While reading surface chemistry, I got

Rate of adsorption is high at begining and then decreases till equilibrium is attained . On the other hand , rate of absorption remains same throughout the process .

Regarding adsorption I have no doubts as logically after some time there will be no surface area for upcoming molecules. But how the rate can be same in case of absorption ? Just if we take an example of cotton ball , it absorbs water and after some time being fully wet by water how further the rate of absorbing be same ? Please help if I am lacking somewhere in the concept ...


Your presumption is reasonable, but not complete. The text states, "remains same throughout the process." In your example, when the cotton ball is fully saturated, the process ceases, so the text appears to be correct. You've described the endpoint, where equilibrium has been reached, after the process ends.

However, you might test absorption to see if it behaves according to that general rule:

  • Obtain some agar gel, phenolphthalein indicator, dilute $\ce{HCl}$ and dilute $\ce{NaOH}$ solution.
  • Dissolve the agar in warm water, mixed with a little phenolphthalein indicator and a drop of $\ce{HCl}$, and pour into a vertical test-tube, leaving a few cm empty at top. Wait for the gel to cool to room temperature.
  • Add a few ml of $\ce{NaOH}$ solution at the top.
  • Every minute (or every ten minutes, if this is slow), mark the lower limit of the pink color.

Does the absorption, and diffusion, proceed uniformly, or does it slow with time?


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