Question

Using the phase diagram for water and Raoult's law, explain why salt is spread on the roads in winter (even when it is below freezing).

Short answer

When salt is spread on roads during winter, it lowers the freezing point of water due to the freezing point depression, as explained by Raoult's law. This law states that the vapor pressure of a solvent in a solution decreases with the increase in solute concentration. In this case, adding salt (solute) to water (solvent) results in a solution with a lower freezing point than pure water. Consequently, the water on the roads remains liquid at lower temperatures, preventing ice formation and making the roads less slippery and safer for driving.

Step-by-step solution

Understanding the Phase Diagram for Water

A phase diagram is a graphical representation of the different phases (solid, liquid, and gas) of a substance under varying conditions, such as temperature and pressure. In this exercise, we are focusing on the phase diagram for water. The most crucial point in the diagram is the freezing point (0°C, 32°F), which is when water turns to ice. As the temperature decreases, water begins to freeze, and ice forms. This is what occurs on roads during winter when the temperature drops, causing ice and slippery conditions.

Understanding Raoult's Law

Raoult's law states that the vapor pressure of a solvent in a solution decreases as the concentration of solute increases. In simpler terms, adding solute (like salt) to a solvent (like water) will lower the vapor pressure of the solvent. This means that a solution of water and salt will have a lower vapor pressure compared to pure water. This decrease in vapor pressure affects the freezing point of the solution, which is known as "freezing point depression."

Applying Raoult's Law to the Phase Diagram

When salt is added to water, the freezing point temperature of the water decreases due to freezing point depression. In other words, the solution of salt and water will freeze at a lower temperature than pure water. This effect can be observed in the phase diagram of water since it shifts the freezing point curve to lower temperatures. This means that the water on the road will remain liquid, rather than turning into ice, at a lower temperature than if no salt was added.

Explaining the Phenomenon

The use of salt in winter road treatment takes advantage of the freezing point depression caused by the presence of solute (salt) in the solvent (water). When salt is spread on the roads, it mixes with the existing water, forming a solution with a lower freezing point than pure water. The freezing point depression allows the water to remain in a liquid state, even in temperatures below the freezing point of pure water. This liquid state prevents ice formation on the road, making it less slippery and safer for vehicles to drive.