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Titanium is not attacked by mineral acids at room temperature or by hot aqueous alkali; it dissolves in hot hydrochloric acid, giving titanium species in the +3 oxidation state, and hot nitric acid converts it into a hydrous oxide that is rather insoluble in acid or base. The best solvents for the metal are hydrofluoric acid or other acids to which fluoride ions have been added; such mediums dissolve titanium and hold it in solution because of the formation of fluoro complexes.
In its compounds, titanium exhibits oxidation states of +2, +3, and +4, as in the oxygen compounds titanium monoxide, TiO, dititanium trioxide, Ti2O3, and titanium dioxide, TiO2, respectively. The +4 oxidation state is the most stable.
The chemistry of titanium in the +2 state is rather restricted. By contrast, many compounds are formed by titanium in the +3 state. One of the more important is the trichloride TiCl3, a crystalline form of which is particularly useful as a catalyst in the stereospecific polymerization of propylene to make the commercially valuable polymer polypropylene.
Of the compounds formed by titanium in its +4 state, the dioxide, TiO2, is the most important. This nontoxic, pure white powder is used extensively as a pigment in paints, enamels, and lacquers. It occurs in nature as the minerals brookite, octahedrite, anatase, and rutile.
Another compound of commercial significance is titanium tetrachloride, a colourless liquid used to obtain titanium metal. It is also utilized for skywriting and producing smoke screens and as a catalyst in many organic reactions.
Titanium combines directly with many nonmetals, such as hydrogen, the halogens, nitrogen, carbon, boron, silicon, and sulfur at elevated temperatures. The resulting nitride (TiN), carbide (TiC), and borides (TiB and TiB2) are interstitial compounds that are very stable, hard, and refractory.