318Calcium salts are colorless from any contribution of the calcium, and ionic solutions of
calcium (Ca2+) are colorless as well.
As with magnesium salts and other alkaline earth metal salts, calcium salts are often quite
soluble in water. Notable exceptions include the hydroxide, the sulfate (unusual for sulfate
salts), the carbonate and the phosphates. With the exception of the sulfate, even the
insoluble ones listed are in general more soluble than its transition metal counterparts.
When in solution, the calcium ion to the human taste varies remarkably, being reported as
mildly salty, sour, "mineral like" or even "soothing." It is apparent that many animals can
taste, or develop a taste, for calcium, and use this sense to detect the mineral in salt licks
or other sources. In human nutrition, soluble calcium salts may be added to tart juices
without much effect to the average palate.
Calcium is the fifth-most-abundant element by mass in the human body, where it is a
common cellular ionic messenger with many functions, and serves also as a structural
element in bone. It is the relatively high-atomic-number calcium in the skeleton that causes
bone to be radio-opaque.
Of the human body's solid components after drying and burning of organics (as for
example, after cremation), about a third of the total "mineral" mass remaining, is the
approximately one kilogram of calcium that composes the average skeleton (the
remainder being mostly phosphorus and oxygen).
Calcium, combined with phosphate to form hydroxylapatite, is the mineral portion of
human and animal bones and teeth. The mineral portion of some corals can also be
transformed into hydroxylapatite.
Calcium hydroxide (slaked lime) is used in many chemical refinery processes and is made
by heating limestone at high temperature (above 825 °C) and then carefully adding water
to it. When lime is mixed with sand, it hardens into a mortar and is turned into plaster by
carbon dioxide uptake. Mixed with other compounds, lime forms an important part of
Portland cement.
Calcium carbonate (CaCO 3 ) is one of the common compounds of calcium. It is heated to
form quicklime (CaO), which is then added to water (H 2 O). This forms another material
known as slaked lime (Ca(OH) 2 ), which is an inexpensive base material used throughout
the chemical industry. Chalk, marble, and limestone are all forms of calcium carbonate.
When water percolates through limestone or other soluble carbonate rocks, it partially
dissolves the rock and causes cave formation and characteristic stalactites and
stalagmites and also forms hard water.
Other important calcium compounds are calcium nitrate, calcium sulfide, calcium chloride,
calcium carbide, calcium cyanamide and calcium hypochlorite. A few calcium compounds
in the oxidation state +1 have also been investigated recently.