During the constant rate period of drying of a solid,

ANS:C - surface evaporation of unbound moisture occurs.

"Surface evaporation of unbound moisture" refers to the process by which moisture present on the surface of a solid material evaporates into the surrounding air when the solid is exposed to suitable drying conditions. This phenomenon typically occurs during the latter stages of the drying process, specifically during the falling rate period. Here's a breakdown of the process:

1. Unbound moisture: Unbound moisture refers to the moisture present on the surface of the solid material that is not chemically or physically bound to the solid structure. This moisture is relatively easy to remove and is typically the first to evaporate during the drying process.
2. Surface evaporation: As the solid material loses moisture during the drying process, the concentration of moisture at the surface decreases. When the moisture concentration at the surface is higher than the equilibrium moisture content corresponding to the surrounding air conditions (e.g., temperature, humidity), evaporation occurs.
3. Falling rate period: Surface evaporation of unbound moisture is most prominent during the falling rate period of drying. In this stage, the drying rate gradually decreases as the moisture content of the solid decreases and approaches equilibrium with the drying conditions. During this period, the rate of moisture removal is limited by factors such as diffusion of moisture from the interior of the solid to the surface.
4. Drying conditions: Surface evaporation of unbound moisture depends on the drying conditions, including temperature, humidity, and airflow. Higher temperatures and lower relative humidity levels enhance evaporation rates by increasing the vapor pressure difference between the solid surface and the surrounding air.