ites, is 2.4 × l0–2, and in basic and ultrabasic rocks, 5 × 10–3. Water migration plays a major role in the history of chlorine in the earth’s crust. In the form of the Cl– ion, chlorine is a component of the earth’s oceans (1.93 percent), underground brines, and salt lakes. There are 97 chlorine minerals, primarily natural chlorides, the most important of which is halite, NaCl (see and ROCK SALT). Numerous extensive deposits of potassium and magnesium chlorides and mixed chlorides are known: sylvite, KCl, sylvinite, (Na,K)Cl, carnallite, KCl · MgCl2 · 6H2Ó, kainite, KCl · MgSO4 · 3H2O, and bischofite MgCl2 · 6H2O. The migration of HCl contained in volcanic gases into the upper portions of the earth’s crust was of great significance in the earth’s geological history.
Physical and chemical properties. The boiling point of chlorine is –34.05°C, and the melting point –101°C. The density of chlorine gas at normal conditions is 3.214 g/liter (g/l), while the density of the saturated vapor at 0°C is 12.21 g/l. The density of liquid chlorine at its boiling point is 1.557 g/cm3, while the density of solid chlorine at –102°C is 1.9 g/cm2. The pressure of saturated chlorine vapor is 0.369 meganewton/m2 (MN/m2), or 3.69 kilograms-force (kgf/cm2), at 0”C, 0.772 MN/m2 (7.72 kgf/cm2) at 25°C, and 3.814 MN/m2 (38.14 kgf/cm2) at 100°C. The heat of fusion is 90.3 kilojoules/kg (kJ/kg), or 21.5 cal/g, while the heat of evaporation is 288 kJ/kg (68.8 cal/g). The heat capacity of chlorine gas at constant pressure is 0.48 kJ/(kg · °K), or 0.11 cal/(g · °C). The critical constants of chlorine are as follows: critical temperature, 144°C; critical pressure, 7.72 MN/m2 (77.2 kgf/cm2); critical density, 573 g/l; and critical volume, 1.745 × 10–3l/g. The solubility of chlorine at a partial pressure of 0.1 MN/m2 (1 kgf/cm2) in water is 14.8 g/l at 0°C, 5.8 g/l at 30°C, and 2.8 g/l at 70°C, while in a 300 g/l NaCl solution, its solubility is 1.42 g/l at 30°C and 0.64 g/l at 70°C.
Below 9.6°C in aqueous solutions, hydrates of chlorine are formed with variable composition Cl2 · nH2O (where n ranges from 6 to 8), which are in the form of yellow crystals of the isometric system that decompose with increasing temperature into chlorine and water. Chlorine is readily soluble in TiCl4, SiCl4, SnCl4, and some organic solvents, especially, hexane, C6H14, and carbon tetrachloride, CCl4. The chlorine molecule is diatomic (Cl2). The degree of thermal dissociation of Cl2 + 243 kJ ⇄ 2Cl is 2.07 × 10–4 percent at 1000°K and 0.909 percent at 2500°K.
The outer electron configuration of the chlorine atom is 3s23p5. Consequently, in its compounds, chlorine may have oxidation states of –1, +1, +3, +4, +5, +6, and +7. The covalent radius of the chlorine atom is 0.99 Å, while the ionic radius of Cl– is 1.82 Å. The electron affinity of the chlorine atom is 3.65 eV, while the ionization energy is 12.97 eV.
Chemically, chlorine is very reactive and combines directly with almost all metals (with some metals, it reacts only in the presence of moisture or upon heating) and with nonmetals (except carbon, nitrogen, oxygen, and the inert gases), forming the corresponding chlorides. It reacts with many compounds, replaces hydrogen in saturated hydrocarbons, and combines with unsaturated compounds. Chlorine replaces bromine and iodine from their compounds with hydrogen and metals and is itself replaced by fluorine from its compounds with these elements.
In the presence of tiny amounts of moisture, alkali metals react with chlorine by combustion. Most metals react with dry chlorine only upon heating. Steel, as well as some metals, are stable in the presence of dry chlorine at moderate temperatures and thus are used for the construction of equipment used with dry chlorine and tanks for storing dry chlorine. Phosphorus ignites in chlorine, forming PCl3, and upon further chlorination, PCl5. Sulfur reacts with chlorine to yield S2Cl2, SCl2, and other compounds with the general formula SnClm. Arsenic, antimony, bismuth, strontium, and tellurium react vigorously with chlorine.
A mixture of