Oxidation = electron loss; Reduction = electron gain. A reducing agent supplies electrons that go on to reduce another species. It is itself oxidised. i.e. read more
Zinc is a useful reducing agent because it is reactive but not too reactive. There are more reducing (i.e. more reactive) metals, but they are not useful as they are too reactive and will react with water in aqueous solution rather than participate in redox reactions (e.g. Group 1 metals). read more
So, to see if an unknown substance is a reducing agent, add acidifies potassium manganate (VII) to see if the pink colour fades. If it does, you know the unknown substance was a reducing agent, because it caused reduction.. Testing for an oxidising agent: . A reducing agent is a substance which causes reduction, but is oxidised itself. read more