Ionic structures and properties
Use the giant lattice and ion mobility to explain high melting temperatures and the electrical conductivity of solid, molten and dissolved ionic compounds.
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Solid sodium chloride contains charged ions, yet it does not conduct electricity in a school conductivity test. Melting it changes that result without changing its formula. To explain this, connect structure, charged particles and whether those particles can move.
Start with forming ions and balancing charges if you need to practise ion formation first.
A giant lattice, rather than separate molecules
An ionic solid contains a repeating three-dimensional arrangement of positive and negative ions: a giant ionic lattice. Strong electrostatic attractions between oppositely charged ions hold it together. In sodium chloride, each ion is surrounded by oppositely charged neighbours; there are no separate NaCl molecules or exclusive sodium–chloride pairs.
NaCl states a 1:1 ion ratio. MgCl₂ states one Mg²⁺ ion for every two Cl⁻ ions. The ions keep their individual charges even though the whole compound is neutral.
The solid panel is a two-dimensional slice of a three-dimensional structure. Use it to explain arrangement and mobility; you are not required to draw an ionic lattice for this course.
Why melting needs a high temperature
Heating makes the ions vibrate more vigorously. To melt the solid, enough energy must be supplied to overcome the attractions that maintain the rigid arrangement, allowing ions to move past one another. The ions remain charged, and attractions between ions still act in the liquid.
Ionic compounds typically have high melting temperatures because the electrostatic attractions are strong. Do not explain this by saying the ions themselves melt or expand, or that covalent bonds inside NaCl molecules break. Some ionic substances decompose before boiling, so a very high boiling point is not a universal observation.
Why conductivity changes with state
Electrical conduction requires mobile charged particles. Having charges present is only the first part of the explanation.
| Sample | Charged particles | Can those particles move through the sample? | Electrical behaviour |
|---|---|---|---|
| Solid sodium chloride | Na⁺ and Cl⁻ ions | No: they vibrate about fixed lattice positions. | Does not conduct in the usual school test. |
| Molten sodium chloride | The same ions | Yes: ions move through the liquid. | Conducts. |
| Sodium chloride dissolved in water | The same ions, surrounded by water molecules | Yes: dissolved ions move through the solution. | Conducts. |
When a salt dissolves, water separates and surrounds its ions. “Aqueous” means dissolved in water; simply adding an insoluble solid to water does not make its ions mobile in solution. Many ionic compounds dissolve, but others do not: use the solubility rules when choosing an example.
Ionic compounds conduct in the molten state, or in solution if they dissolve, because ions can move and carry charge. Their conduction is not a flow of electrons through the ionic material.
Test sodium chloride yourself: switch on the circuit, heat it until it melts, cool it and add water, then strike the solid.
Sodium chloride, a giant ionic structure, at 25 °C: a solid. The circuit is off.
- State
- solid
- Melting point
- —
- Boiling point
- —
- Bulb
- switched off
- Melting overcomes
- —
Try this
0 of 4 doneMelt a giant structure and a simple molecular substance. (not done yet)
Melting a giant structure breaks strong bonds all through it; melting a simple molecular substance only overcomes weak forces between molecules, which takes far less energy.
Test sodium chloride as a solid, as a melt and in water. (not done yet)
Ions carry the current, but only when they can move: in the melt or the solution, not when held in the lattice.
Strike sodium chloride, then copper. (not done yet)
A blow pushes ions of the same charge next to each other in NaCl, so they repel and the crystal shatters. In copper the layers of ions slide in the sea of electrons, so the metal bends instead.
Test diamond and graphite for conductivity. (not done yet)
Each carbon in graphite bonds to three others, leaving one electron per atom delocalised along its layer. Diamond uses all four outer electrons in bonds, so nothing can carry a current.
Compare the explanations
Examiner practice 1
Conductivity Explanation (Solid vs Molten)
Examination question
Explain why solid sodium chloride does not conduct electricity but molten sodium chloride does. [4 marks]
Compare the charge carriers in both states
View solution step by step
Describe solid sodium chloride
2 marksMethod
State the position and mobility of its ions.Reason
The ions are held in fixed positions in the giant lattice and cannot move to carry charge.
Working
Solid NaCl does not conduct.Describe molten sodium chloride
2 marksMethod
State what melting changes.Reason
The lattice breaks down sufficiently for ions to become mobile and carry charge.
Working
Molten NaCl conducts through moving ions.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark the charge carriers and their mobility in each state.
Try these independently
Mind stretcher 1: The same compound in different statesExtension
Question: Two samples of sodium chloride have formula NaCl. One is molten and conducts electricity. The other is solid and does not. Explain why the conductivity differs, even though the formula is the same.
Show Answer
Both samples contain Na⁺ and Cl⁻ ions. Conductivity depends on whether ions are mobile.
Solid: ions fixed in the lattice → no conduction.
Molten: lattice broken; ions mobile → conduction.
Mind stretcher 2: Does adding water always make a salt conduct?Extension
Question: A solid ionic compound has very low solubility in water. A learner predicts that stirring it into water will produce a highly conducting solution because “all ionic compounds conduct when wet”. What is missing from that reasoning?
Show answer
The ions must enter the solution and become mobile. Merely wetting an insoluble solid does not release all its ions. Very low solubility means few dissolved ions, so it does not support the prediction of high conductivity. Other dissolved substances and the actual ion concentration also affect a measured result.
Mind stretcher 3: Predict properties from a supplied structureExtension
Question: Material Q is described as a giant lattice of positive and negative ions, with strong electrostatic attractions. Predict its typical melting behaviour and its conductivity as a solid and as a melt. Explain each prediction.
Show answer
A high melting temperature is expected because much energy is needed to overcome attractions maintaining the lattice. The solid does not conduct in the usual test because its ions are held at fixed positions. The melt conducts because its charged ions can move. These are model-based predictions; use supplied observations if Q decomposes or has unusual behaviour.
Practise and check
See what you know across this topic, then go back to anything you got wrong.
Syllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
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