When prepared in this way it is a soft white powder often called precipitated chalk, and is much used as a polishing powder. It is insoluble in water, but dissolves in water saturated with carbon dioxide, owing to the formation of the acid calcium carbonate which is slightly soluble: CaCO_{3} + H_{2}CO_{3} = Ca(HCO_{3})_{2}. The natural varieties of calcium carbonate find many uses, such as in the preparation of lime and carbon dioxide; in metallurgical operations, especially in the blast furnaces; in the manufacture of soda, glass, and crayon (which, in addition to chalk, usually contains clay and calcium sulphate); for building stone and ballast for roads. ~Calcium chloride~ (CaCl_{2}). This salt occurs in considerable quantity in sea water. It is obtained as a by-product in many technical processes, as in the Solvay soda process. When crystallized from its saturated solutions it forms colorless needles of the composition CaCl_{2}.6H_{2}O. By evaporating a solution to dryness and heating to a moderate temperature calcium chloride is obtained anhydrous as a white porous mass. In this condition it absorbs water with great energy and is a valuable drying agent. ~Bleaching powder~ (CaOCl_{2}). When chlorine acts upon a solution of calcium hydroxide the reaction is similar to that which occurs between chlorine and potassium hydroxide: 2 Ca(OH)_{2} + 4 Cl = CaCl_{2} + Ca(ClO)_{2} + 2 H_{2}O. If, however, chlorine is conducted over calcium hydroxide in the form of a dry powder, it is absorbed and a substance is formed which appears to have the composition represented in the formula CaOCl_{2}. This substance is called bleaching powder, or hypochlorite of lime. It is probably the calcium salt of both hydrochloric and hypochlorous acids, so that its structure is represented by the formula /ClO Ca \Cl. In solution this substance acts exactly like a mixture of calcium chloride (CaCl_{2}) and calcium hypochlorite (Ca(ClO)_{2}), since it dissociates to form the ions Ca^{++}, Cl^{-}, and ClO^{-}. Bleaching powder undergoes a number of reactions which make it an important substance. 1. When treated with an acid it evolves chlorine: /ClO Ca + H_{2}SO_{4} = CaSO_{4} + HCl + HClO, \Cl HCl + HClO = H_{2}O + 2Cl. This reaction can be employed in the preparation of chlorine, or the nascent chlorine may be used as a bleaching agent. 2. It is slowly decomposed by the carbon dioxide of the air, yielding calcium carbonate and chlorine: CaOCl_{2} + CO_{2} = CaCO_{3} + 2Cl. Owing to this slow action the substance is a good disinfectant. 3. When its solution is boiled the substance breaks down into calcium chloride and chlorate: 6CaOCl_{2} = 5CaCl_{2} + Ca(ClO_{3})_{2}. This reaction is used in the preparation of potassium chlorate.