Explanation for Equilibrium Example Two

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Naturally occuring iodine consists only of the single isotope I-127. However, I-131, a radioactive isotope is available commercially. Some methyl iodide, CH3I (which is a liquid at room temperature and one atmosphere pressure) was prepared using radioactive iodine and then used in the following experiment.

Side A is filled with radioactive CH3I while side B is filled with the same volume of non-radioactive CH3I and the beaker is left to sit after being tightly covered.

During the course of the experiment, the liquid levels in compartment A and B do not change. After several hours have elapsed, liquid in compartment B is removed and found to be radioactive.

Explain how the non-radioactive CH3I came to have some radioactive CH3I in it, even though the levels of liquid in both compartments did not change.

There are two reactions happening and they are the reverse of each other:

1) Radioactive CH3I molecules are evaporating, then diffusing to side B and dissolving.
2) Non-radioactive CH3I molecules are evaporating, then diffusing to side A and dissolving.

The result over time is that radioactive CH3I moves from side A to side B and non-radioactive CH3I moves in the opposite direction (side B to side A).

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