In a chemical reaction, a more reactive metal displaces a less reactive metal from its salt. From the reactivity series of metal, we know that zinc is more reactive than iron. So iron, being less reactive than zinc, can't replace zinc from its salt zinc sulphate.
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According to the chemical reactivity series(in decreasing order of reactivity),Fe is below Zn,and therefore it is unable to replace Zn from any Zn salt,in this case,ZnSO₄ Fe is less reactive than Zn.
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