The inexorable rise in atmospheric carbon dioxide impacts not only global warming but also the acidity of the ocean. Increasing ocean acidity causes a decline in carbonate (as it is protonated), which in turn will negatively impact the ability of calcifying marine organisms to build their calcium carbonate shells. A simple set of equilibrium thermodynamic calculations appropriate for first-year chemistry students shows that, based on carbonate concentration predictions from reliable models, calcium carbonate shell deposition could become nonspontaneous in the Southern Ocean before the end of this century. The resulting decline in calcifying plankton could in turn lead to ecosystem collapse in this region. For physical chemistry students, effects of seawater salinity on equilibrium concentrations of carbonate can be explained using activity coefficients and ionic strength. Also, differences in the carbon dioxide/carbonate system in polar vs tropical oceans are explained by a combination of thermodynamics and the fact that carbon dioxide and carbonate concentrations are maintained out of equilibrium by biological and physical processes.