Iodine is formed when $KI$ reacts with a solution of

The highest oxidation state of $Mn$ is shown by

Identify the correct statement.

Which of the following metals becomes passive when treated with concentrated $HNO_3$?

All $Cu(II)$ halides are known except $CuI_2$,because ....

$(a)$ Transition metals can act as catalysts because they can change their oxidation state. How does $Fe(III)$ catalyze the reaction between iodide and persulphate ions?
$(b)$ Mention any three processes where transition metals act as catalysts.