GCSE · Chemistry · AQA · Spec 8462

Properties of ionic compounds

Sodium chloride is made entirely of charged particles, yet solid salt won't conduct electricity. Melt it or dissolve it, and suddenly it does. Same particles. So what changed?

Heat up a giant ionic lattice

Energy supplied by heating: 0. State: Solid: ions locked in place. static

Energy supplied by heating0

Every ion sits in a fixed spot in a regular, repeating pattern. Each one is surrounded by ions of the opposite charge, and they pull on it from every side. This model shows where the ions are and how they move; it doesn't colour the positive and negative ions differently.

Each dot is one ion. Slide the energy up and watch what the ions do. The slider is a relative scale showing how much energy has gone in, not a real measurement.

Melting and boiling points

Reason it through

Why do ionic compounds have high melting points and high boiling points?

Link 1 of 4

First link · your turn

Start with the structure. How are the particles arranged?

Locked — reveal the link above first

What carries the charge?

Why does the melt conduct?

Molten sodium chloride conducts electricity. Solid sodium chloride does not.

Which of these is closest to what you think explains it right now?

In the melt the ions can move around, so they can carry charge from one place to another.Melting turns it into ions. The solid didn't have any ions until then.Melting frees electrons, and they flow through the liquid like they do in a metal wire.

How sure are you?

I'm sureI think soI'm guessing

In the melt the ions can move around, so they can carry charge from one place to another.

This matches the evidence

Spot on. The charge is carried by the ions themselves, moving through the liquid. In the solid, the same ions are locked in the lattice, so nothing can carry the charge anywhere.

Melting turns it into ions. The solid didn't have any ions until then.

This is a common misconception

The ions were there all along. A solid ionic compound is a lattice made of ions. Melting doesn't create them; it frees them. The solid fails to conduct because its ions can't move, not because it has none.

Melting frees electrons, and they flow through the liquid like they do in a metal wire.

This is a common misconception

That's a very natural guess, because current in a wire is a flow of electrons. But in a molten ionic compound there are no free electrons drifting about. The charged particles that move are the ions, and they carry the charge with them.

In an ionic compound, charge flows only when the ions themselves are free to move.

Solid, molten, dissolved

Build the conduction grid

All three samples contain exactly the same ions. For each one, tick every statement that is true, then check your grid.

Solid ionic compound

Ions held in fixed positionsIons free to moveConducts electricity

Molten ionic compound

Ions held in fixed positionsIons free to moveConducts electricity

Ionic compound dissolved in water

Ions held in fixed positionsIons free to moveConducts electricity

Put it in writing

Your turn: the full explanation

Sodium chloride is an ionic compound. Explain why sodium chloride has a high melting point, and why it conducts electricity when molten but not when solid. [5 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Sodium chloride has a giant lattice structure made of (oppositely charged) ions. (1 mark)
There are strong electrostatic forces of attraction between oppositely charged ions, acting in all directions. (1 mark)
A large amount of energy is needed to break the many strong bonds, so the melting point is high. (1 mark)
In the solid, the ions are held in fixed positions and cannot move, so it does not conduct. (1 mark)
When molten, the ions are free to move, so charge can flow. (1 mark)

Model answer

Sodium chloride has a giant lattice of positive and negative ions. Strong electrostatic forces attract the oppositely charged ions to each other in all directions. It takes a large amount of energy to break these many strong bonds, so sodium chloride has a high melting point. In the solid, the ions are held in fixed positions and cannot move, so no charge can flow. When it is molten, the ions are free to move and can carry charge, so it conducts electricity.

The words that earn credit are 'ions', 'giant lattice', 'strong electrostatic forces in all directions', 'a large amount of energy' and 'free to move'. Writing 'molecules', 'intermolecular forces' or 'electrons' gets no credit here.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

Ionic compounds have a giant ionic lattice: a regular structure of oppositely charged ions.
Strong electrostatic forces of attraction act in all directions between the oppositely charged ions.
Ionic compounds have high melting points and high boiling points, because a large amount of energy is needed to break the many strong bonds.
Ionic compounds conduct electricity when melted or dissolved in water, because the ions are free to move and so charge can flow.
Solid ionic compounds do not conduct, because their ions are held in fixed positions.

The big picture

Ionic compounds are giant lattices of oppositely charged ions, held together by strong electrostatic forces acting in all directions. Breaking so many strong bonds takes a large amount of energy, so melting and boiling points are high. The solid doesn't conduct because its ions are stuck in place. When the compound is melted or dissolved in water, the ions are free to move and can carry charge, so it conducts.

Key points

1Structure first: a giant lattice of oppositely charged ions, pulled together from every direction.

2Many strong bonds → a large amount of energy to break them → high melting and boiling points.

3Solid: ions stuck in place, so no conduction.

4Molten or dissolved: ions free to move, so charge can flow.

5The charge is carried by moving ions, not electrons, and there are no molecules in an ionic lattice.

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Worked example

Problem

A student wrote: "Sodium chloride has a high melting point because it has strong intermolecular forces between its molecules." Rewrite this into an answer that would earn full credit.

⚠ Watch out

Saying a solid ionic compound doesn't conduct because it has no ions or no charged particles. The ions are there in the solid, held in the lattice. The solid doesn't conduct because those ions can't move.

🧠

Memory hook

Locked ions, no current. Free the ions by melting or dissolving, and charge can flow.

✓

Check yourself

Sodium chloride conducts when dissolved in water, with no heating at all. Explain why in one sentence. (Dissolving breaks up the lattice, so the ions are free to move and charge can flow.)

Flashcards

(8)

What kind of structure does an ionic compound have?

A giant ionic lattice: a regular, repeating arrangement of oppositely charged ions.

What holds the ions together in an ionic lattice?

Strong electrostatic forces of attraction between oppositely charged ions, acting in all directions.

Why do ionic compounds have high melting and boiling points?

A large amount of energy is needed to break the many strong bonds in the giant lattice.

Under what two conditions do ionic compounds conduct electricity?

When they are melted (molten) or dissolved in water.

Why can a molten ionic compound conduct electricity?

Its ions are free to move, so charge can flow.

What happens to the ions in a solid ionic compound as it is heated but before it melts?

They vibrate more, but they stay in fixed positions in the lattice.

What carries the charge through a molten ionic compound: ions or electrons?

Ions. They move through the liquid and carry the charge with them.

Why is 'intermolecular forces' the wrong phrase for an ionic compound?

There are no molecules. The forces are electrostatic attractions between ions throughout a giant lattice.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Heat up a giant ionic lattice

Why does the melt conduct?

Build the conduction grid

Your turn: the full explanation

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