Radioactive Tracers

An equilibrium is first established with normal hydrogen, iodine, and hydrogen iodide in the flask on the left.  The flask on the right is a storage flask, used to keep some radioactive iodine.  Since it is hard to see the molecules while they are moving around, let's stop their motion for a moment.

You can see that there are only molecules of hydrogen, iodine, and hydrogen iodide on the left, and only molecules of radioactive iodine on the right.

Now, let's open the valve for a moment, which will allow some radioactive iodine to move into the flask on the left.  In order to maintain equal pressures, some non-radioactive iodine will also move to the right hand flask.  Now, let's close the valve.  before any chemical reaction has occurred, the system would look like this: a mixture of non-radioactive hydrogen, iodine and hydrogen iodide, with a small amount of radioactive iodine in the same container.  Equilibrium is still in place.

If we let the system sit for a time, then freeze the molecular motion once more, we can see that some of the radioactive iodine has become part of the hydrogen iodide.  This is only possible if the reaction has continued to occur, even though the reaction was at equilibrium the whole time.