Question

Moisture contained by a substance in excess of equilibrium moisture is called______

• A. unbound moisture
• B. critical moisture
• C. bound moisture
• D. free moisture

Hint:

Remember that "free moisture" represents the maximum amount of water that can be evaporated from a solid under a given set of drying air conditions.
It is a relative property, whereas "bound" and "unbound" moisture are absolute properties of the material.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks for the terminology used in drying operations to describe moisture in a solid that is in excess of the equilibrium moisture content for a given humidity of the drying gas.

Step 2: Key Formula or Approach:
The total moisture content of a wet solid (\( X_{\text{total}} \)) can be divided into different categories based on thermodynamic equilibrium and ease of removal:
\[ X_{\text{free}} = X_{\text{total}} - X^* \]
where \( X^* \) is the equilibrium moisture content under the specified environmental conditions.

Step 3: Detailed Explanation:

• Free Moisture (\( X_{\text{free}} \)): This is the moisture in excess of the equilibrium moisture content at the given air temperature and humidity.
It is the only moisture that can be removed during a drying process under those specific conditions.

• Equilibrium Moisture (\( X^* \)): This is the moisture content of a solid when it is in thermodynamic equilibrium with the surrounding air of a given temperature and humidity.
It cannot be removed by further drying under these conditions.

• Bound Moisture: This is the moisture trapped inside capillary walls or chemically bound to the solid, exerting a vapor pressure lower than that of pure liquid water at the same temperature.

• Unbound Moisture: This is the liquid moisture held in voids or large capillaries that exerts its full vapor pressure, equal to that of pure water.


Step 4: Final Answer:
Moisture in a solid in excess of the equilibrium moisture is defined as free moisture.