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Silver compounds precipitated in test tubes. From left to right, these are silver chloride (AgCl), silver bromide (AgBr) and silver iodide (AgI). These were formed by reacting silver nitrate (AgNO3) with solutions of halides. During the reactions the nitrate group becomes substituted with the halide forming a precipitate. (KEYSTONE/SCIENCE PHOTO LIBRARY/ANDREW LAMBERT PHOTOGRAPHY)