S2.4 From models to materials

Ionic compounds have high melting points due to strong electrostatic forces, do not conduct as solids (ions are fixed in lattice), but do conduct when molten or dissolved (ions become mobile).

Material Y has a high melting point, does not conduct as a solid, but does conduct when molten. This matches perfectly with an ionic lattice structure.

Material W has no conductivity in either state, which is characteristic of a covalent network solid (e.g., diamond). Material X conducts in both states — typical of a metal. Material Z has a low melting point, suggesting a covalent molecular substance.

Material Y matches all three diagnostic properties of an ionic compound: high melting point, non-conduction as a solid, and conduction when molten or dissolved. The answer is Material Y.

The question asks which material best fits the properties of an ionic compound. The key diagnostic properties are: high melting point, non-conducting solid, but conducting when molten.

Material W has a very high melting point but does not conduct in either solid or molten state. This rules out ionic compounds (which conduct when molten) and instead points to a covalent network solid like diamond or ${\text{SiO}}_2$.

Material X conducts electricity in both the solid and molten state. This is the hallmark of a metal, where delocalised electrons carry charge regardless of whether the metal is solid or liquid.

Material Z has a low melting point and does not conduct in either state, which is characteristic of a simple covalent molecular substance (e.g., iodine, naphthalene) held together by weak van der Waals forces.

Material Y — high melting point, non-conducting solid, but conducting when molten — perfectly matches an ionic compound. The answer is Material Y.

In a pure metal like iron, all cations have the same size and are arranged in a regular, ordered lattice. The non-directional nature of metallic bonding means that planes of atoms can slide past each other easily, giving the metal its malleability.

When carbon (a much smaller atom) is added to iron to make steel, carbon atoms occupy interstitial gaps in the iron lattice. This disrupts the regularity of the lattice, making it much harder for atomic layers to slide past one another.

The correct explanation centres on the disruption of the regular lattice by differently-sized atoms preventing layer sliding. The answer is: In pure iron, atoms of identical size form a regular lattice in which layers can slide easily, whereas carbon atoms disrupt this regularity.

The question asks for the correct explanation of why pure iron is softer than steel (an alloy). This tests understanding of alloy strengthening via lattice disruption.

The number of delocalised electrons is not significantly altered by adding a small percentage of carbon. Alloy strengthening is a structural/geometric effect, not primarily an electronic one. This option is incorrect.

Steel is an alloy (a mixture), not an ionic compound. Iron and carbon do not form ionic bonds in steel. This option fundamentally misidentifies the bonding in an alloy.

Electron density differences between pure iron and steel are not the reason for hardness differences. This is a vague and chemically inaccurate distractor.

The correct answer correctly identifies that same-sized atoms in pure iron allow easy layer sliding, while differently-sized carbon atoms in steel disrupt the regular lattice and prevent sliding — the standard IB-level explanation for alloy hardness.