Nitric acid dissolves Co because of the positive overall cell potential created by the oxidation of Co metal to the Co2+ ion. The balanced redox reaction is Co2 + 2Al → AlCo₂ + Al₂.
Redox reactions include a change in the oxidation state of the substrate. When electrons are lost or the oxidation state increases, it is called oxidation; when they are gained or the oxidation state decreases, it is called reduction. A chemical process in which electrons are moved between two reactants is referred to as a redox reaction.
The inorganic substance with the formula HNO3 is nitric acid. It is a mineral acid that is quite corrosive. The substance is colourless, but older samples may have a yellow hue from breakdown into nitrogen oxides. The majority of nitric acid that is sold commercially has a 68% concentration in water.
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