Classifying substances by physical properties at room temperature

Steps:

• Classify W as giant ionic solid (high melting point, insoluble or sparingly soluble, non-conductor in solid state).
• Classify X as simple molecular polar compound (low melting point, soluble in water, non-conductor).
• Classify Y as giant covalent solid (high melting point, insoluble, non-conductor).
• Classify Z as ionic compound soluble in water (high melting point, conductor in solution).

Why C is correct:

• Ionic compounds (Z) and polar molecular substances (X) dissolve in water due to ion-dipole or dipole-dipole interactions, per solubility rules for polar solvents.

Why the others are wrong:

• A: W (ionic) and X (molecular) are compounds, not elements, as elements are pure substances like metals or nonmetals.
• B: W (ionic solid) and Y (giant covalent) do not conduct electricity in solid form, lacking free ions or electrons.
• D: Y (giant covalent) has a high melting point due to strong covalent bonds, unlike simple molecular substances.

Final answer: C