Falling rate period of drying can be explained by one/more of the following:
Options:
(A) Capillary rise of water
(B) Molecular diffusion of water
(C) Surface cooling
(D) Surface evaporation
Official Answer Key:
A, B
Step-by-Step Solution & Explanation
During the falling rate period, the surface is no longer fully wetted; the drying rate is no longer set by external conditions but by how fast moisture can be transported from within the solid to the surface. The controlling internal mechanisms are:
(A) Capillary rise (capillary flow) ✔ — liquid water is drawn through the pore network by capillary forces towards the drying surface; this dominates the early falling-rate period in porous materials.
(B) Molecular diffusion ✔ — as moisture content drops further, liquid and vapour diffusion through the solid matrix becomes the governing mechanism (the second falling-rate period).
Why the others are excluded:
(C) Surface cooling and (D) Surface evaporation are the mechanisms of the constant rate period, in which the saturated surface behaves like a free water surface, evaporates at the wet-bulb temperature, and the rate is set by external heat and mass transfer — not by internal movement.
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