4.2.2—Describe, interpret and predict the effect

Giant ionic substances have high melting and boiling points because strong electrostatic attractions hold oppositely charged ions throughout the lattice. They are brittle because shifting layers can bring like charges together and split the crystal. Ions are fixed in a solid but mobile when molten or dissolved, so conductivity depends on state; ion–dipole interactions can support solubility in water.

Giant metallic substances are strong and generally have high melting and boiling points because positive metal ions attract delocalised electrons. Mobile electrons allow conduction in both solid and liquid states. Layers can slide while the metallic attraction is maintained, giving malleability; more delocalised electrons and smaller cations strengthen the attraction in the source-supported comparison.

Simple molecular substances generally have low melting and boiling points because only weak intermolecular forces need to be overcome between molecules; they usually do not conduct because they lack mobile charged particles. Polarity or hydrogen bonding can change solubility and attraction strength. The covalent bonds inside each molecule are not broken during melting or boiling.

Giant covalent substances have high melting and boiling points because many strong covalent bonds extend through the network. Diamond and silicon(IV) oxide are hard and lack mobile charge carriers, whereas graphite is soft because its layers attract weakly and can slide, and it conducts along the layers because it has delocalised electrons. Use these as structure-based exceptions, not as a rule that all carbon forms behave alike.

Explain a property by naming the structure, the relevant attraction or mobile particle, and the condition being tested. Do not infer ionic structure from melting point alone, say that every ionic solid conducts, or treat ‘insoluble’ and ‘non-conducting’ as interchangeable evidence.