Problem 80 - Chemistry Matter and Change

Question

Snow Why does a pile of snow slowly shrink even on days when the temperature never rises above the freezing point of water?

Step-by-Step Solution

Answer

Sublimation causes snow to shrink even on cold days, as ice turns directly to vapor without melting.

Step 1: Understand the Process of Sublimation

Sublimation is the process where a solid turns directly into a gas without first becoming liquid. In the context of snow, sublimation occurs when ice in the snow turns directly into water vapor, bypassing the melting phase. This can occur even at temperatures below freezing.

Step 2: Examine Temperature and Atmospheric Conditions

Even when the air temperature is below freezing, factors like sunlight, wind, or low humidity can cause the ice in the snow to sublimate. Sunlight provides energy, while wind can carry away water vapor, and low humidity increases the rate of sublimation by increasing evaporation.

Step 3: Analyze the Effect of Energy Input

Energy from sunlight or the air can provide enough heat energy to overcome the latent heat of the ice, allowing sublimation to proceed at temperatures where melting isn't possible. This energy causes individual water molecules at the surface of the snow to escape and enter the atmosphere as vapor.

Step 4: Conclude Based on Observed Phenomena

Due to sublimation, snow can decrease in size even when temperatures are below freezing. This is why snow piles often shrink on cold days, despite the lack of melting, as the snow transitions directly to water vapor.

Key Concepts

Freezing Point

When we discuss the freezing point, we refer to the temperature at which a liquid turns into a solid. For water, this is 0°C or 32°F. However, snow can decrease in size even when the temperature is below this freezing point. This is due to the process of sublimation. Despite temperatures being low enough to keep snow frozen, sublimation allows for the ice to transform directly into vapor.
This means that melting isn't needed for snow to shrink. So, even in freezing conditions, a snow pile can shrink without turning to liquid first.

Phase Transition

A phase transition involves a substance changing from one form to another – like solid ice turning directly into vapor during sublimation. Typically, we expect water to shift from solid to liquid, then to gas. But in sublimation, the phase transition skips the liquid phase.
This unusual transition is key in understanding snow reduction at freezing temperatures.

• No liquid water is created
• Direct transformation from solid ice to vapor
• Occurs below the usual melting point

Such a transition requires specific conditions and energy input, which we'll discuss in further sections.

Energy Input

Energy is crucial for any phase transition to occur, including sublimation. Even when it's cold, energy comes into play from sources like sunlight and the air.
This energy doesn't raise the temperature above freezing but does provide enough heat to overcome the latent heat of the ice in snow. The result is the escape of water molecules as vapor.

• Sunlight delivers direct heat to surfaces
• Air, though cool, contains energy that aids sublimation
• Energy grants molecules enough momentum to change phase

This energy interaction proves why snow still shrinks without a rise in ambient temperature.

Atmospheric Conditions

Atmospheric conditions significantly influence sublimation rates. Even below freezing, snow can vanish due to interaction with environmental factors.

• Sunlight provides the essential energy
• Wind increases sublimation by carrying away released vapor
• Low humidity favors higher sublimation rates as the air is less saturated

These factors create an environment where snow decreases in size. It's a delicate balance of elements that may seem counterintuitive in cold conditions. Thus, even on the chilliest days, the atmosphere orchestrates snow reduction through sublimation.