Clothes Drying Explained

The question describes a scenario where wet clothes, containing water, are hung outdoors in conditions where the temperature is below the freezing point of water (0°C or 32°F). After a period, the clothes are observed to be dry. We need to identify the physical process responsible for this drying.

Sub-Zero Weather Physics

Let's examine the given options in the context of the described situation:

• Vaporization: This is the general term for the phase transition of a substance from a liquid to a gas (vapor). While drying does involve water turning into vapor, vaporization typically refers to the process occurring at or above the boiling point, or evaporation which occurs at lower temperatures but usually requires the substance to be in liquid form. In sub-zero weather, the water in the clothes will freeze into ice first.
• Sublimation: This is the transition of a substance directly from the solid state to the gas state, without passing through the liquid state. In this case, the water in the clothes freezes into ice due to the sub-zero temperatures. This ice can then turn directly into water vapor and escape into the atmosphere. This process explains how solids disappear without melting.
• Melting: This is the phase transition from solid to liquid. This process requires temperatures above the melting point. Since the weather is sub-zero, melting is not the relevant process for drying; in fact, the water freezes.
• Condensation: This is the phase transition from gas to liquid. This process would cause moisture to form on the clothes, making them wetter, not drier.

Sublimation Process

When wet clothes are exposed to sub-zero temperatures, the liquid water within them freezes, forming ice. The key to the clothes becoming dry lies in how this frozen water disappears. While evaporation (liquid to gas) is common for drying, in freezing conditions, the frozen water (ice) can undergo sublimation. Sublimation is the process where ice turns directly into water vapor without first melting into liquid water. This direct conversion from solid ice to gaseous water vapor allows the moisture to leave the clothes and disperse into the air, resulting in dry clothes, even though the temperature remains below freezing.

Therefore, the process of drying wet clothes in sub-zero weather is best explained by sublimation, where the ice in the fabric transforms directly into water vapor.