Q1: Pick out the correct statement.

ANS:D - The driving force for mass transfer in a wet solid is its free moisture content.

The driving force for mass transfer in a wet solid is indeed its free moisture content. When a solid material contains moisture, there exists a concentration gradient of moisture between the surface of the solid and the surrounding environment. This concentration gradient creates a potential for moisture to move from regions of higher concentration (the interior of the solid) to regions of lower concentration (the surrounding environment). The free moisture content, also known as the unbound moisture or excess moisture, refers to the moisture content of the solid that is not bound to the solid matrix and can be readily removed during drying. This free moisture content acts as the driving force for moisture transfer from the solid to the surrounding environment. During the drying process, energy is supplied to the wet solid (typically in the form of heat) to increase the temperature and facilitate the transfer of moisture. As the temperature increases, the vapor pressure of the moisture within the solid also increases, promoting the release of moisture from the solid surface into the surrounding air. Therefore, the free moisture content represents the driving force for mass transfer in a wet solid, as it determines the rate at which moisture can be transferred from the solid to the surrounding environment during the drying process.