Which ion produces an orange-brown precipitate with sodium hydroxide?

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Multiple Choice

Which ion produces an orange-brown precipitate with sodium hydroxide?

Explanation:
When a metal ion in solution reacts with sodium hydroxide, the OH- ions form a metal hydroxide precipitate. The color of that precipitate depends on the metal. Iron(III) ions form Fe(OH)3, which is a rusty orange-brown solid, so observing an orange-brown precipitate points to Fe3+. By contrast, aluminium gives a white Al(OH)3 precipitate (which dissolves in excess base to form a soluble complex), copper gives blue Cu(OH)2, and calcium gives a white Ca(OH)2 precipitate. So the orange-brown precipitate specifically identifies iron(III).

When a metal ion in solution reacts with sodium hydroxide, the OH- ions form a metal hydroxide precipitate. The color of that precipitate depends on the metal. Iron(III) ions form Fe(OH)3, which is a rusty orange-brown solid, so observing an orange-brown precipitate points to Fe3+. By contrast, aluminium gives a white Al(OH)3 precipitate (which dissolves in excess base to form a soluble complex), copper gives blue Cu(OH)2, and calcium gives a white Ca(OH)2 precipitate. So the orange-brown precipitate specifically identifies iron(III).

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