What is the role of metal or reagents written on arrows in the given chemical reactions?

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(a) Ni acts as a catalyst

(b) Concentrated Sulphuric acid (Conc. H₂SO₄) acts as a catalyst.

(c) Alkaline Potassium permanganate (Alkaline KMNO₄) acts as an oxidising agent in the above reaction.

(Note: Catalysts are defined as substances that alter the rate of reaction by changing the path of reaction.

An oxidizing agent is a chemical species that tend to oxidize other substances, i.e. cause an increase in the oxidation state of the substance by making it lose electrons.)