Compounds of Calcium

Some Important compounds of calcium are:

1. Calcium oxide or Quicklime (CaO)
2. Calcium hydroxide or Slaked lime  (Ca(OH)₂)
3. Calcium Tetraoxosulphate (VI), CaSO₄
4. Calcium Trioxonitrate (V), Ca(NO₃)₂
5. Calcium Sulphate or Plaster of Paris, CaSO₄.2H₂O or \( \scriptsize CaSO_4.\normalsize \frac{1}{2} \scriptsize H_2 O \)

Calcium compounds are of great economic importance, especially in extraction of metals and in the building industry.

Calcium Oxide:

Preparation of Quick Lime:

Calcium oxide, CaO, is commonly called quicklime. Calcium oxide is prepared by the calcination process. Limestone i.e. calcium trioxocarbonate (IV) is heated to a temperature between 900^oC and 1300^oC in Kilns and it decomposes to quicklime and carbon dioxide. The kilns have steel shells line with refracting bricks. CaO is removed from the bottom of the Kiln as it is formed.

CaCO_3(s)\( \scriptsize \overset{900^oC}{\rightarrow}\) CaO_(s) + CO_2(g)

Properties of Quick Lime:

1. CaO is a white amorphous solid.

2. It has a high melting pointThe temperature at which a solid changes its state to liquid at atmospheric pressure is called the melting point of that liquid. The melting point is usually defined as the point... More of 2600^oC.

3. It can withstand very high temperatures.

4. It is highly stable and even fusion cannot decompose it.

5. It is very hygroscopic. It is used to dry ammonia gas.

Chemical Properties:

1. When water is added to CaO, it cracks to form a white powder. This process is accompanied by the liberation of a lot of heat. This is called slaking of lime and the end product is calcium hydroxide, Ca(OH)₂, commonly known as slaked lime.

CaO_(s) + H₂O_(l) \( \rightarrow\) Ca(OH)_2(s)

2. Calcium oxide is a basic oxide. It can react with acids to give calcium salts.

3. It forms silicates and phosphates with acidic oxides like silicon dioxide and phosphorus pentoxide. This property makes lime useful as a flux in metallurgy to remove impurities.

4. Calcium oxide reacts with ammonium salts to liberate ammonia gas.

5. Calcium carbide is produced industrially in an electric arc furnace from a mixture of lime and coke at approximately 2000 °C.

CaO_(s) + 3C_(s)→ CaC_2(s) + CO_(g)

The major industrial use of calcium carbide, CaC₂, is in the production of acetylene.

Uses of CaO:

1. It is used in making mortar and plaster.
2. It is used for medicinal purposes, insecticides and plant and animal food.
3. It is used as a laboratory reagent for gas absorptionAbsorption is the movement of digested food molecules from the digestive system into the blood (glucose and amino acids) and lymph (fatty acids and glycerol). The small intestine absorbs most of... More, precipitationPrecipitation is any liquid or frozen water that forms in the atmosphere and falls back to the Earth. It comes in many forms, like rain, sleet, and snow. More, dehydration etc.
4. It is used in the manufacture of paper, steel and cement. It is also used in the preparation of calcium carbide, basic calcium trioxonitrate (V).
5. It can be used for softening hard water and in the recovery of ammonia (by-product of Solvay process).
6. It finds enormous use in the manufacture of soap, rubber, varnish, refractories and lime bricks.

Calcium Tetraoxosulphate (VI), CaSO₄

Calcium tetraoxosulphate (VI), CaSO₄ is found in nature as anhydrite, CaSO₄, and gypsum, CaSO₄.2H₂O. It may be prepared in the laboratory by the reaction between a soluble calcium salt with sodium tetraoxosulphate (VI) solution. The CaSO₄ formed is only slightly soluble in water and it comes out as a white precipitate.

CaCl_2(aq) + Na₂SO_4(aq) \( \rightarrow\) CaSO_4(s) + 2NaCl_(aq)

Calcium tetraoxosulphate (VI) causes permanent hardness of water. 

Calcium tetraoxosulphate (VI) is used in the making compounds such as tetraoxosulphate (VI) acid and ammonium tetraoxosulphate (VI).

Plaster of Paris:

Calcium tetraoxosulphate (VI) with half a molecule of water per molecule of the salt (hemi-hydrate) is called plaster of Paris.

Preparation of Plaster of Paris:

Plaster of Paris is prepared by heating gypsum (CaSO₄.2H₂O) at 120°C in rotary kilns, where it gets partially dehydrated. The temperature should be kept below 140°C otherwise further dehydration will take place and the setting property of the plaster will be partially reduced.

Properties of Plaster of Paris:

It is a white powder. When mixed with water (1/3 of its mass), it evolves heat and quickly sets to a hard porous mass within 5 to 15 minutes. During setting, a slight expansion (about 1%) in volume occurs so that it fills the mould completely and takes a sharp impression. 

Uses of Plaster of Paris

1. In surgery for setting broken or fractured bones
2. For making casts for statues, in dentistry, for surgical instruments, and toys, etc.
3. In making blackboard chalks, and statues
4. In construction industry

Calcium Trioxonitrate (V), Ca(NO₃)₂

Calcium trioxonitrate (V), Ca(NO₃)₂ is prepared in the laboratory by the reaction between CaCO₃ or CaO with trioxonitrate (V) acid.

CaCO₃(s) + 2HNO₃(aq) →  Ca(NO₃)₂(aq) + CO₂(g) + H₂O(l)

It can also be prepared from an aqueous solutionA solution in which the solvent is water. is called an aqueous solution. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a... More of ammonium nitrate, and calcium hydroxide:

2NH₄NO₃ + Ca(OH)₂    →   Ca(NO₃)₂ + 2NH₄OH

Calcium trioxonitrate (V) decomposes on heating to release nitrogen (IV) oxide. 

2Ca(NO₃)₂    →    2CaO + 4NO₂ + O₂ 

Calcium trioxonitrate (V) is used in making fertilizers.

Calcium Chloride, CaCl₂:

Calcium chloride is found in seawater and soil water. It is prepared by the action of hydrochloric acid on calcium trioxocarbonate(IV).

CaCO_3(s) + 2HCl_(aq) → CaCl_2(aq) + H₂O_(l) + CO_2(g)

The crystals of CaCl₂ are very deliquescent and white.

Calcium chloride is used as a drying agent for all gases except ammonia.

The anhydrous salt of CaCl₂ is used in desiccators.

Calcium Trioxocarbonate(IV), CaCO₃

Calcium trioxocarbonate(IV), CaCO₃ occurs abundantly naturally as limestone, chalk and marble.

It is also found in coastal caves as stalagmites (deposits of CaCO₃ which grow upwards from the floor of the cave to its top). and stalactites (deposits of CaCO₃ which grow downwards from the top of the cave to its floor).

It is also an important constituent in the bones of animals, in the external skeleton of marine organisms. It is found in natural ores, such as calcite, dolomite and Iceland spar.

CaCO₃ can be produced in the laboratory as a precipitate when trioxocarbonate(IV), Na₂CO₃ solution is added to a solution of Calcium chloride, CaCl₂.

Na₂CO_3(aq) + CaCl_2(aq) → CaCO_3(s) + 2NaCl_(aq)

Properties of Calcium Trioxocarbonate(IV)

Below are properties shown by calcium trioxocarbonate(IV)

1. It is a white solid which is insoluble in pure distilled water.

2. It dissolves in water that contains dissolved carbon(IV)oxide, CO₂ , to form calcium hydrogen tri-oxocarbonate(IV)

CaCO_3(s) + H₂O_(l) + CO_2(g)  → Ca(HCO₃)_2(aq)

The presence of calcium hydrogen trioxocarbonate(IV), Ca(HCO₃)₂, is the cause of temporary hardness in water.

3. Calcium Trioxocarbonate(IV) decomposes to give calcium oxide and carbon (IV) oxide, when strongly heated.

4. The action of dilute hydrochloric acid on CaCO₃ liberates. CO₂

CaCO_3(s) + 2HCl_(aq) → CaCl_2(aq) + H₂O_(l) + CO_2(g)

Uses of Trioxocarbonate(IV)

Trioxocarbonate(IV) is useful in the following ways:

1. Limestone is a major raw material used in making sodium trioxo-carbonate(IV), glass, steel, quicklime and cement.

2. Marble and limestone are used as building materials.

4. Finely ground limestone is used in neutralizing excess acid in acidic soils.

5. In the extraction of iron. Calcium oxide is provided by limestone which is used to remove impurities as slag.

6. As building materials.

7. In the manufacture of pigments, putty and pap.

8. Cement is mixed with sand and gravel to form concrete, which is an important building material because of its hardness and strength.