Ocean acidification explained with dry ice

Dear imaginary readers,

It took ages for me to write a post as I am about to embark in a very exciting new adventure in the very near future.

I wanted to write this post straight after I was back from Into the blue in Manchester last October, but my worst enemy, the God of Postponing won the battle.

If you remember I was there with the Environmental Chemistry Group of the Royal Society of Chemistry working as a volunteer to introduce the public to the real problems the environmental chemists work on.

We had three stalls with different “hands on” interactive activities for kids (even though adults seemed to enjoy as well).

The first and the second were about environmental pollution, with a tray full of soil and small pieces of plastic dispersed in it and simple nitrates and phosphates testing kits.

into the blue 2

The real queen of the activities we proposed was the ocean acidification demonstration. It does involve smoke and colour changes so it does automatically attract the public attention.

In the rest of the article you can find the procedure, materials and adaptations you can use to realize the experiments in secondary schools, lab or even in the streets, as we did!

First of all we assessed what the audit knows about pH, asking questions and showing a pH colour scale. Then we encourage people to test different thing like drinking water, “fake” pre-made ocean water (pH about 8.2, adjusted with NaOH), coke, orange juice, milk and soap solution with bromothymol blue, litmus paper or any other pH indicator.

Ask the spectators if they know where seawater fits in on the spectrum. Explain that pH is measured on a logarithmic scale like earthquakes and a small change in pH from 8.2 to 8.1 corresponds to an increase in acidity of about 30%.

For this activity, the audience should understand that when humans release carbon dioxide into the atmosphere by burning fossil fuels, some of the carbon dioxide is absorbed by the ocean. This changes the chemistry of seawater making it more acidic. About one third of the carbon dioxide released into the atmosphere over the past 200 years has been taken up by the ocean.

Then we started to explain why ocean acidification is a current and future problem. The average coastal ocean pH is 8.2, but it is changing because of the addition of carbon dioxide to the atmosphere and the subsequent absorption of that carbon dioxide into the ocean. About 30% of the carbon dioxide produced daily is absorbed by the ocean. The pH of the ocean has decreased 0.1 in the last century; it is becoming more acidic (less basic). Some of the organisms at greatest risk include larva and shell-forming animals at the base of the food web that provide food for larger species. Organisms faced with the stress of ocean acidification can migrate, acclimate or go extinct and values of 7.8 are expected by 2100 for the average ocean water, representing a doubling of acidity. Additional stressors that increase the impact include temperature increase and habitat loss.

Then you show the spectators the dry ice in a watch glass pointing out that is the solid form of CO2 and that is changing to a gas (sublimation) at room temperature, so its temperature is about -78 C so they should never touch it with bare hands. When it is added to water it rapidly changes to the gas form and some of this gas becomes dissolved in the water.

We then added enough dry ice to a conical flask containing the “ocean water” and bromothymol blue and the solution will turn progressively yellow as the pH decreases because of the formation of carbonic acid that acidifies the solution.

Because of the quick reaction and the big temperature difference between the “sea water” and the dry ice there will be quite a bit of what looks like smoke or fog billow away from the conical flask (it is actually vapour). The kids will go really excited (some of them even scared) and you will definitely get some “oooooooooooooooh”.



When the reaction has finished and the spectators have regained their concentration, we tried to recap what they saw and why it matters to the ocean’s environment. Adding carbon dioxide to water makes it more acidic or less basic. This is what is happening to the ocean. Humans add carbon dioxide to the atmosphere by burning fossil fuels (driving cars, creating electricity, etc), deforestation, and in many other ways. The ocean then absorbs some of what gets emitted into the atmosphere, sometimes we say that the ocean acts as a “sink” for carbon dioxide. This effect is worsened by the global warming of the planet. The change in the chemical average composition of the ocean’s water, have consequences on many marine creatures, such as shell forming organisms such a corals, bivalves (clams, mussels and oysters) and pteropods (free swimming snails) that are sensitive to changes in pH.

If you want to do this demo as part of a wider lesson in schools, you can add a detailed explanation of the different reactions the CO2 gives in different environments and related demonstrations (acidic rain, weathering of rocks, cave formation)

For the ocean acidification the chemical reaction is the following.

CO2 + H2O      →        H2CO3

As shown in the chemical reaction scheme, when carbon dioxide (CO2) dissolves in seawater, it creates carbonic acid, which releases bicarbonate ions (HCO3-) and hydrogen (H+) ions into the water. The hydrogen ions make the seawater more acidic (lowering its pH). In addition, some of the hydrogen ions react with carbonate ions (CO3=) already in the seawater to create more bicarbonate. This reduces the amount of carbonate (an important mineral for building shells) dissolved in the seawater. The impacts of ocean acidification and lower carbonate ion concentrations on marine ecosystems include reduced growth of organisms that form calcareous skeletons or shells.

Ocean acidification and global warming are relegated to the science fiction shelves by one of the more powerful politician of the world, at a point in the life of our planet when the consequences of the increase of CO2 released into the atmosphere are about to spiral up culminating in a very hostile environment for humans and every other living thing.

For this reason is more important than ever to engage in outreach activities trying to explain to as much person as possible that our choices have a very big and dangerous impact on the place where we live, and that it is important to act on a personal and a political basis.

Time is now!

“I think that once people understand the great risks that climate change poses, they will naturally want to choose products and services that cause little or no emissions of greenhouse gases, which means ‘low-carbon consumption.’ This will apply across the board, including electricity, heating, transport and food.” Nicholas Stern, IG Patel Professor of Economics and Government, Chairman of the Grantham Research Institute on Climate Change and the Environment and Head of the India Observatory at the London School of Economics.

Have a lovely end of summer everyone, but always try to be conscious! Hasta la vista




Femmes of STEM Podcast Shares Stories of Women in Science

Free Radicals

by Sophie Duncan

An illustration of femme scientists from a variety of fields using a microphone with the words ‘Femmes of STEM’ to break through a glass ceiling of Science History. Artwork by Keshy Jeong.

Breaking down the false narrative around the absence of women in STEM.

August 2, 2017

Although I know that white cis-males are not the only historical scientists of note (because that would be absurd), that doesn’t mean I know the names and stories of the scientists left out of his tory. However with my new favorite podcast, I am learning the names and stories I knew had to exist, but did not know where to find. Femmes of Stem, a bi-weekly podcast, tells the story of all-star femme scientists from history. In March I was lucky enough to interview the creator of the podcast, Michelle Barboza-Ramirez, who is retelling science history, one episode at a…

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