Most students will be told that copper is very inert and it will not react with acid. However, it can be easily shown that copper will react with dilute nitric acid (2M) with a little heating to provide the activation energy required for the reaction. The left test tube shows the beginning of the reaction with the acid slightly warmed up and bubbles begin to appear. These are bubbles of nitrogen dioxide which will appear as a brown gas when they enter the air above the solution.

Note: no hydrogen is produced, instead water is produced as one of the 3 products.

Cu + HNO₃ → copper nitrate + nitrogen dioxide + water