Why does the pH of neutral water drop when it is left out in an uncovered container on a worktop for a long period of time?

This question tests your knowledge on; the properties of non-metal oxides in solution, the concept of equilibrium, pH.

Non-metal oxides such as (CO2, P4O10, SO2, etc) are acidic in solution. When left out on the worktop, some of the CO2 dissolves in the water to reach the position of equilibium as both the CO2 and H2O are present in high concentrations. As this position of equilibrium is approached, more CO2 overall has dissolved and so the neutral solution has become more acidic. This is due to the concentration of H+ increasing and by using the equation for pH it can be deduced that the pH decreases.