It can be shown that, if some solid radioactive iodine is added to the system:I2(s) ⇆ I2(g) the iodine vapour also becomes radioactive. Why is this so?

When solid radioactive iodine is added to the system, it undergoes a phase change from a solid (I2(s)) to a gas (I2(g)). During this phase change, the iodine atoms gain enough energy to break the intermolecular forces holding them together in the solid state. As a result, the iodine atoms become more mobile and are able to move freely in the gas phase.

Since the solid radioactive iodine is added to the system, the radioactive atoms are also present in the solid phase. When the solid iodine undergoes vaporization, the radioactive atoms are carried along with the iodine vapor. This means that the iodine vapor also becomes radioactive.

The reason for this is that the radioactive atoms in the solid iodine have unstable nuclei, which undergo radioactive decay. During this decay process, the unstable nuclei emit radiation in the form of alpha particles, beta particles, or gamma rays. When the solid iodine vaporizes, the radioactive atoms are released into the gas phase, and they continue to undergo radioactive decay.

As a result, the iodine vapor becomes radioactive because it contains the radioactive atoms that were originally present in the solid iodine. This phenomenon is observed when solid radioactive iodine is added to the system and it undergoes vaporization.