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Sulfur trioxide
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<p>Sulfur trioxide (alternative spelling sulphur trioxide) is the chemical compound with the formula SO3. It has been described as "unquestionably the most [economically] important sulfur oxide". It is prepared on an industrial scale as a <a href="/facts/Precursor_(chemistry)/dUw8Mx2Y">precursor</a> to <a href="/facts/Sulfuric_acid/Wom6fymJ">sulfuric acid</a>. </p><p>Sulfur trioxide exists in several forms: gaseous monomer, crystalline trimer, and solid polymer. Sulfur trioxide is a solid at just below room temperature with a relatively narrow liquid range. Gaseous SO3 is the primary precursor to <a href="/facts/Acid_rain/f769GyNz">acid rain</a>. </p>