Problem 35 - Chemistry: An Atoms-Focused Approach

Question

A brief winter storm leaves a dusting of snow on the ground. During the sunny but very cold day after the storm, the snow disappears even though the air temperature never gets above freezing. If the snow didn't melt, where did it go?

Step-by-Step Solution

Answer

Question: Explain how the snow can disappear even though the air temperature is below freezing. Answer: The snow can disappear without melting due to a process called sublimation, where it directly transitions from a solid state to a gaseous state (water vapor). This occurs when conditions such as low relative humidity, wind, and sunlight are present. These factors enable the snow to absorb enough energy to break down intermolecular bonds and sublimate, even when the air temperature is below freezing.

Step 1: Introduction

Sublimation is the process where a substance changes directly from a solid state to a gas state, without passing through a liquid phase. In this exercise, the snow disappears by sublimating from its solid state (snow) into a gaseous state (water vapor).

Step 2: Factors affecting sublimation

Sublimation of snow can occur more quickly when the following conditions are present: 1. Low relative humidity: Relative humidity is the amount of moisture in the air compared to the maximum amount of moisture the air can hold at that temperature. When the relative humidity is low, there's less moisture in the air, which allows the snow to vaporize more quickly. 2. Wind: Wind can help to increase the sublimation process by removing the water vapor generated by sublimation from the surface of the snow, allowing for more snow to vaporize. 3. Sunny conditions: Sunlight can also help to increase the sublimation process by providing additional energy for the process.

Step 3: Sublimation process

During the sunny, cold day after the winter storm, snow on the ground absorbs energy from sunlight. This energy is used to break down the intermolecular bonds between water molecules in the snow. As the bonds weaken, the water molecules transition directly from the solid state into the gas state, without passing through the liquid state. This causes the snow to turn into water vapor and enter the atmosphere, making the snow appear to disappear.

Step 4: Effects of temperature

Even though the air temperature was below freezing, sublimation can still occur because the process depends on the difference in vapor pressure between the snow and the surrounding air. Since the air had lower humidity and the snow was absorbing sunlight, the snow's vapor pressure was higher than the surrounding air, allowing sublimation to occur.

Step 5: Conclusion

The snow disappeared even though the temperature was below freezing because it underwent sublimation. Due to low humidity, wind, and sunlight providing energy, the snow was able to transform directly from a solid state to a gaseous state, causing it to disappear without melting.