Time To Read
Date Last Modified
Place 01 · The Great Barrier Reef · Chapter 5 of 7
Building With a Dissolved Gas
The reef is made of air. Follow the carbon and you can watch it happen.
About 20 minutes · the longest chapter · worth it
Start here
A gas goes in, and rock comes out
Carbon dioxide dissolves into seawater, runs through a short chain of reactions, and comes out the other end as a solid. Corals grab it and lay it down as limestone. This is the chapter with real chemistry in it, and it is the one that explains the news headlines.
Watch this
See it move
Carbon Brief — How does ocean acidification affect coral reefs? A short, clear explainer.
While you watch, look for
- Where the extra carbon dioxide comes from
- What it does to the building blocks corals need
- Something you did not expect. Hold onto it.
The short version
Three things going on here
Step one
Gas becomes acid
Carbon dioxide dissolves and makes carbonic acid, the same acid in soda. That acid lets go of a hydrogen ion, and that loose ion causes all the trouble.
Step two
pH is a headcount
pH is just a count of loose hydrogen ions. More ions means a lower pH. Surface seawater has dropped about 0.1 since the 1800s, which is about 30% more hydrogen ions.
Step three
Omega is the budget
Saturation state, written with the Greek letter omega, is how easy the water makes it to build. Healthy reefs sit near 3.5 to 4. Below about 3, building slows.
Photos: Fuka jaz, Wes Baker, Tom Fisk / Pexels
Play with it
Test the pH of real things
Another real simulator. Pick a liquid, add water, and watch the pH change. Then look at how many hydrogen ions are floating around.
Simulation: pH Scale, PhET Interactive Simulations, University of Colorado Boulder (CC BY 4.0).
Two questions to answer while you are in there
- Which liquid in the simulator is closest to seawater’s pH of about 8.1?
- When pH drops by one whole number, what happens to the number of hydrogen ions?
Try it yourself
Check yourself
Try it
Follow the Carbon
Drag the words into the story to follow one carbon atom from the sky into a coral skeleton, and see what happens when we add too much.
Try it · interactive
Carbonate Balance
Turn up the carbon dioxide and watch what happens to the water and the coral.
Open the Carbonate Balance in its own window (opens in a new tab).
Words you will hear
Four terms, that is all
Carbonate ion
The coral’s building block
Bicarbonate
What carbonate becomes when hydrogen grabs it
pH
A count of loose hydrogen ions
Saturation state
Omega, the coral’s building budget
Check your understanding
Practice the whole chapter
Everything in this chapter, in eight quick questions. Retry as often as you like. Read the feedback on every answer, right or wrong, because that is where the learning is.
Practice
Practice: Building With a Dissolved Gas
Eight questions covering the video, the three cards, the four terms and the interactive above.
If you want more
Go deeper
- Webb 5.5 — Carbon Dioxide, pH and Ocean Acidification — LibreTexts · 10 minutes · read it twice
- NOAA PMEL — What is ocean acidification? — Plain-language and well illustrated · optional
Photo credits (Pexels): Жанна Алимкулова, hartono subagio, Fuka jaz, Wes Baker, Tom Fisk.