Metal Nitrate Test - NO2 Gas
Nitric acid is not the only nitrate that falls apart on heating — every metal nitrate (other than the Group 1 and ammonium ones, which behave differently) does the same thing: metal oxide left behind, nitrogen dioxide and oxygen driven off. Copper nitrate makes the point with a colour change to match — blue crystals give way to a black solid, copper oxide, as the same reddish-brown fumes seen from nitric acid itself pour out of the flask.
4 steps · Nitric Acid · Study of Compounds
What you will use
Apparatus
- Flask
Chemicals
- Copper Nitrate
The steps
-
Add copper nitrate crystals to the flask.
Blue crystals. Nothing happens until they are heated.
-
Attach a burner under the flask.
The flame is lit; the burner slider now controls how hot the crystals get.
-
Heat the crystals.
The blue crystals begin to change as they heat.
-
Watch the crystals and the gas together.
The blue solid turns black, and reddish-brown fumes rise off it — nitrogen dioxide, escaping as the nitrate breaks down.
What it shows
Copper nitrate decomposes on strong heating: 2Cu(NO₃)₂ → 2CuO + 4NO₂ + O₂. The black solid left behind is copper oxide; the reddish-brown gas is nitrogen dioxide, the same gas nitric acid itself gives off when heated alone. Any metal nitrate in this family follows the same pattern — 2M(NO₃)₂ → 2MO + 4NO₂ + O₂ — which is why heating a nitrate and watching for reddish-brown fumes is a standard way to tell one apart from a chloride or a sulphate.