Copper Initiated nitric acid fountain
"A few copper cents are dropped into concentrated (14M) Nitric Acid. Nitric acid is an oxidizing acid and reacts with copper by the following exothermic (heat producing) redox reaction:
3Cu(s) + 8HNO3(aq) → 3Cu(NO3)2(aq) + 2NO(g) + 4H2O(l)
When exposed to oxygen, NO is converted into nitrogen dioxide.
2 NO + O2 → 2 NO2
The other possible reaction where NO2 is formed instead of NO:
4 HNO3 + Cu → Cu(NO3)2 + 2 NO2 +2 H2O
The expanding gas (bubbles) displace water from the tank. When the gas cools, the water is drawn back into the flask, dissolving the nitric oxide and forming a blue solution with the copper ions still there.”