Cold, dilute nitric acid reacts with copper to form _____(hydrogen, nitrogen dioxide, nitric oxide).

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Hint: When nitric acid reacts with copper at two different temperatures then they will give two different types of products. These are frequently asked questions so try to remember perfectly.

Complete answer:
Copper is a reddish-brown metal.
Copper is oxidized by nitric acid and forms $C{u^{ + 2}}$ ions.
When copper is reacted with cold, dilute nitric acid it yields copper nitrate, water, and nitric oxide.
And again it undergoes a reaction with air, which is nitric acid reacts with oxygen and gives nitrogen dioxide.
Nitrogen dioxide is a poisonous brown gas with an irritating odor.
The overall balanced reaction is
$4HN{O_3} + Cu \to Cu{(N{O_3})_2} + 2N{O_2} + 2{H_2}O$
So the answer is nitrogen dioxide.
Additional Information:-From the above reaction the main product is copper nitrate which is a blue crystalline solid. Nitrogen dioxide is one of the major atmospheric pollutants that absorb UV light and stops to reach it to the earth’s surface. It is used as a catalyst and acts as an oxidizing agent. It is also used in the making of explosives. Nitric oxide and nitrogen dioxide are the two most toxic and dangerous nitrogen oxides.

In some chemical reactions we are unable to write the correct answer so learn thoroughly some important reactions like the Synthesis of ammonia, Combustion of hydrogen / Electrolysis of water, etc
because there will be exceptional cases due to exposure to the air like that.