This activity illustrates how the diffusion of a gas into a liquid can cause ocean acidification. It also models part of the short-term carbon cycle—specifically the interaction between our atmosphere and the ocean’s surface.
Mixing vinegar and baking soda together in the paper cup creates carbon dioxide gas (CO2). The CO2 gas then diffuses into the liquid below. When CO2 gas diffuses into water, the following chemical reaction takes place and results in carbonic acid (H2CO3):
CO2 (aq) + H2O → H2CO3
Carbonic acid dissociates into H+ and HCO3-. The increase in H+ causes the solution to become more acidic.
Carbonic acid is a weak acid. Even so, the presence of this acid affects the pH of the solution. Thus, after a short time, the surface of the indicator solution changes color: from blue to yellow if you’re using bromothymol blue or from purple to pale pink if you’re using cabbage-juice indicator. This color change indicates a pH change caused by the diffusion of CO2 gas into the liquid.
Outside of your paper cup, on a much larger scale, atmospheric CO2 diffuses into the oceans.¹ Oceans are the primary regulator of atmospheric CO2. Human activities such as burning fossil fuels and changes in land use have increased the amount of carbon dioxide (CO2) in the atmosphere from 540 gigatons of carbon (Gt C) in pre-industrial times to 800 Gt C in 2015.
Current atmospheric CO2 levels are greater than they have been in 800,000 years, and as a result, the fast carbon cycle is no longer in balance. From 1860 to 2009, the oceans absorbed an additional 150 Gt C from the atmosphere.
The CO2 taken up by the oceans reduces oceanic pH through a series of chemical reactions. The first of these is the reaction you just observed: the creation of carbonic acid via the diffusion of CO2 gas into water.²
In pre-industrial times, the pH of the oceans was close to 8.2. In 2005, it was approximately 8.1.³ While the pH of the ocean is still basic, it is more acidic than it used to be. Since the pH scale is logarithmic, this means that the oceans are 30% more acidic now than they were in pre-Industrial times.⁴