Heating a metal nitrate gives oxygen, and for most metals also the oxide and brown nitrogen dioxide, for example 2Cu(NO3)2(s) → 2CuO(s) + 4NO2(g) + O2(g); Group 1 nitrates give only a nitrite and oxygen.
Nitrates all decompose on heating, but they split in two different ways depending on how reactive the metal is. This makes thermal decomposition of nitrates a favourite 4541 question, because the products, the gases and the colour changes all test whether you understand the reactivity link.
The balanced equation
For the very reactive Group 1 metals (except lithium), the nitrate loses only oxygen and becomes a nitrite:
2NaNO3(s) → 2NaNO2(s) + O2(g)
2KNO3(s) → 2KNO2(s) + O2(g)
For most other metals, the nitrate decomposes all the way to the oxide, giving brown nitrogen dioxide as well as oxygen:
2Cu(NO3)2(s) → 2CuO(s) + 4NO2(g) + O2(g)
2Pb(NO3)2(s) → 2PbO(s) + 4NO2(g) + O2(g)
For a very unreactive metal such as silver, even the oxide is unstable and the metal itself is left:
2AgNO3(s) → 2Ag(s) + 2NO2(g) + O2(g)
Balancing the 4 : 1 ratio of nitrogen dioxide to oxygen is the step most often done wrong.
Conditions required
Strong, direct heating of the solid nitrate is required, usually in a boiling tube or crucible over a hot Bunsen flame. No catalyst or other reactant is involved. As with carbonates, the more reactive the metal, the more stable the nitrate, so a Group 1 nitrate resists full decomposition while copper, lead and silver nitrates break down completely.
What you observe
Two gases give the reaction away. Oxygen is colourless and relights a glowing wooden splint. Nitrogen dioxide is a brown, pungent gas, its appearance tells you at once that the metal is not in Group 1. Colour changes in the residue are useful: blue copper(II) nitrate leaves black copper(II) oxide; white lead(II) nitrate crackles as it decomposes and leaves lead(II) oxide, which is yellow when hot; silver nitrate leaves grey silver metal. A Group 1 nitrate gives only oxygen and no brown gas.
Where it appears in the SPM exam
In 4541/1 this is tested as the products of heating a nitrate, the identification of brown nitrogen dioxide, and the oxygen test with a glowing splint. In 4541/2 it appears as equation writing, especially balancing the copper or lead nitrate equation, and as questions that ask you to distinguish a Group 1 nitrate from the rest. In 4541/3 you may heat a nitrate and identify the gases evolved.
How we teach it
We split nitrates into three groups: Group 1 (nitrite plus oxygen only), most metals (oxide plus brown NO2 plus oxygen), and silver (metal plus NO2 plus oxygen). Students most often lose marks by writing the same products for sodium as for copper, or by mis-balancing the NO2 and O2. Fixing the two gas tests, glowing splint for oxygen, brown colour for nitrogen dioxide, gives you reliable observation marks every time.
Quick summary
Ask one question first: is the metal in Group 1? If yes, expect a nitrite and oxygen with no brown gas. If no, expect the oxide (or silver metal), brown nitrogen dioxide and oxygen. Confirm oxygen with a glowing splint that relights, note the brown gas, and give the residue colour. Balancing the 4NO2 to 1O2 ratio correctly turns a good answer into a full-mark one.
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