Question

Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): During constant-rate drying period, surface moisture is completely removed.
Reason (R): The rate of drying is controlled by internal diffusion of moisture.
Choose the correct answer from the options below:

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A)
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A)
• C. (A) is correct but (R) is not correct
• D. (A) is not correct but (R) is correct

Hint:

Constant-rate → external control, Falling-rate → internal diffusion control.

Answer 1

The Correct Option is C

Concept: Drying occurs in two major stages:
• Constant-rate period
• Falling-rate period

Step 1: Understanding constant-rate period.
In this stage:
• Surface of the material remains saturated with water
• Evaporation occurs as if from a free water surface
• Surface moisture is continuously supplied from inside Thus, surface moisture is effectively removed continuously. Hence, Assertion (A) is correct.

Step 2: Understanding rate control.
During constant-rate period:
• Drying rate is controlled by external conditions (air velocity, temperature, humidity) Internal diffusion becomes important only in:
• Falling-rate period

Step 3: Evaluating Reason (R).
Reason states drying is controlled by internal diffusion. This is incorrect for constant-rate period.

Step 4: Final reasoning.
Thus:
• Assertion is correct
• Reason is incorrect Final Conclusion:
Option (3) is correct.