Topic 5 of 5
Work backwards from the evidence
Use melting point and conductivity together to identify and explain a likely structure.
SEC G2 2027
Work backwards from the evidence
Use several properties together, then explain the particles, attractions and charge carriers.
Worked example
Identify 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?
- 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.
Answer
A giant ionic structure. Strong electrostatic attractions require much energy to overcome; ions carry charge only when free to move.
Check your understandingA pure substance melts at a low temperature and does not conduct as a solid or liquid. Suggest a structure and explain the low melting point.Think it through, then reveal the answer
A simple molecular structure is likely. Relatively little energy is needed to overcome weak intermolecular forces. The covalent bonds within the molecules remain intact during melting.
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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