Reduction of aqueous nitrous acid hno2 to gaseous nitric oxide no in acidic aqueous solution. be sure to add physical state symbols where appropriate.

Reduction is only one half of the reaction of a redox (reduction-oxidation reaction). It is characterized by the reduction of oxidation number or the gain of electrons. So, you would expect the reaction to have moles of electrons in the reactant side to depict gaining of electrons. The reduction reaction is as follows:

HNO₂ + e⁻ --> NO

Why only 1 e-? Compute the oxidation number of N in the reactant side.
1+x+2(-2) = 0; x = +3
Then, compute the oxidation number of N in the product side.
x -2 = 0; x = +2

So, there is a difference of 1 electron. Hence, 1e-.

The correct answer would be 1. precipitation