Ruby ($Al_2O_3$ with $Cr^{3+}$) and Emerald ($Be_3Al_2Si_6O_{18}$ with $Cr^{3+}$) both owe their colors to $Cr^{3+}$ impurities, but the different crystal fields result in different colors.
To determine the incorrect statement among the given options, let’s analyze each statement one by one:
VOSO₄ is a reducing agent: Vanadyl sulfate (VOSO₄) is known to be a reducing agent. Vanadium in the +4 oxidation state can be further oxidized, hence it acts as a reducing agent. Therefore, this statement is correct.
RuO₄ is an oxidizing agent: Ruthenium tetroxide (RuO₄) is a powerful oxidizing agent. It can oxidize a wide variety of substances due to ruthenium's high oxidation state of +8. This statement is correct.
Cr₂O₃ is an amphoteric oxide: Chromium(III) oxide (Cr₂O₃) exhibits amphoteric behavior, meaning it can react with both acids and bases to form salts. Therefore, this statement is also correct.
Red colour of ruby is due to the presence of Co³⁺: The red color of ruby is actually due to the presence of chromium ions (Cr³⁺), not cobalt ions (Co³⁺). Rubies are composed mainly of aluminum oxide with traces of chromium, which imparts the red color. This statement is incorrect.
Thus, the incorrect statement is: "Red colour of ruby is due to the presence of Co³⁺". The correct reason for the red color is the presence of Cr³⁺ ions.
