 A system can only come to equilibrium if nothing is lost from it. Consider a pot of water boiling on the stove. The water is at 100 degrees Celsius and the water molecules are evaporating. That is, they're moving from the liquid state to the gaseous state. If we put a lid on the pot, then gaseous water molecules are trapped inside. Some of those water molecules are going to condense back into liquid. They might do it on the inside of the lid or the walls of the pot or directly back into the water at the bottom of the pan. This system can come to equilibrium. When it does, the rate of evaporation will equal the rate of condensation. So as long as no water escapes, this can go on indefinitely. However, if the lid is taken off the pot, gaseous water can escape into the surrounding air. The chance that these molecules, these escaped molecules, will condense back into the pot are effectively zero. So for the water in the pot, the rate of condensation will drop very low and will never equal the rate of evaporation. This means the system can't come to equilibrium. The effect is that the liquid water will gradually boil away until the pot is dry. We summarise this by saying that equilibrium can only be reached in a closed system. An open system, that is one where there can be an exchange of matter in or out, cannot come to equilibrium. This applies to both physical and chemical systems. A chemical reaction involving gases, for instance, will never come to equilibrium if the gases are able to escape. It would have to be held in a sealed container to reach equilibrium. On the other hand, a chemical reaction in which all the species are aqueous is very convenient for studying equilibrium, because the reactants and products are effectively trapped or hemmed in by the water in which they are dissolved.