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When I mix ferrous sulfate crystals with water, the solution almost immediately turns orange. However, when I mix ferrous sulfate with vinegar, it doesn’t change colors.

Why is this?

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    $\begingroup$ Welcome to the chemistry site of the StackExchange (SE) network. Be aware that laconic, not elaborated questions are usually closed on the SE. The site expects that you include the compact summary of your related current knowledge, involving your conclusion of searching for existing related info or answers. It would prevent others to tell you what you already know or what you could easily find yourself. Effort not shown may be considered as effort not done, possibly leading to the question closure. How do I ask a good question?. $\endgroup$
    – Poutnik
    Apr 2, 2022 at 5:39
  • $\begingroup$ What have you searched and found about salt hydrolysis, ferrous ion oxidation and how hydrolysis is affected by acids? $\endgroup$
    – Poutnik
    Apr 2, 2022 at 5:44

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As Poutnik suggests, water contains a small proportion of dissolved oxygen, about $\pu{10 mg/L}$. These $\ce{O2}$ molecules oxidize quickly some $\ce{Fe^{2+}}$ ions, and produce some $\ce{Fe^{3+}}$ ions which have a brownish color when included in aqueous complexes like $\ce{[Fe(OH)]^{2+}}$ according to : $$\ce{4 Fe^{2+} + O2 + 2 H2O -> 4 Fe^{3+} + 4 OH^- -> 4 [Fe(OH)]^{2+}}$$In vineggar, or in any acidic solution, the brown complex $\ce{[Fe(OH)]^{2+}}$ is not formed, because the oxidation equation is $$\ce{4 Fe^{2+} + O2 + 4 H^+ -> 4 Fe^{3+} + 2 H2O}$$ The ferric ion $\ce{Fe^{3+}}$ may be included in a water complex like $\ce{[Fe(H2O)6]^{3+}}$, which is not brownish.

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  • $\begingroup$ Additionally, many impure or old lots of iron(II) sulphate already contain some iron(III) too, which happily hydrolyzes unless water is acidified. $\endgroup$
    – Poutnik
    Apr 2, 2022 at 9:13

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