Road-maintenance crews sometimes place large blocks of compressed sodium chloride ("salt licks") on icy pavements to accelerate melting. When a heavy truck tire rolls over a salt block, the block often fractures suddenly into many smaller pieces. In contrast, steel reinforcement bars in concrete react to the same wheel load by bending rather than breaking.
Analyze these observations at both the macroscopic and particulate levels.
(a) Describe one macroscopic property that classifies sodium chloride as brittle and one that classifies steel as ductile, based on the scenario provided.
(b) Using particulate-level reasoning, explain why a small lateral shift of ions inside a sodium chloride crystal under shear stress makes fracture more likely than plastic deformation.
(c) Using particulate-level reasoning, explain how delocalized electrons in steel permit layers of iron atoms to slide without causing fracture.
(d) Suppose the steel bar were replaced by cast iron that contains a higher percentage of carbon. Predict how the bar would respond to the same wheel load and justify your prediction in terms of crystal structure and bonding.