States of Matter: Question 3

Worked solution

Part (a): Identifying the lattice type of each solid

• P: an extremely high melting point, combined with no electrical conductivity in either the solid or the molten state, rules out ionic bonding (which would conduct once molten) and metallic bonding (which would conduct even as a solid). This pattern, a very high melting point but never conducting, is the signature of a giant covalent (giant molecular / macromolecular) lattice, in which every atom is joined to its neighbours by a continuous network of strong covalent bonds.
• Q: a fairly high melting point, no conductivity as a solid, but good conductivity when molten, together with solubility in water, is the classic signature of a giant ionic lattice.
• R: a very low melting point (well below room temperature), no conductivity in either state, and insolubility in water but solubility in a non-polar solvent, is the signature of a simple molecular lattice held together by weak intermolecular forces.
• S: a high melting point with good electrical conductivity in both the solid and molten states is the signature of a giant metallic lattice.

Part (b): Why solid P remains solid up to such a high temperature

In a giant covalent lattice, every atom is bonded to several neighbouring atoms by strong covalent bonds that extend throughout the entire structure. There are no separate small molecules. Melting a giant covalent solid means breaking a very large number of these strong covalent bonds (not just overcoming weak intermolecular forces), which requires a great deal of energy. This is why P’s melting point is so much higher than a simple molecular solid’s.

Part (c): Why solid Q only conducts once molten

In solid Q, the positive and negative ions are held in fixed positions in a rigid lattice by strong electrostatic forces of attraction, so although the ions carry charge, they cannot move from place to place and no current can flow. When Q is melted (or dissolved in water), the lattice breaks down and the ions become free to move throughout the liquid; the mobile ions can then carry charge, so molten (or aqueous) Q conducts electricity.

Part (d): Why solid R is insoluble in water but dissolves in a non-polar solvent

Solid R is a simple molecular lattice held together only by weak intermolecular forces (van der Waals and/or permanent dipole–dipole forces) between individual, non-polar or only weakly polar molecules. Water is a strongly polar solvent whose molecules are held together by hydrogen bonds; for R to dissolve in water, new attractions between R and water molecules would need to be at least as strong as the hydrogen bonds broken in water and the intermolecular forces broken in R, but R’s molecules cannot form strong ion–dipole or hydrogen-bonding interactions with water, so it does not dissolve. A non-polar organic solvent, however, is held together by van der Waals forces of a similar strength and type to those within R’s own lattice, so R’s molecules can be surrounded by solvent molecules with a comparable energy change, following the “like dissolves like” principle, and R dissolves readily.

Final answers

• (a) P = giant covalent (giant molecular) lattice; Q = giant ionic lattice; R = simple molecular lattice; S = giant metallic lattice
• (b) A very large number of strong covalent bonds throughout the lattice must be broken to melt P, requiring a large amount of energy
• (c) The ions in solid Q are fixed in position and cannot move, but become mobile and free to carry charge once the lattice breaks down on melting
• (d) R is held together by weak, non-polar-type intermolecular forces that cannot be replaced by strong interactions with polar water molecules, but can be replaced by similar-strength van der Waals interactions with a non-polar solvent