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2
Too much or too little carbon dioxide: acidification and shells
Integrated Science Sec 1 Term 1, Unit 1: pp. 43–55
GoalExplain how extra CO₂ acidifies water and weakens shells, and what too little CO₂ does.
New words
acidification · a fall in the pH of water towards neutral, caused here by dissolved CO₂carbonic acid · H₂CO₃, a weak acid formed when CO₂ dissolves in watercalcification · building a shell or skeleton from calcium carbonatecalcium carbonate · CaCO₃, the slightly soluble substance of shells, corals and chalk
Explanation
When more CO₂ dissolves in water, it forms carbonic acid: CO₂ + H₂O ⇌ H₂CO₃. The pH falls. Sea water is slightly basic, about pH 8.1 (extra), so “acidification” means that it moves towards neutral, not that it turns into an acid. Lower pH harms many species, especially eggs and larvae. Corals and oysters have an outer skeleton or shell of calcium carbonate, which they build by calcification. Extra CO₂ turns slightly soluble CaCO₃ into soluble calcium bicarbonate: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂, so building and keeping shells gets harder. Shrimp shells are mainly chitin. Bony fish have an inside skeleton of calcium phosphate and collagen, and extra CO₂ upsets their calcium balance and slows growth. Too little CO₂ is also harmful: algae and plants make less sugar by photosynthesis, so less energy reaches the food chain, and the pH rises. The harm of extra CO₂ is the acidity, not a lack of oxygen.
more CO₂ dissolves→more carbonic acid, lower pH→shell material dissolves more easily→shells and coral skeletons harder to build
Worked examples
CaCO3 + CO2 + H2O→Ca(HCO3)2
shell material turns into a soluble substance
Check that CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂ is balanced. Left: Ca 1, C 1 + 1 = 2, H 2, O 3 + 2 + 1 = 6. Right: Ca 1, H 2, C 2, O 3 × 2 = 6. Both sides match.
A pupil writes: “More CO₂ in the sea kills fish because it uses up their oxygen.” Correct this. Gases dissolve independently, so extra CO₂ does not remove the oxygen. The harm is that CO₂ forms carbonic acid, which lowers pH, harms eggs and larvae, and makes shells harder to build.
Which is the “acid” and which is the “carbonate”: H₂CO₃ and CaCO₃? H₂CO₃ is carbonic acid, formed when CO₂ dissolves. CaCO₃ is calcium carbonate, the hard material of shells and chalk.
Class activity
Chalk and breath, with a grown-up. (1) Put a small piece of chalk or eggshell in a cup of vinegar and another in a cup of plain water. Describe both after a few minutes. (2) With the red cabbage indicator from week 2, an adult pours a little into a clear cup. You blow gently through a straw into it for a minute (blow out only, never suck in). Describe any colour change and link it to CO₂ forming carbonic acid. Do not taste or sniff.
Practice
1
Write the reaction of CO₂ with water and name the acid.
CO2 + H2O⇄?
CO₂ + H₂O ⇌ H₂CO₃; carbonic acid.
2
Why can extra CO₂ make it harder for oysters to build shells?
It turns calcium carbonate into soluble calcium bicarbonate, so the shell material dissolves more easily.
3
Why is it better to say sea water is “becoming less basic” than “becoming an acid”?
Sea water has a pH of about 8.1; acidification moves it towards 7 but it stays above 7.
4
What happens to the pH of water and to the food chain when there is too little CO₂?
The pH rises; algae and plants make less sugar, so less energy reaches the food chain.
