The aqueous solution of transition metal salt changes colour from pink to blue, when concentrated hydrochloric acid is added to it. The change In colour is due to
evolution of hydrogen that changes the oxidation state of the metal ion
change in the coordination number of the metal ion from $6$ to $4$ and formation of new species in solution
formation of a coordination complex of the metal ion with hydrochloric acid
protonation of the metal ion
Which of the following element has maximum density
Iron becomes passive by ..................... due to formation of .....................
In $d-$ block $3d$ series, which of the following element has maximum size ?
Passing $H_2S$ gas into a mixture of $Mn^{+2}, Ni^{+2}, Cu^{+2}$ and $Hg^{2+}$ ions in an acidified aqueous solution precipitates