A student is given a sample of a pure, white crystalline substance. Which of the following would be — Bonding Chemistry Question

STEPS:

1. Understand the defining physical property of ionic compounds: Ionic compounds consist of a rigid, three-dimensional lattice of alternating positive cations and negative anions held together by strong electrostatic forces.
2. Recall the conditions required for electrical conductivity: For a substance to conduct electricity, it must contain charged particles that are free to move (mobile).
3. Analyze the conductivity of ionic compounds in different states:
* In the solid state, the ions are tightly locked in fixed positions within the crystal lattice and cannot move; therefore, solid ionic compounds do not conduct electricity.
* In the aqueous state (when dissolved in water), the water molecules separate the ions from the lattice, allowing them to move freely as mobile carriers of electric charge (electrolytes).
4. Compare with molecular (covalent) substances: Many soluble molecular substances (such as sugar, $\text{C}_{12}\text{H}_{22}\text{O}_{11}$) also form white crystalline solids and dissolve in water. However, when molecular solids dissolve, they separate into neutral, uncharged molecules rather than ions. As a result, their aqueous solutions do not conduct electricity.
5. Formulate a diagnostic test: Testing whether an aqueous solution of the substance conducts electricity is the most definitive way to distinguish an ionic compound from a molecular compound, making Option D the correct choice.

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WHY_OTHERS_WRONG:

• Option A is incorrect: Many covalent molecular solids (like table sugar or citric acid) and covalent network solids (like quartz) also form highly ordered, white crystalline structures. Examining the geometric shape of the crystals under a microscope cannot reveal whether the particles making up the lattice are ions, molecules, or atoms.
• Option B is incorrect: While density is an intrinsic physical property of matter, both ionic and covalent solids have wide, overlapping ranges of densities. Determining the density of an unknown substance does not provide diagnostic information about the underlying chemical bonding (ionic vs. covalent) within the substance.
• Option C is incorrect: In the solid state, both ionic compounds and molecular compounds are electrical insulators because neither possesses mobile charged particles. Therefore, a solid conductivity test will yield a negative result for both types of compounds, making it impossible to distinguish between them.