In countries where the temperature fall below 273k, salt always spilled on the road in order to

Analysis

The question asks for the scientific reason why salt (sodium chloride) is spread on icy roads in cold climates (where temperatures drop below 273K, which is 0°C).

Concept: Freezing Point Depression

This phenomenon is based on a colligative property of solutions known as Freezing Point Depression.

1. What is Freezing Point Depression? When a solute (like salt) is dissolved in a solvent (like water), the freezing point of the resulting solution becomes lower than that of the pure solvent.
2. How it works: The salt particles interfere with the ability of water molecules to arrange themselves into the rigid, crystalline structure of ice. Because the salt disrupts this process, the water needs to be cooled to a much lower temperature than 0°C before it can successfully turn into ice.
3. Application: By spreading salt on the road, the freezing point of the water/ice mixture is lowered. If the ambient temperature is, for example, -2°C, the salt might lower the freezing point of the ice to -5°C. Since the ambient temperature is now above the new freezing point, the ice melts into liquid water.

Step-by-Step Solution

• Step 1: Identify the state of the road. It is covered in ice (solid water) at 273K (0°C) or below.
• Step 2: Recognize the effect of adding salt. Salt acts as an impurity that dissolves in the thin layer of water on the ice surface.
• Step 3: Apply the principle of colligative properties. The presence of the solute (salt) lowers the chemical potential of the liquid phase, making it harder for the water to freeze and easier for the ice to melt.
• Step 4: Conclude the result. The freezing point of the ice is reduced, causing it to melt even if the air temperature remains below the standard freezing point of pure water.

Conclusion

Exam Tip: Remember that adding a solute to a solvent always lowers the freezing point and raises the boiling point. This is why we add salt to water when boiling pasta (to raise the boiling point) and why we add salt to icy roads (to lower the freezing point).