Physical Properties
Calcium carbonate is a fine, white crystalline powder that is tasteless and odorless. It exists in both amorphous and crystalline forms. The crystalline forms include orthorhombic and hexagonal systems (anhydrous calcium carbonate appears as colorless orthorhombic crystals, while calcium carbonate hexahydrate appears as colorless monoclinic crystals [3]); the crystals are columnar or rhombohedral in shape, with a density of 2.93 g/cm³. Its melting point is 1339°C (decomposition occurs between 825°C and 896.6°C), and the melting point is 1289°C at a pressure of 10.7 MPa. It is virtually insoluble in water-though it dissolves in water containing ammonium salts or ferric oxide-and is insoluble in alcohol.
Molecular Structure
Its crystal structure belongs to the orthorhombic system. Each calcium carbonate unit consists of one carbon atom and three oxygen atoms, with each oxygen atom bonded to a calcium ion.
Calcium carbonate is composed of calcium ions and carbonate ions held together by ionic bonds, while the internal structure of the carbonate ion features covalent carbon-oxygen bonds. The carbonate ion involves sp² hybridization; the central carbon atom utilizes three hybrid orbitals and one p-orbital.According to the VSEPR model, it is classified as an AY3-type molecule with an ideal trigonal planar geometry, featuring three C-O bonds arranged in a trigonal planar configuration. Additionally, it possesses a delocalized p-p pi-bond system involving four orbitals and six electrons [5]. Within the crystal, calcium carbonate units are arranged in a layered structure parallel to the a-axis and c-axis. These layers are interconnected via coplanar oxygen atoms, forming a three-dimensional network structure. This structural arrangement imparts high stability and hardness to calcium carbonate.
Chemical Properties
1. Calcium carbonate decomposes into calcium oxide and carbon dioxide at 825-896.6℃. (Industrial production of CO₂):
2. Calcium carbonate reacts with dilute acids (such as dilute acetic acid, dilute hydrochloric acid, dilute nitric acid, etc.) to effervesce and dissolve. The reaction releases carbon dioxide, making it an exothermic reaction. For example, it reacts with dilute hydrochloric acid to produce calcium chloride, water, and carbon dioxide (laboratory production of CO₂):
3. Passing excess carbon dioxide into water mixed with CaCO₃ will produce a calcium bicarbonate solution. Calcium carbonate reacts with a carbonic acid solution (rainwater) to produce calcium bicarbonate. Passing CO₂ into cloudy limewater will cause the precipitate to disappear. The principle behind these phenomena is:
4. Anhydrous calcium carbonate (a white, odorless, non-toxic powdery substance, light calcium carbonate) heated to 1000K transforms into calcite (trigonal crystal system, heavy calcium carbonate).