We have used the word equilibrium a number of times in the course.
When we looked at vapor pressure we introduced the idea that in a closed container liquid molecules which escaped into the gas phase might eventually return to the liquid phase to be replaced by others.
When we discussed various solution conditions we noted that in a saturated solution the undissolved solute was always slowly dissolving even as dissolved solute in the solution was slowly reprecipitating.
Both of these situations illustrate some important ideas concerning equilibrium but they do not reveal all of its ins-and-outs. These are physical equilibria. There is no change in substance occurring. But chemical equilibrium also exists and it is to that topic we now turn our attention as we begin to answer the third of the four BIG questions posed at the beginning of the course: if a reaction occurs, when will it stop?