Heat Transfer & Thermodynamics Relative to Water and Wood

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In our ChemE lab, we were asked to place a 25 cm x 25 cm x 1 cm (thick) block of wood in a water bath.....the rate of water absorption into the wood block is known to be 8 grams/100 cm^2 at 20 degrees C --- and the wood block is assumed to be at normalized ambient temp as well and completely dry or free of moisture before entering the water bath.

Now...when we change the temperature of the water bath from 20 degrees C to 30 degrees C, the rate of water absorption changed and the wood absorbed more water in the same unit time. Why? Help me understand?

In this case, it's got to do with the diffusion coefficient. As the Fick's law on diffusion clearly states:
The diffusion coefficient at different temperatures is often found to be well predicted by

An approximate dependence of the diffusion coefficient on temperature can often be found using Stokes-Einstein equation, which predicts that:

where:
T1 and T2 denote temperatures 1 and 2, respectively
D is the diffusion coefficient (m²/s)
T is the absolute temperature (K),
μ is the dynamic viscosity of the solvent (Pa?s)
As you can see, the higher the temperature, the higher the diffusion coefficient of water into the wood block so you have more absorption of water into the wood.
http://en.wikipedia.org/wiki/Fick%27s_law_of_diffusion#Temperature_dependence_of_the_diffusion_coefficient

Second....When we pre-conditioned the block of wood and raised its temp to 30 degrees C before placing it in the 20 degree C water bath, it also absorbed more water than the original control....again why?

The same concept. Either raising the wood or the water temperature will raise the equilibrium temperature and thus increasing the absorption coefficient, leading to increasing absorption.

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