. In which reaction is nitric acid acting as an oxidising agent? A. CuO + 2HNO3 → Cu(NO3)2 + H2O B. Cu + 4HNO3 → Cu(NO3)2 + 2H2O + 2NO2 C. Na2CO3 + 2HNO3 → 2NaNO3 + H2O + CO2 D. NaOH + HNO3 → NaNO3 + H2O

B. Cu + 4HNO3 → Cu(NO3)2 + 2H2O

Explanation:

A redox reaction is one in which oxidation and reduction reactions are occuring simultaneously.

In a redox reaction, an oxidizing agent is reduced ( its oxidation state decreases) whereas a reducing agent is oxidized (it's oxidation state increases).

The nitrogen in the nitrate (v) ion, NO3-, exists in the +5 oxidation state. Therefore it would be acting as an oxidizing agent in any reaction in which it's oxidation state becomes less than +5 after the reaction.

Let us then consider each of the reactions:

A. Reaction 1: in this reaction, the oxidation state of nitrogen remains +5 as what is simply an exchange of radicals in a neutralization reaction.

B. Reaction 2: in this reaction, trioxocarbonate (v) acid oxidizes Copper atom to Copper (ii) ion, while itself it is reduced to Nitrogen (IV) oxide. The oxidation state of nitrogen changes from +5 to +4

C. Reaction 3: in this reaction, what occurs is an exchange of radicals by a displacement reaction. The oxidation state of nitrogen remains the same

D. Reaction 4: in the reaction, what occurs is a neutralization reaction, and the oxidation state of nitrogen remains the same.

Therefore, the correct option is B.

metals are good conductor of electricity. it has free electrons on their surface. according to electron pool theory, each atom present in a metal crystal losses all of its valence electron and as a result a pool of free is form. these pool of electrons are free to move and   conduct electricity easily   while making metal as good conductor.

the correct answer is a. the ability of electrons to flow throughout the metal.

if it's not useful, feel free to report for mistake in answer.