UNIT 2 CHEMISTRY · PEARSON AAQ · B1.4
Melting Points and Electrical Conductivity
FREE CHEMISTRY REVISION · TOPIC 13
Explain properties from structure and mobile charge carriers.
Compare metallic, ionic, molecular and giant covalent substances without confusing bond strength, melting and electrical conduction.
Pearson BTEC Level 3 AAQ Applied Science
Unit 2: Principles and Applications of Chemistry · B1.4
What you will learn
Compare melting points and electrical conductivity of metallic, ionic, simple molecular and giant covalent substances; explain observations using the particles, attractions and mobile charge carriers; account for graphite as a conducting giant covalent exception.
This is B1.4: apply the structures from B1.1–B1.3 to physical properties. State the physical condition (solid, molten or aqueous) when discussing conduction.
1. Use structure → forces → property
A useful explanation starts with the particles and their arrangement, identifies the attraction that must be overcome, and links that attraction to the observed property. Avoid simply saying “the bonds are strong” without naming which bonds or forces are relevant.
Melting point is the temperature at which solid and liquid coexist at a specified pressure. More energy is generally needed to disrupt stronger attractions, but melting point also depends on structure and packing. It is not a universal numerical scale of individual bond strength.
Electrical conductivity requires charged particles that can move through the substance. Having charged particles is insufficient if they cannot migrate. Electrons and ions are both possible carriers, but the carriers depend on the structure and state.
2. Comparison table
| Structure | Particles and attractions | Melting | Conduction as solid | Conduction when molten |
|---|---|---|---|---|
| Metallic | Positive cores attracted to delocalised electrons | Often relatively high, but wide variation | Yes: mobile electrons | Yes: mobile electrons remain |
| Ionic | Opposite ions in a giant lattice | Usually high: strong electrostatic attractions | No appreciable ionic conduction in the usual school model: ions fixed at sites | Yes: mobile ions |
| Simple molecular | Molecules with covalent bonds inside; intermolecular attractions between | Often low compared with giant structures | Usually no mobile charges | Usually no mobile charges in the pure liquid |
| Giant covalent | Atoms joined in an extended covalent network | Generally very high thermal stability; may sublime or decompose | Usually poor; graphite and graphene conduct | No single blanket rule: phase and chemical changes must be considered |
“High” and “low” are qualitative comparisons, not fixed boundaries. Do not assume that every metal melts above every molecular substance or that every giant covalent material can be melted cleanly in a school experiment.
3. Metals: delocalised electrons remain mobile
Strong attraction between the positive ion cores and delocalised electrons holds the giant structure together. Significant energy is often needed to overcome enough of this attraction for the solid to become liquid. Metallic bonding varies with electronic structure, ion-core charge and separation, so metal melting points span a large range; mercury is liquid near room temperature.
In a solid metal the positive cores vibrate about fixed positions, but the delocalised electrons are mobile. An applied potential difference produces a net electron drift and an electric current. The cores do not need to travel through the wire for it to conduct.
A molten metal also has mobile electrons and normally conducts. Loss of the regular crystalline lattice does not remove the electron system or all metallic attraction. In the ordinary model, electronic conduction does not involve a chemical change to the metal.
4. Ionic substances: state changes the carrier mobility
A giant ionic lattice contains many strong attractions between opposite ions in three dimensions. Melting requires enough energy to overcome the rigid lattice arrangement, so ionic substances typically have high melting points. It is not necessary to remove electrons from the ions or convert all ions to neutral atoms.
In the solid, ions are charged but cannot migrate through the lattice in the usual introductory model, so the solid does not conduct appreciably. When molten, both cations and anions are free to move, allowing charge transport. Do not explain this by inventing delocalised electrons in molten NaCl.
An ionic compound can conduct in aqueous solution if it dissolves sufficiently to produce mobile ions. Water separates and hydrates the ions; the solid has not necessarily melted. Insoluble compounds supply few ions to the solution, so “all ionic compounds conduct when put in water” is false. Ionic conduction can be accompanied by electrode reactions (electrolysis).
For a simple lattice comparison, higher ion charges and smaller separations often mean stronger electrostatic attractions. This can help explain why MgO is more strongly bound than NaCl, but melting-point predictions across different crystal structures require care; charge alone is not a universal ranking rule.
5. Simple molecular substances: distinguish two strengths
Within a molecule, strong covalent bonds hold its atoms together. Between molecules are intermolecular attractions. On ordinary melting or boiling, it is mainly intermolecular attractions that are overcome. The molecules normally remain intact, so a low melting point does not imply weak bonds inside the molecule.
Simple molecular substances often melt at comparatively low temperatures because intermolecular attractions are generally weaker than the attractions in giant lattices or networks. Molecular size, polarizability, polarity, hydrogen bonding and packing can change this behaviour. Larger molecules can have stronger dispersion interactions; “all molecules are gases” is wrong.
Pure molecular solids and liquids usually lack mobile ions or delocalised electrons, so they are poor electrical conductors. Dissolving a neutral molecular solute such as sucrose does not automatically produce ions. By contrast, some molecular substances react or ionise in water: hydrogen chloride produces ions in aqueous solution and the solution conducts. Explain the actual particles in the solution, not just the original substance’s label.
6. Giant covalent substances and the graphite exception
Diamond and silica have extensive covalent networks. Disrupting the framework involves many strong covalent bonds, giving very high thermal stability. At ordinary pressure some network substances sublime, transform or decompose rather than undergo a straightforward melting transition; carbon is a useful example. Avoid giving an ordinary-pressure “melting point” for diamond as if it behaved like ice.
In diamond, each carbon forms four covalent bonds and there are no mobile delocalised electrons in the simple model. Diamond is an electrical insulator. Silica also has localised bonding and is a poor conductor under normal conditions.
In graphite, each carbon forms three σ bonds in a layer. Electrons in the extended π system are delocalised along the layers, enabling electrical conduction, especially along the sheets. Graphene also conducts. Thus, “all covalent solids are insulators” is false.
Weak attractions between graphite’s layers allow layers to slide; they do not make the in-layer covalent network easy to destroy. The reason graphite can be soft is distinct from its high thermal stability. Its conductivity is due to mobile electrons, not carbon ions moving through a lattice.
Do not assume every molten covalent material conducts or fails to conduct for the same reason. High-temperature phases, decomposition and semiconductor behaviour require substance-specific evidence. For B1.4 questions, explain the named examples and specified state.
Worked examples
Example 1 · Identify an unknown from two states
A solid does not conduct, has a high melting point and conducts after melting. A giant ionic structure is the best fit: strong lattice attractions explain the melting point, fixed ions explain the solid, and mobile ions explain the melt. State that property patterns support an identification rather than proving composition uniquely.
Example 2 · Explain solid and molten sodium chloride
Solid NaCl contains Na⁺ and Cl⁻ ions fixed about lattice sites, so they cannot transport charge through it. Molten NaCl still contains ions, but they can migrate in an electric field. The change is ion mobility; electrons have not become a metallic sea.
Example 3 · Explain why iodine can melt relatively easily
I₂ molecules have strong I–I covalent bonds. Melting mainly overcomes attractions between different I₂ molecules, leaving the molecules intact. Therefore the melting process does not require breaking every I–I bond. A molecular solid can still be solid at room temperature.
Example 4 · Compare graphite and diamond
Both are giant covalent forms of carbon with strong networks. Diamond uses four localised bonds per carbon and lacks mobile electrons. Graphite has three σ bonds per carbon plus a delocalised electron system along layers, so it conducts. The explanation must use electron mobility, not just “one has layers”.
Example 5 · Compare a metal and an ionic crystal
Both may have high melting points, so this alone does not distinguish them. Test the solid: a metal conducts via delocalised electrons; a typical ionic solid does not because its ions cannot migrate. On melting, both may conduct, but the charge carriers differ.
Example 6 · Explain two aqueous solutions
Sucrose dissolves as neutral molecules, so the solution has no substantial supply of solute ions. NaCl dissolves to give mobile Na⁺ and Cl⁻, so its solution conducts. HCl is molecular before dissolving but forms ions in water; classify the solution by its actual particles.
Common misconceptions and exam tips
“Charged particles always conduct.” They must be mobile.
“Melting NaCl releases free electrons.” The carriers are mobile ions.
“Molecular melting breaks all covalent bonds.” It mainly overcomes intermolecular attractions.
“Graphite has weak covalent bonds.” Its in-layer bonds are strong; weaker attractions lie between layers.
“All covalent substances are insulators.” Graphite and graphene are important exceptions.
“All metals have very high melting points.” Metal melting points vary widely.
Write a complete chain: structure → attraction overcome → energy needed → melting behaviour. For conduction: name carrier → say whether it can move → state the consequence. Always include the state and, for solutions, whether mobile ions are actually present.
Your revision summary
Melting behaviour depends on which attractions are disrupted. Molecular melting largely overcomes intermolecular forces; giant structures involve stronger extended attractions or networks. Conduction requires mobile charge carriers: electrons in metals and graphite, ions in molten or sufficiently dissolved ionic substances. Pure molecular substances and diamond usually lack mobile carriers.
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