by the Rev. Philip Foster
I have written about this before but it bears repeating. When you were at school and were doing chemistry, I hope you were paying attention at least to the simple test used for detecting CO2. The gas was tested for by bubbling it through lime water (a solution of Ca(OH)2 otherwise known as Calcium hydroxide).
Now you will remember that this clear solution of limewater would turn cloudy white if CO2 was the gas bubbling through it. This I trust you all remember, but hopefully, your chemistry teacher would have explained that the Calcium hydroxide (an alkali) had reacted with the mildly acidic carbonic acid (CO2 dissolved in water) to make Calcium Carbonate which was the insoluble white cloudiness. Calcium carbonate is a very common mineral – limestone – made by sea creatures in the past and present: such as corals as well as sea shells etc. One of the commonest rocks on the planet and very common on seabeds and, as mentioned before, is the main constituent of coral reefs both dead and living. So far so good.
Now, if you didn’t fall asleep (and your chemistry teacher was up to snuff), the teacher would say, “Pay attention as I go on bubbling CO2 through the cloudy limewater. Watch what happens.” and, behold, a strange thing happened: the cloudiness disappeared and the limewater once more became clear. Why? Because a second reaction took place as more CO2 was added. The insoluble Calcium carbonate was attacked by more carbonic acid (CO2 and water, remember) to make a new chemical called Calcium bicarbonate (sometimes called Calcium hydrogen carbonate) which is soluble in water and – here is the key thing – it is alkaline in solution. As more CO2 dissolves in seawater, particularly around coral reefs, then a little of the the solid Calcium carbonate in the reefs will dissolve in the seawater making it more alkaline, not less.
This aids new growth as Calcium using organisms can easily absorb the Calcium bicarbonate to make more living coral and more seashells generally. That is why coral reefs have never suffered from ‘too much’ CO2 in the past many millions of years of growth. They can suffer from too little!