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Aluminium chloride (AlCl₃) is a fascinating and industrially vital inorganic compound that exists in two primary forms. In its anhydrous (water-free) state, it is a white or pale yellow solid notorious for its vigorous reactivity with moisture, fuming in air as it hydrolyzes to release hydrogen chloride gas. This form is a classic and powerful Lewis acid, meaning it readily accepts an electron pair from another molecule. This property makes it an indispensable catalyst in the chemical industry, most famously in the Friedel-Crafts alkylation and acylation reactions used to build complex organic molecules for pharmaceuticals, plastics, and dyes.