[VISUAL] Substance | Melting Point (°C) | Electrical Conductivity in Solid Phase | Electrical Conduc — Bonding Chemistry Question

STEPS:

1. Identify the core chemistry concepts being tested: This question tests your ability to relate a substance's macroscopic physical properties—specifically melting point and electrical conductivity in different states of matter—to its microscopic bonding type (network covalent, ionic, metallic, or molecular).
2. Analyze Substance X:
* Properties: It has an extremely high melting point ($1600^\circ\text{C}$) and does not conduct electricity in either the solid or liquid phase.
* Reasoning: A very high melting point indicates that incredibly strong chemical bonds must be broken to melt the substance. The lack of electrical conductivity in both phases means there are no mobile charged particles (neither free-moving ions nor a sea of mobile valence electrons) present. These combined properties are classic hallmarks of a network covalent solid (such as silicon dioxide, $\text{SiO}_2$, or diamond, $\text{C}$).
3. Analyze Substance Y:
* Properties: It has a moderately high melting point ($650^\circ\text{C}$) and exhibits high electrical conductivity in both the solid and liquid phases.
* Reasoning: High conductivity in both the solid and molten liquid phases indicates that the valence electrons are highly delocalized and free to flow under an electric potential in any state of matter (the "sea of electrons" model). This behavior is the defining characteristics of metallic bonding.
4. Analyze Substance Z:
* Properties: It has a high melting point ($1418^\circ\text{C}$), does not conduct electricity as a solid, but is highly conductive in the liquid phase.
* Reasoning: In the solid state, the charged particles are held rigidly in a three-dimensional crystalline lattice and cannot migrate to carry an electric current. Once melted into the liquid phase, the lattice is disrupted, allowing the charged particles to move freely and conduct electricity. This behavior represents a classic ionic solid (such as $\text{NaCl}$ or $\text{CaF}_2$).
5. Match the substances to the correct option:
* Substance X: Network covalent
* Substance Y: Metallic
* Substance Z: Ionic
* This sequence corresponds perfectly to Option D.

*

WHY_OTHERS_WRONG:

• Option A is incorrect: It swaps the identities of Substance Y and Substance Z, claiming Y is ionic and Z is metallic. If Y were ionic, it would not conduct electricity in its solid phase. If Z were metallic, it would conduct electricity in both the solid and liquid phases.
• Option B is incorrect: It misclassifies all three. It claims X is ionic, but ionic substances conduct when molten, which X does not. It claims Y is molecular, but molecular substances have low melting points and do not conduct electricity in either phase. It claims Z is network covalent, but network covalent solids do not conduct electricity in the liquid phase.
• Option C is incorrect: It misidentifies X as molecular, which is incorrect because molecular substances have weak intermolecular forces and typically have very low melting points (well below $1600^\circ\text{C}$). It also incorrectly claims Y is network covalent (which would not conduct) and Z is metallic (which would conduct as a solid).