ANS:B - sublimation

The disappearance of snow in subzero weather exemplifies the process of sublimation. Sublimation is the process by which a substance transitions directly from the solid phase to the gas phase without passing through the intermediate liquid phase. In the case of snow, when the temperature is below freezing (subzero), the snow can sublimate directly into water vapor without melting into liquid water first. This process is commonly observed in cold, dry conditions where snow can slowly disappear even when temperatures remain below the freezing point. Sublimation is a phase transition process in which a substance transitions directly from the solid phase to the gas phase without passing through the intermediate liquid phase. This means that when a solid undergoes sublimation, it converts directly into a vapor without melting into a liquid first. Sublimation occurs when the vapor pressure of the solid phase exceeds the atmospheric pressure at a given temperature. At this point, molecules of the solid gain enough kinetic energy to break free from the solid lattice and enter the gas phase. Unlike melting or evaporation, sublimation occurs at temperatures below the substance's melting point and without the input of external heat. Several substances exhibit sublimation under certain conditions. Common examples include:

1. Dry Ice (Solid Carbon Dioxide): Dry ice is a well-known example of a substance that sublimes. At atmospheric pressure, dry ice sublimes at -78.5°C (-109.3°F), transitioning directly from a solid to a gas.
2. Iodine: Solid iodine sublimes at room temperature and atmospheric pressure, producing a violet-colored vapor.
3. Camphor: Solid camphor sublimes at room temperature, producing a distinctive aroma.

Sublimation has various applications in industries such as freeze-drying (lyophilization), where it is used to preserve delicate substances like food and pharmaceuticals, as well as in the production of certain chemicals and in scientific processes. In summary, sublimation is the process by which a solid substance transitions directly into a gas phase without passing through the liquid phase.