Deduce structure and bonding from physical evidence
Combine melting, conductivity and solubility observations to infer likely structure and bonding, and state limits of the evidence.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Keep pressure, volume, amount and absolute temperature consistent in gas calculations. Written practice is self-checked
Before you begin
- Rearrange a formula and convert between common metric volume units.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which evidence best distinguishes an ionic solid from a simple molecular solid?
What you will learn
- Deduce structure type from provided physical properties
- Use more than one observation and avoid claiming a unique structure from weak evidence
Use several observations to narrow the structure
A high melting point suggests strong interactions throughout a giant structure but does not alone distinguish ionic, giant covalent or metallic solids. Electrical conductivity, solubility, physical state and details of the sample provide additional evidence.
A solid that does not conduct but conducts when molten is consistent with an ionic lattice: ions are fixed in the solid and mobile in the melt. A solid that conducts as a metal is consistent with mobile delocalised electrons; graphite can also conduct, so combine the evidence.
State what the evidence supports
Low melting and boiling points with non-conductivity are consistent with a simple molecular substance, but solubility may vary. Very high melting point and insolubility can support a giant covalent network, while conductivity data may distinguish graphite from diamond.
Deductions are strongest when multiple independent observations agree. If properties are incomplete or overlap between structure types, state the most consistent model and the evidence limitation.
Pause and explain
A solid conducts electricity in its solid state. Which structure is strongly supported?
Worked example
A crystalline solid has a high melting point, does not conduct when solid, and conducts when molten. Deduce its likely structure.
Show the worked solution
- The high melting point is consistent with strong attractions in a lattice.
- In the solid, charge carriers are fixed.
- In the melt, mobile ions conduct; the pattern supports a giant ionic lattice.
Answer A giant ionic lattice is most consistent: fixed ions do not conduct in the solid, but mobile ions conduct when molten.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A solid has a high melting point, is soluble in water and its solution conducts electricity. Give the most likely structure and evidence.
Give me a hint
Use lattice strength and mobile charge in solution together.
Compare my reasoning
- High melting point supports strong lattice attractions.
- Conducting solution indicates mobile ions are present.
- Together the evidence supports a giant ionic solid.
A giant ionic lattice is most likely: strong ionic attractions support a high melting point, and mobile ions in solution carry charge.
Look for these in your work
- Structure inferred
- Melting evidence interpreted
- Solution conductivity linked to ions
Substance X has a low melting point, does not conduct and dissolves in a non-polar solvent. Deduce a likely structure and explain the evidence cautiously.
Give me a hint
Look for separate neutral particles and weak attractions between them.
Compare my reasoning
- Low melting point is consistent with weak intermolecular attractions.
- Non-conductivity supports the absence of mobile ions or electrons.
- Solubility in a non-polar solvent is consistent with a non-polar molecular substance.
A simple molecular structure is most consistent: weak intermolecular attractions, no mobile charge carriers and non-polar-solvent solubility support neutral molecules.
Look for these in your work
- Simple molecular structure identified
- Three observations used
- Conclusion phrased as an evidence-based inference
A very hard, insoluble solid has a very high melting point and does not conduct. State a likely structure and one limit of the deduction.
Give me a hint
Consider a continuous covalent network, then acknowledge that some properties overlap.
Compare my reasoning
- A giant covalent network has strong covalent bonds throughout.
- This can explain hardness, insolubility and a very high melting point.
- Non-conductivity is consistent with diamond-like electrons, but more evidence may be needed to distinguish all structures.
A giant covalent structure such as diamond is likely. The observations support it, but without further evidence they do not identify a unique lattice or exclude every other structure conclusively.
Look for these in your work
- Likely structure named
- Properties linked to network bonds
- Inference limit stated
Common mistakes
- Inferring a unique structure from melting point alone
- Calling a solid ionic solely because it dissolves in water
- Ignoring phase when interpreting conductivity
- Failing to mention ambiguous or overlapping evidence
What can you explain now?
Which evidence best distinguishes an ionic solid from a simple molecular solid?
Compare with the explanation
Conductivity when molten plus melting behaviour
Mobile ions in the melt and lattice interactions help identify ionic structure.
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Make it stick
Which combination of phase-specific conductivity, melting point and solubility best supports a structure deduction?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.