A student investigates three pure substances: sodium chloride (NaCl), carbon dioxide in its solid form (dry ice, CO₂(s)), and copper metal (Cu). For each substance the student records the following observations at room temperature (25 °C):
• NaCl is a hard, brittle crystalline solid that does not conduct electricity.
• CO₂(s) sublimes readily and does not conduct electricity.
• Cu is malleable and an excellent electrical conductor.
Using principles of chemical bonding and structure, address the questions below.
(a) Identify the dominant type of particle–particle interaction present within each solid at the particulate level.
(b) Rank the three substances from highest to lowest melting point. Clearly justify your ranking in terms of the interactions identified in part (a).
(c) Explain why NaCl conducts electricity when melted but not in the solid state, whereas Cu conducts electricity in both states.