Lesson 5 of 5
Work backwards from the evidence
Use melting point and conductivity together to identify and explain a likely structure.
O-Level 2026 SEC G3 2027
I can explain ionic physical properties.Syllabus
Syllabus K326 / K328, 3.1(c). Explain ionic physical properties
I can connect covalent structure and properties.Syllabus
Syllabus K326 / K328, 3.2(d). Connect covalent structure and properties
Work backwards from the evidence
Use several properties together, then explain the particles, attractions and charge carriers.
Worked exampleIdentify a likely structure
An unknown solid melts at a high temperature. It does not conduct when solid, but conducts after melting. What structure is most likely?
Try it first. Then show the worked stepsHide the worked steps
- The high melting point suggests strong attractions extending through the solid.
- The molten material has mobile charged particles.
- The solid lacks mobile charge carriers. That makes a metal unlikely.
- An ionic lattice fits both observations: fixed ions in the solid, mobile ions after melting.
A giant ionic structure. Strong electrostatic attractions require much energy to overcome; ions carry charge only when free to move.
Check your understanding
A pure substance melts at a low temperature and does not conduct as a solid or liquid. Which structure fits?
Show the answer
Simple molecular: melting overcomes only weak forces between molecules.
Low melting point means little energy is needed, which fits weak intermolecular forces. The covalent bonds inside the molecules stay intact. No ions or free electrons means no conduction.
Exam-style question
Magnesium oxide melts at 2852 °C. Chlorine is a gas at room temperature. Explain this difference in terms of structure and bonding. [3]
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| Term | Definition to write |
|---|---|
| Element | A substance that cannot be broken down into simpler substances by chemical methods. It is made of only one type of atom. |
| Compound | A substance made of two or more elements chemically combined in a fixed ratio. |
| Mixture | Two or more substances that are not chemically combined. They can be separated by physical methods. |
| Ionic bond | The electrostatic attraction between oppositely charged ions. |
| Covalent bond | The electrostatic attraction between a shared pair of electrons and the nuclei of the two bonded atoms. |
| Alloy | A mixture of a metal with one or more other elements, usually other metals. |