the classic AQA question: "Explain why sodium oxide has a high melting point" (P1 Nov21 Q04.6, 3 marks).

1Giant ionic lattice — a regular structure of oppositely charged ions

2Strong electrostatic forces of attraction between oppositely charged ions

3Large amounts of energy needed to break the forces

VERBATIM MARK SCHEME (P1 NOV21 Q04.6)

Mark 1: "giant structure" (allow "giant ionic lattice")

Mark 2: "strong (electrostatic) forces of attraction between (oppositely charged) ions"

Mark 3: "large amounts of energy needed to break the bonds / forces" (allow "separate the ions")

PRECISION CHECK

The forces act in all directions across the lattice (spec language). Use "break the forces" or "break the bonds" — both are accepted; "separate the ions" also scores.

Conductivity — The Big Rule

This is the most-tested ionic property. Memorise the exact wording for both states.

SOLID — DOES NOT CONDUCT

Ions are in fixed positions in the lattice and cannot move. No moving charge → no current.

MOLTEN / DISSOLVED — CONDUCTS

Ions are free to move and carry charge to the electrodes.

Examiner Alert — Ions, Not Electrons

This is the most repeated precision error on ionic-conductivity questions in Paper 1.

EXAMINER ALERT — P1 Jun 2022 Q06.3

MCQ asked: "Which type of particle passes through the mesh in the electrolysis of molten sodium chloride?" Only 60% chose "ion". "Nearly all other responses were 'electrons'. Many students seem to think that an electric current can only be conducted by electrons moving, even through a solution in which there are no free electrons."

CORRECT ANSWER

Solid: the ions are in fixed positions in the lattice and cannot move.

Molten or dissolved: the ions are free to move and carry charge.

DON'T CONFUSE WITH METALS

Metals conduct because of delocalised electrons. Ionic compounds conduct because of moving ions. Different mechanism, different particle. Never use "electrons" to explain ionic conductivity.

Brittle & Soluble in Water

Two further properties — both follow from the regular arrangement of charged ions.

BRITTLE

If the layers of an ionic crystal are pushed sideways, like-charged ions line up with like-charged ions. They repel, and the crystal shatters along the layer.

OFTEN SOLUBLE IN WATER

Water molecules can surround the ions and pull them out of the lattice — separating positive from negative — so many ionic compounds dissolve readily.

Simple Molecules — 4 Key Properties

Examples: H₂O, CO₂, CH₄, O₂, NH₃, propane (C₃H₈). Each is a small molecule made of atoms held by strong covalent bonds — but the molecules themselves are only weakly attracted to each other.

1Low melting and boiling points

2Often liquid or gas at room temperature

3Do not conduct electricity

4Bigger molecules → higher melting point

The Central Distinction — Bonds vs Forces

This is the single most-tested precision point in simple molecular substances. There are two types of interaction in any sample of small molecules — and only one is overcome during melting and boiling.

REMEMBER THIS

Within the molecule → covalent bonds → STRONG

Between molecules → intermolecular forces → WEAK

When a simple molecular substance melts or boils, it is the intermolecular forces that are overcome. The covalent bonds remain intact.

CONCRETE EXAMPLE

Steam (H₂O gas) still contains H₂O molecules. Boiling water did not break the O–H covalent bonds. What was overcome was the attraction between separate water molecules.

Common Error

Writing "covalent bonds are weak" or "little energy needed to break the bonds." Both are wrong. Covalent bonds are strong, and they are not what is broken.

Why Low Melting Points?

The mechanism behind every low-bp explanation. Three steps — and you must include all three to score full marks.

1Substance consists of small molecules

2Weak intermolecular forces between the molecules

3Little energy needed to overcome them

MARK BREAKDOWN (3-MARK QUESTIONS)

One mark per stage. Examiners look for the structural type (small molecules), the location and weakness of the force (intermolecular, weak), and the energy consequence (little energy / easily overcome). Drop any one and you drop a mark.

EXAM-WORDING CHECKLIST

"Small molecules" — names the structure type

"Weak intermolecular forces" — names what's weak and where it acts

"Little energy needed to overcome" — never "break bonds"

Bigger Molecules — Higher Boiling Points

Within a related group of substances, larger molecules have stronger intermolecular forces — so more energy is needed to overcome them, and the boiling point is higher.

MoleculeFormulaSizeBoiling point (°C)

MethaneCH₄smallest−162

EthaneC₂H₆↓−89

PropaneC₃H₈↓−42

ButaneC₄H₁₀largest−1

REASONING

Larger molecules have more points of contact with neighbouring molecules. More contact means stronger total intermolecular attraction. So more energy is needed to separate them — boiling point rises.

PRECISION CHECK

The covalent bonds in butane are not stronger than those in methane. What's stronger is the intermolecular forces between the molecules.

PEE Framework — Propane Low Boiling Point

The 3-mark question that "only one in seven" students scored full marks on (P1 Jun24 Q08.1). Here is the model answer using the PEE framework.

Q: Explain why propane (C₃H₈) has a low boiling point. 3 MARKS

P

Propane has a low boiling point because it consists of small molecules with weak intermolecular forces between them.

E

The atoms within each propane molecule are held by strong covalent bonds, but the forces between separate propane molecules — the intermolecular forces — are weak.

E

Only a small amount of energy is needed to overcome these weak intermolecular forces and separate the molecules into a gas. The covalent bonds within each molecule are not broken when propane boils.

MARKING POINTS

Mark 1: propane consists of small molecules

Mark 2: intermolecular forces (between the molecules) are weak

Mark 3: little energy needed to overcome the intermolecular forces

Examiner Alert — Name the Structure

The most common reason students lost marks on the propane question was missing the structural type. Most went straight to "weak intermolecular forces" without saying what propane is structurally.

EXAMINER ALERT — P1 Jun 2024 Q08.1

"Only one in seven scored all three" marks. Few students recognised that it is the structure of propane as small molecules that is important — most did not mention the structure at all. State the structural type before describing the forces.

STATE THE STRUCTURE FIRST

Open every "low boiling point" answer with: "<substance> consists of small molecules…" Only then describe the intermolecular forces. Without naming the structure, the rest of your reasoning has no anchor.

SAFE OPENERS

"Propane consists of small molecules…"

"Methane is a simple molecular substance made of small molecules…"

"Carbon dioxide exists as small molecules…"

Examiner Alert — Language Traps

Two further phrases the examiner has flagged as wrong every year. Saying any of these will cancel the mark.

EXAMINER ALERT — P1 Jun 2024 Q08.1

From the examiner report: "Some, who correctly referred to intermolecular forces being weak, then described little energy being needed to break bonds rather than to break the intermolecular forces." Covalent bonds are strong and are not what is broken on melting. Always say "intermolecular forces" — never "bonds".

EXAMINER ALERT — P1 Jun 2024 Q08.1

From the same question: "many students used the term intermolecular forces, [but] it was clear that many did not know what this phrase means as they referred to 'intermolecular forces between atoms' or 'between bonds.'" These students lost the marks for "weak forces" and "little energy" because of this language slip alone.

CORRECT LANGUAGE

✓ Intermolecular forces between molecules are weak.

✓ Little energy needed to overcome the intermolecular forces.

✗ Bonds are weak.

Side-by-Side Comparison

One table that locks in the contrast between ionic and simple molecular structures.

PropertyIonic compound (e.g. NaCl)Simple molecular (e.g. propane)

StructureGiant ionic lattice — ions in 3D regular arrangementSmall molecules with weak forces between them

Melting / boiling pointHighLow

State at room temperatureSolidOften liquid or gas

What is overcome on meltingStrong electrostatic forces between ions (in all directions)Weak intermolecular forces between molecules — covalent bonds stay intact

Conducts as a solid?No — ions in fixed positionsNo — molecules do not have an overall electric charge

Conducts when molten / dissolved?Yes — ions are free to moveNo — molecules do not have an overall electric charge

Charge carrierIons (never electrons)None

Practice Question

Sodium chloride conducts electricity when molten but propane (C₃H₈) does not conduct electricity at all.

Explain these observations.4 MARKS

P

Sodium chloride conducts when molten because its ions are free to move and carry charge.

E

In solid NaCl the ions are in fixed positions in the giant lattice; on melting, the ions become free to move, so charge can flow.

P

Propane does not conduct because it consists of small molecules that do not have an overall electric charge.

E

There are no free charged particles — no ions, no delocalised electrons — so there is nothing to carry charge in any state.

MARK ALLOCATION

2 marks for NaCl conduction (ions free to move; from the lattice on melting). 2 marks for propane non-conduction (small molecules; no free charged particles). Watch out for the trap: never say "electrons" for ionic conductivity.

Summary

The Two Structures, Two Mechanisms

Ionic compounds — giant lattice + strong electrostatic forces in all directions → high mp/bp, solid, brittle, often soluble. Conduct when molten or dissolved (ions free to move). Never electrons — always ions.

Simple molecular substances — small molecules + weak intermolecular forces between them → low mp/bp, often gas/liquid. Don't conduct (neutral molecules). Covalent bonds within each molecule stay intact during melting and boiling.

3-MARK PROPANE ANSWER

(1) small molecules; (2) weak intermolecular forces between molecules; (3) little energy needed to overcome them.

IONIC CONDUCTIVITY

Charge carrier = ions. Solid: ions fixed → no conduction. Molten / dissolved: ions free → conduction. Never write "electrons".

EXAM TIP — THE THREE LANGUAGE TRAPS

(1) Don't say "bonds break" — say "intermolecular forces are overcome". (2) Don't say "intermolecular forces between atoms" — they act between molecules. (3) Don't say "electrons carry charge" in ionic conductivity — it's ions.