Metal precipitates. Four test tubes showing the colours of metal ion solutions and the formation of the solid precipitates of the metal hydroxides. From left: iron (Fe3+, Fe[OH]3), copper (Cu2+, Cu[OH]2), chromium (Cr3+, Cr[OH]3) and cobalt (Co2+, Co[OH]2). These are all transition metal elements with specific oxidation states that can explain the colours produced. The formation of the solid hydroxide of a metal is often used to test for that metal. Strictly speaking, the hydroxide is not always formed first. Cobalt hydroxide is pink, and the blue precipitate seen here is an intermediate form known as a basic salt.

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