AQA Chemistry 5.2.2.3 · Contrast
Properties of ionic compounds
Salt is made entirely of charged particles, yet a salt crystal will not light a bulb. Melt it, or dissolve it, and it will. What changed?
Combined Science · Foundation tier
Start here
Two probes, one bulb, a pile of salt crystals.
Push two carbon probes into a heap of dry salt crystals and connect them to a battery and a bulb. The bulb stays dark. Every crystal is packed with Na⁺ and Cl⁻ ions: billions of charged particles, right between the probes.
Commit first
Why does no current flow through the crystals?
A current is a flow of charge, so it needs charged particles that can move. Having charged particles is not enough. In solid sodium chloride every ion is held in the lattice by strong electrostatic forces from all its neighbours. The ions vibrate but stay put. That leaves one question to test: what happens when something sets them free? Compare the solid with each of the other forms, one at a time, and call the bulb first.
Side by side · solid against the rest
Same compound. Change one thing. Watch the ions.
0 of 2 compared
Commit first. The salt is heated until it melts. Will the bulb light?
Think again
“The molten salt conducts because electrons flow through it, like in a metal wire.”
Spot the flaw
ExplainLook at the right-hand beaker again. What is wrong with that sentence?
Linked comparison
Solid does not conduct; molten does. Build the reason.
An examiner wants each state described, then the difference linked to the ions. Put four links in order and leave the two false ones out.
4 links belong. 2 do not.
The links
Your chain · 0 of 4
Be the examiner · 3 marks
Mark this answer.
Question. Explain why sodium chloride conducts electricity when dissolved in water but not when solid. [3 marks]
Your mark
How many of the 3 marks does it earn?
Command words in this lesson
- Explain
- Structure first, then the property, joined with because.
- Compare
- Say what happens in each case and how they differ, in the same answer.
- Predict
- Give a plausible outcome. Add the reason from the structure when the question also says explain.
Key fact
Ionic compounds conduct only when molten or dissolved, because only then are the ions free to move and carry charge. It is the ions that move, never electrons.
Examiner tip
Key note · AQA 5.2.2.3
Properties of ionic compounds
- Ionic compounds are giant lattices of oppositely charged ions with strong electrostatic forces in all directions.
- High melting and boiling points: a lot of energy is needed to overcome the many strong forces.
- Solid: does not conduct. The ions are in fixed positions.
- Molten or dissolved in water: conducts. The ions are free to move and carry charge.
- It is the ions that move, never electrons.
- In an answer, say "free to move". "Has charged particles" alone scores nothing.
Photograph this card. Tomorrow, cover it and say each line first.
Previous and next
Connects to
Ask Mr Badmus AI
Still thinking electrons carry the current in salt water?
The beakers are a model. Molten sodium chloride is above 801 °C and is heated in a crucible, not a glass beaker. In solution, water molecules surround each ion; they are left out of the drawing so the ions can be seen. Ion drift is drawn far faster than it really is.