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Ocean Acidification in a Cup: Complete Activity Guide

Change the atmosphere to change the water below.

Create a carbon dioxide-rich atmosphere in a cup and watch how it changes the water beneath it. This model of ocean–atmosphere interaction shows how carbon dioxide gas diffuses into water, causing the water to become more acidic. Ocean acidification is a change that can have big consequences.

Tools and Materials

  • Safety goggles
  • An acid-base indicator, such as Bromothymol Blue or purple cabbage juice indicator, that is diluted with water: 10 ml Bromothymol Blue (0.04% aqueous) to 1 liter of water (see the Teaching Tips section below for alternatives)
  • Two clear 10 oz plastic cups (the tall ones)
  • Paper cups—3 oz size (you’ll only use one in the experiment, but keep a few extras at hand just in case)
  • Masking tape
  • Plain white paper
  • Permanent marker
  • Baking soda
  • White vinegar
  • Two Petri dishes—to use as lids on the plastic cups
  • Graduated cylinder (or measuring spoons)
  • Gram scale (or measuring spoons)


First things first: put on your safety goggles.
Pour 40–50 ml of acid-base indicator into each of the two clear plastic cups. One cup will be your control.
Add 2 g (about half a tsp) of baking soda to the paper cup.
Tape the paper cup inside one of the clear plastic cups containing the acid-base indicator so that the top of the paper cup is about 1 cm below the top of the clear cup. Make sure the bottom of the paper cup is not touching the surface of the liquid in the plastic cup—you don’t want it to get wet!
Place both clear plastic cups onto a sheet of white paper and arrange another piece of white paper behind the cups as a backdrop—this makes it easier to see the change.
Add 6 ml (a little more that 1 tsp) of white vinegar to the paper cup holding the baking soda. Be very careful not to spill any vinegar into the acid-base indicator. Immediately place a Petri dish over the top of each plastic cup.

To Do and Notice

Move your eye to the surface level of the acid-base indicator and observe the liquid’s surface. What do you see? Where is the color change taking place?
After a few minutes have passed, you should notice a distinct color change at the surface of the liquid. As you continue to observe the cup, the liquid in other parts of the cup will also begin to change color.

What’s Going On?

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.
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 now. Because the CO2 amounts in the atmosphere are greater than they have been in 800,000 years, the carbon cycle is not in balance. As a result, from 1860 to 2009, the oceans absorbed an additional 150 Gt C from the atmosphere.
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 result is carbonic acid (H2CO3) —the following chemical reaction takes place: 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 pH indicator at the surface 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. They have absorbed half of all the CO2 from anthropogenic sources produced between 1800–1994 and one-third of the anthropogenic carbon produced between 1980–2000.2 The CO2 taken up by the oceans reduces oceanic pH through a series of chemical reactions. The first of these is the reaction you’ve just observed: the creation of carbonic acid via the diffusion of CO2 gas into water.2
The pH of the oceans was close to 8.2 in pre-industrial times. 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

Going Further

Diffusion goes both ways—from the atmosphere into the liquid and from the liquid into the atmosphere. This experiment shows passive diffusion: the CO2 gas diffuses into the liquid. What experiment might you try in order to show that diffusion also goes the other way—from the liquid back into the atmosphere?
In March 2015, the global monthly average of the atmospheric concentration of CO2  was around 400 parts per million (ppm)—or 0.04%. It is a small amount, but it is increasing by more than 2 ppm every year due to the combustion of fossil fuels—oil, gasoline, natural gas, coal—and land use changes, such as deforestation.
Increases in the concentration of atmospheric CO2 have led to increases in the concentration of CO2 and other carbon-containing molecules in seawater. The CO2 added to seawater reacts with the water molecules to form carbonic acid in a process known as ocean acidification. The oceans are absorbing about 25% of the CO2 we release into the atmosphere each year. Additionally, as more CO2 gas enters the atmosphere, the atmosphere gets warmer, causing global temperatures to rise.
Ocean acidification is expected to impact ocean species to varying degrees. Photosynthetic algae and seagrasses may benefit from higher CO2 conditions in the ocean, as they require CO2 to live (just like plants on land). On the other hand, studies have shown that a more acidic ocean environment has a dramatic effect on some calcifying species, including oysters, shellfish, clams, sea urchins, shallow water corals, deep sea corals, and calcareous plankton. When shelled organisms are at risk, the entire food web may also be at risk.

Teaching Tips

Prior to trying Ocean Acidification in a Cup, learners should be familiar with acid-base indicators and know that baking soda and vinegar create CO2 gas when mixed. This lesson dovetails with prior lessons on surface interactions and diffusion.
Making your use cabbage juice indicator
If Bromothymol Blue indicator is hard to come by, or if you’d prefer not to use this chemical in your classroom, you can use cabbage juice indicator instead. It’s easy to make: just take a quarter head of purple cabbage, place it in a blender with water to cover, and blend until you get a uniform puree. Strain the resulting mixture—the purple liquid you’re left with is your cabbage juice indicator. Dilute it with some water and proceed with the experiment, using it instead of Bromothymol Blue. You will need to experiment with the ratio of water to cabbage juice to see what dilution gives you good results. Unlike Bromothymol Blue, cabbage juice indicator turns pink, not yellow, in the presence of an acid.


Thanks to Chris Sabine of NOAA’s Pacific Marine Environmental Lab for his expertise.
Thanks to Jim Butler of University of California at Berkeley for his expertise.

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