Which of the following could be the identity of a white crystalline solid that exhibits the followin — Bonding Chemistry Question

STEPS:

1. Analyze the conductivity of the solid: The unknown substance does not conduct electricity as a solid. In metallic solids, highly mobile valence electrons (often described as a "sea of electrons") allow the solid to conduct electricity. Since this white crystalline solid is a non-conductor in its solid state, we can immediately eliminate any metallic elements such as copper ($\text{Cu}$).
2. Analyze the conductivity of the aqueous solution: The substance conducts electricity in an aqueous solution. To conduct electricity in water, a substance must be soluble and dissociate into free-moving charged particles (ions) that can carry an electric current.
* Ionic compounds (like $\text{NaOH}$) are strong electrolytes that dissociate into free ions in water and conduct electricity.
* Molecular compounds (like glucose, $\text{C}_6\text{H}_{12}\text{O}_6$) dissolve as intact, neutral molecules and do not conduct electricity (non-electrolytes).
* Network covalent solids (like $\text{SiO}_2$) are completely insoluble in water and do not dissolve to form ions.
Therefore, this property strongly points to an ionic compound, identifying Option B as the prime candidate.
3. Correlate with the melting point (320°C): The substance melts at 320°C.
* Ionic compounds have moderately high to high melting points due to the strong electrostatic attractions (lattice energy) holding the ions in place. The melting point of sodium hydroxide ($\text{NaOH}$) is approximately $318^\circ\text{C}$, which is extremely close to $320^\circ\text{C}$.
* Molecular solids like glucose have much lower melting points (around $146^\circ\text{C}$) because they are held together by weaker intermolecular forces (like hydrogen bonding and dispersion forces) rather than chemical bonds.
* Network covalent solids like $\text{SiO}_2$ require breaking highly stable covalent bonds throughout a 3D network to melt, resulting in an extremely high melting point (over $1700^\circ\text{C}$).
This confirms that Option B is the correct answer.

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WHY_OTHERS_WRONG:

• A is incorrect: Glucose ($\text{C}_6\text{H}_{12}\text{O}_6$) is a molecular solid. It has a much lower melting point than $320^\circ\text{C}$ and dissolves in water as neutral, uncharged molecules rather than ions, meaning its aqueous solution does not conduct electricity.
• C is incorrect: Silicon dioxide ($\text{SiO}_2$) is a network covalent solid. Due to the highly stable covalent bonds extending throughout its continuous 3D structure, it has an exceptionally high melting point (exceeding $1700^\circ\text{C}$) and is highly insoluble in water, meaning it cannot form a conductive aqueous solution.
• D is incorrect: Copper ($\text{Cu}$) is a metallic solid. Because it features a highly mobile "sea of valence electrons," copper is an excellent conductor of electricity in its solid state, which directly contradicts the second clue. Additionally, its melting point ($1085^\circ\text{C}$) is far higher than $320^\circ\text{C}$.