A rather neglected element on the Association’s Periodic Table, calcium is in fact classed as a minor metal. Say ‘calcium’ and a metallic substance does not immediately spring to mind, rather a chalky powdery rock or possibly a bottle of milk. Calcium is, however, a metal in its pure form, though rarely seen in this state due to its instability. In air, calcium decomposes rapidly into calcium hydroxide and calcium carbonate. Calcium carbonate is the chalky substance we associate with calcium, with the white cliffs of Dover (see right) being composed of this compound.
Although calcium is the fifth most abundant element (and 3rd most abundant metal) in the earth’s crust, present at a level of about 3% in the oceans and soil, it is never found free in nature. Calcium easily forms compounds by reacting with not only air but also with water and acid (on contact with water or acid, hydrogen gas is made in a fairly controlled manner, in comparison to the other alkali metals).
Originally, the scientist Antoine Lavoisier had classified calcium as an ‘earth’ because it seemed impossible to reduce it further, but he suspected it was the oxide of an unknown element. He was correct, and metallic calcium was first isolated by Sir Humphry Davy in 1808 through the electrolysis of a mixture of lime (CaO) and mercuric oxide (HgO). Today, metallic calcium is obtained by displacing calcium atoms in lime with atoms of aluminium in hot, low-pressure containers.
Due to its high reactivity with common materials, there is very little demand for metallic calcium. One application is in some chemical processes to refine thorium, uranium and zirconium as well as being used to remove oxygen, sulphur and carbon from certain alloys. Calcium can be alloyed with aluminium, beryllium, copper, lead and magnesium and is also used in vacuum tubes as a ‘getter’, a material that combines with and removes trace gases from these tubes.
However, calcium compounds are widely used. There are vast deposits of limestone (calcium carbonate) used directly as a building stone and indirectly for cement. When limestone is heated in kilns it gives off carbon dioxide gas leaving behind quicklime (calcium oxide). This reacts vigorously with water to give slaked lime (calcium hydroxide). Slaked lime is used to make cement, as a soil conditioner and in water treatment to reduce acidity, as well as in the chemicals industry. It is also used in steel making to remove impurities from the molten iron ore. When mixed with sand, slaked lime takes up carbon dioxide from the air and hardens as lime plaster.
Calcium carbonate is also used to make white paint, cleaning powder, toothpaste and stomach antacids. Gypsum (calcium sulphate) is used by builders as plaster and by nurses for setting bones, i.e. ‘plaster of Paris’.
Calcium is essential to all living things, particularly for the growth of healthy teeth and bones. Calcium phosphate is the main component of bone. The average human contains about 1 kilogram of calcium. Children and pregnant women are encouraged to eat foods rich in calcium, such as dairy products, leafy green vegetables, fish, nuts and seeds.
While calcium and its compounds are not considered to be toxic, ingesting too many calcium carbonate dietary supplements or antacids can cause milk-alkali syndrome, which is associated with hypercalcemia sometimes leading to fatal renal failure. Excessive consumption would be in the order of 10 g calcium carbonate per day, though symptoms have been reported upon ingesting as little as 2.5 g calcium carbonate daily.
With thanks to Karolina Jackiewicz, Lipmann Walton & Co
Nature’s Building Blocks: An A-Z Guide to the Elements, John Emsley, 2001, Oxford University Press