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When iron (II) ions react with sodium hydroxide solution, they form a green precipitate known as iron (II) hydroxide, represented by the chemical formula Fe(OH)₂. This occurs because the hydroxide ions from the sodium hydroxide interact with the iron (II) ions in solution, leading to the formation of the insoluble precipitate.
It is important to note that the colors associated with precipitates can be characteristic of specific metal ions. In the case of iron (II), the specific shade of green can vary based on concentration and other conditions, but it is generally accepted as a light green color indicative of iron (II) compounds.
The other colors listed do not correspond to iron (II) when precipitated with sodium hydroxide: blue is characteristic of copper (II) hydroxide, white typically corresponds to precipitates like zinc hydroxide, and brown generally indicates the presence of iron (III) ions or other hydroxides such as iron (III) hydroxide. Thus, the formation of a green precipitate confirms the presence of iron (II) ions in the reaction with sodium hydroxide.