Exercise 3 solution

Substance P matches the pages' description of an ionic compound:
- brittle: "Most salts are very brittle" (Ionic compound article)
- high melting point: "Ionic compounds generally have a high melting point" (Ionic bonding)
- does not conduct as a solid but conducts when molten or dissolved:
  "Ionic compounds conduct electricity when molten or in solution, typically not when
  solid" (Ionic bonding). The Ionic compound article gives the cause: the ions become
  mobile when the compound is melted or dissolved.
(The statement is "generally" and "typically", so these properties suggest an ionic
compound but do not prove it.)

Substance Q does not match: it conducts when solid, which the pages say ionic
compounds typically do not. Its softness also does not match "brittle". The pages read
for this chapter do not say what Q is, so I cannot name its kind of bonding from them
(its being a conductor as a solid fits metals, from general knowledge - check).

Why a lattice picture suits P: the pages say ionic solids form extended
three-dimensional networks in which "it is usually not possible to distinguish discrete
molecular units". The conduction on melting is explained by ions freed from that
lattice.

What the pages do not tell you about P: which elements it contains, its exact formula,
its charges, or how soluble it is. Those would need more information. Properties alone
point towards ionic bonding without identifying the compound.