Multiple choice The gas liberated during the reaction of copper with dilute nitric acid is _____. NO2 N2O5 O2 NO Reveal answer Fill a bubble to check yourself D Correct answer Explanation Copper reacts with dilute nitric acid to form nitrogen monoxide (NO), copper nitrate, and water. The NO is initially colorless but quickly oxidizes in air to brown NO2. With concentrated nitric acid, NO2 is produced directly.