Final answer:
Bridges, overpasses, and shaded areas are likely to freeze first due to less heat retention. Salt is used on roads to prevent or melt ice through freezing point depression. Darker pavements absorb more heat, melting ice more effectively.
Step-by-step explanation:
Areas that are likely to freeze before other road surfaces include bridges, overpasses, and shaded spots. These areas freeze first because they are exposed to the cold air from all sides, which causes the temperature of the road surface to drop more rapidly than that of the surrounding ground, which can retain more heat. Trucks put salt on the roads when snow or ice is forecast because salt lowers the freezing point of water, which helps to prevent the formation of ice or to melt existing ice. Sodium chloride (NaCl) and calcium chloride (CaCl₂) are commonly used. The process of salt dissolving into ions in water disrupts the formation of ice, thereby lowering the freezing point of the water, a phenomenon known as freezing point depression.
The use of these salts is based on the scientific principle that a solute (like salt) can affect the boiling and freezing points of a solution. For ice melting applications, determining the concentrations of NaCl and CaCl₂ in a solution can enable one to calculate the new freezing points of these solutions, thus assessing their effectiveness for deicing. Darker pavement tends to absorb more heat and thus, when in sunlight, will melt ice more effectively than lighter pavement, as indicated by Figure 1.30 where more ice has melted on the darker pavement despite equal sunlight exposure.