Ocean acidification and shells

Ocean acidification has been called the evil twin of climate change. It is the other carbon dioxide problem. Around a quarter of the carbon dioxide produced by burning fossil fuels dissolves into the ocean where it combines with water to form carbonic acid. While this acid is extremely weak it is nevertheless strong enough to dissolve the calcium carbonate shells of corals, microscopic molluscs or effect other animals. As the acid dissolves calcium carbonate, it releases more carbon dioxide that, in turn, makes more carbonic acid.

You will need

  • 2 eggs
  • 1 litre vinegar
  • 2 small bowls
  • large bowl
  • jug
  • measuring cup
  • spoon
  • salt
  • water
  • ruler

Scientists are starting to see evidence and the consequences of ocean acidification and what it means for marine life.

Did you know that egg shells are made of the same compound from which coral and mollusc shells are made ie. calcium carbonate?

Try the following activity and see if you can make a shell disappear.

What to do

Day one:

  • Put the eggs in the large bowl
  • Pour vinegar into the bowl until the eggs are completely covered. The eggs will start to bubble.
  • Leave the eggs to stand overnight.
  • Create a salt solution in a jug by dissolving as much salt as possible into 500 mls Keep adding salt until a few salt crystals are left on the bottom that will not dissolve, no matter how long you stir.

Day two:

  • The eggs should be soft. Remove from the vinegar and gently brush any remaining shell off the egg until you can see the yolk through the membrane (clear covering).
  • Measure each egg with a ruler.
  • Carefully put one egg into each of the small bowls.
  • Gently cover one of the eggs with the salt solution from the jug.
  • Cover the other egg with the salt solution from the jug.
  • Leave both eggs overnight. Measure the eggs. Have they changed in size?

What’s happening?

Eggshells contain calcium carbonate (CaCO3), and vinegar’s active ingredient is acetic acid (CH3COOH). When these chemicals react together, the result is a salt called calcium ethanoate, some water, and bubbly carbon dioxide gas. The reaction for this equation looks like this:

CaCO3 + 2CH3COOH → Ca(CH3COO)2 + CO2 + H2O

Once an egg’s shell is dissolved by this reaction, its membrane is revealed. This membrane is ‘selectively permeable’, which means it will allow some things through but not others. An egg’s membrane will allow small molecules like water to pass through but not large ones like salt.

The egg left in water will look very different to the one in saltwater due to a process called osmosis. Osmosis occurs when two solutions are separated by a selectively permeable membrane. Water moves by osmosis from a weak (dilute) solution to a strong (concentrated) solution, such as the solution inside the egg. So, when the egg is left in water only, water flows in through the membrane, making the egg expand.

The egg in the saltwater shrinks. This is because the solution outside the egg is more concentrated, so the water flowed out from the dilute solution to the concentrated solution.


(Copyright: Australian Science Teachers Association 2020, except where indicated otherwise. This work is licensed under a Creative Commons Attribution NonCommercial 4.0 International licence.)