Acid and shells

 Although the oceans won’t become acid (pH less than 7) the amount of acid in the ocean is increasing. As you’ll have read on the Science pages this is harmful to any animals or plants that make shells out of calcium carbonate because calcium carbonate actually begins to dissolve in seawater before the seawater reaches “acidic” conditions. Although this dissolving of shells at pH levels between 7 and 7.4 does happen slowly it can still be a problem for many organisms.

Try an experiment for yourself:

1. Get a can of cola (soft drink) and pour it into a jar

2. Add a chicken egg to the jar (uncooked)

3. Leave the egg soaking in the cola over night

4. See what has happend to the shell of the egg…

The egg shell should have dissolved away leaving just a fine, almost transparent, membrane that holds the insides of the egg in place. 

The shell of these chicken eggs are made up of approximately 95% calcium carbonate. Shells of snails, crabs, algae, sea urchins and many other marine creatures are made up of different amounts of calcium carbonate and so they all dissolve at different rates, this is why different creatures seem to react differently to changing pH levels, with some organisms (like sea urchins) seemingly being more vulnerable than other organisms (like crabs).

Cola is actually quite acidic (pH around 3) so you could also try this experiment with other household products that have different levels lof acidity to see whether the egg shell dissolves more quickly or more slowly; how long does it take at each pH level. 

An approximate pH scale using household items is:

Lemon juice pH = 2; Cola pH = 3 (vinegar pH = 3); Tomato juice pH = 4; Black coffee pH = 5; Urine pH = 6; freshwater pH = 7; seawater pH = 8; Bicarbonate of soda pH = 9; Household ammonia pH = 11; Household bleach pH = 12.

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