Is calcium carbonate really a superstuff? We meet it in chemistry lessons as a white powder that fizzes in acids giving off carbon dioxide. It doesn’t seem special and yet it is closely involved with the evolution of life, the changing Earth and the history of mankind.
Calcium compounds in the Earth’s crust slowly dissolved as oceans began to condense. At the same time carbon dioxide in the air dissolved in the water. The salty oceans acquired small concentrations of calcium ions (Ca2+) and hydrogen carbonate ion (HCO3-). Single-celled life-forms took in these ions and, converting the hydrogen carbonate to carbonate (CO32-), precipitated out calcium carbonate (CaCO¬3 ) as a protective coating. Later, multi-celled creatures performed the same feat becoming shell-fish. When all these creatures died they fell to the bottom of the ocean. Over millions of years the sediment of calcium carbonate was compressed into thick layers of limestone and chalk. Sometimes heat and pressure deep in the crust partially melted the calcium carbonate and when it cooled it became marble. Occasionally the calcium carbonate melted completely and solidified as clear crystals of calcite.
Today about 10% of the rocks on the Earth’s surface are forms of calcium carbonate; almost all of it made by living creatures over hundreds of millions of years. The landscapes we see around us from the white cliffs of Dover to the limestone pavements of Yorkshire are a result of the combination of calcium ions and carbonate ions in living cells.
You might think of rocks as being insoluble but water only needs to be slightly acidic and calcium carbonate will start to dissolve. Rainwater containing dissolved carbon dioxide dissolves limestone and chalk to make potholes and caves, some of which formed the homes of early humans. When the water evaporates the calcium carbonate precipitates forming stalactites and stalagmites.
In the 1750s the Scottish scientist, Joseph Black, investigated the reactions of calcium carbonate. His discoveries, which included the gas carbon dioxide, led to a new understanding of gases and the birth of modern chemistry
Mankind has found uses for calcium carbonate since the earliest civilisations. The pyramids of Egypt were built over four thousand years ago from blocks of limestone and covered with a hard, white, polished surface of a rock halfway to becoming marble. Great buildings across the world were built from limestone or marble right up to the present.
Today we are more familiar with concrete as the building material of choice. Concrete is made from pieces of rock and cement. Cement is a mixture of clay and limestone heated together in a kiln. Almost 3 billion tonnes of cement are produced worldwide each year. Unfortunately the process releases almost the same mass of carbon dioxide and is a major contributor to global warming.
Calcium carbonate has other uses. Farmers spread it on fields to neutralise acidic soil and coal-fired power stations react it with sulfur dioxide to prevent the formation of acid rain. It is also used as an anti-acid in indigestion remedies. In fact calcium carbonate is used in many foods to neutralise acidic ingredients, as a raising agent (giving off carbon dioxide when mixed with a weak acid), to add bulk to foods, but most importantly it is a source of calcium, an essential element for cells and for building bones.
Perhaps you will agree that calcium carbonate is pretty super but as a final example of its special properties, have a look through a calcite crystal. It has the property called “birefringence”. This means it produces a double image.
Activities
- As well as sea creatures, birds also use calcium carbonate to make shells. Investigate the reactions of egg shells with an acid such as vinegar. (Wear goggles when doing experiments.)
- Find out about Joseph Black and his experiments with chalk (calcium carbonate), lime (calcium oxide) and “fixed air” (carbon dioxide). How important do you think his work was?
- Make a list of foods that contain calcium carbonate (chalk) and find out the reason for it being added.
- Explore the optical properties of calcite crystals. If you can’t get a crystal yourself you can search for images using the terms birefringence and calcite.
- Hardly any calcium carbonate existed on Earth before life evolved. Discuss how life has contributed to changes on the Earth.
- Many of the uses of calcium carbonate result in the release of carbon dioxide gas. Discuss the contribution of these processes to global warming and suggest solutions.
- (Advanced level). (a) Find out the structure and bonding of calcium carbonate and how the atoms are arranged in calcite crystals. (b) The chance of shells of sea creatures reaching the bottom of the sea depends on the pH of the water. Find out how pH affects the solubility of calcium carbonate and the consequences of increasing acidity of the oceans caused by carbon dioxide emissions.
Peter Ellis,
August 2012.
