Problem 81 - Chemistry

Question

A student hangs wet clothes outdoors on a winter day when the temperature is \(-15^{\circ} \mathrm{C}\). After a few hours, the clothes are found to be fairly dry. Describe the phase changes in this drying process.

Step-by-Step Solution

Answer

The clothes with liquid water on them were exposed to a temperature below freezing, causing the water to freeze into a solid. This ice on the clothes sublimated (i.e., went directly from the solid phase to the gaseous phase without becoming a liquid first), leaving the clothes dry. This process of water in liquid form turning into a solid (ice) then directly into a gas is called sublimation.

Step 1: Initial Condition

Initially, the wet clothes are hung outside, and the water on the clothes is in its liquid state. The outside temperature is given as \(-15^{\circ} \mathrm{C}\), which is well below the freezing point of water, \(0^{\circ} \mathrm{C}\). Thus, the water on the clothes will quickly freeze into ice, converting the phase from liquid to solid.

Step 2: Transformation Process

Once the liquid water has turned to ice, it doesn't normally go back to being liquid in such cold temperatures. Meanwhile, the air isn't 100% humidity, so the ice on the clothes will start to sublimate. Sublimation is the transition of a substance from the solid to the gas phase without passing through the intermediate liquid phase. This process will continue until all the ice has transformed into water vapor.

Step 3: End Results

At the end of this process, the clothes will be dry as all the water (initially as a liquid, then froze into solid ice) has now sublimated into the gaseous phase. The clothes went from being wet (having liquid water on them), to having ice, to being dry (no water left as it's changed to gaseous form)

Key Concepts

Sublimation

Sublimation is a unique phase change where a substance directly transitions from a solid state to a gaseous state, without becoming a liquid first. This is an unusual but fascinating process that occurs under specific conditions.

• Occurs when the substance is exposed to conditions where the pressure and temperature enable such a change.
• For water, this often happens at very low temperatures when the air is dry, allowing solid ice to turn into water vapor.

The drying of wet clothes outdoors on a cold winter day incorporates sublimation. Any ice formed from the water in the clothes transitions directly into vapor, allowing the clothes to dry even if the air is frosty.

Freezing

Freezing is a common phase change where a liquid transitions into a solid. When temperatures drop below the freezing point of water, which is 0°C, liquid water becomes ice.

• Under cold conditions, like \(-15^{\circ} \mathrm{C}\), water on the clothes will freeze.
• This creates a solid layer of ice instead of liquid water.

In the context of drying clothes outside in winter, the initial freezing of liquid water on clothes is crucial for the following sublimation to take place. This phase change is the first step in taking wet clothes to a fully dry state.

Water Vapor

Water vapor is the gaseous form of water. It results from the evaporation or sublimation of ice into the air. Even on cold days, water vapor can be present due to sublimation.

• It is invisible and becomes the endpoint for water as it transitions through sublimation.
• Replacing the ice on clothes, the vapor mixes with the surrounding air.

Through this process, water in its gaseous state ensures the clothes become free from any liquid or solid water, resulting in them being dry.

Solid to Gas Transition

The transition from a solid directly to a gas involves specific phase change processes known as sublimation. This phenomenon bypasses the liquid state entirely and plays a crucial role in various natural and technological scenarios.

• In the winter drying of clothes, ice on the fabric sublimates due to the low humidity in the air.
• The process is slow yet effective in removing moisture without needing the ice to melt first.

Understanding this transition helps explain how substances behave under different conditions, especially at varying pressures and temperatures, offering insights into phase behavior beyond typical liquid-solid-gas changes.