Ammonium Dichromate Decomposition (the Chemical Volcano)
Heating ammonium dichromate is the classic chemical volcano demonstration: a single decomposition reaction, (NH4)2Cr2O7 to Cr2O3 + N2 + 4H2O, produces a voluminous green solid and a gas, and both products are exam favourites in their own right.
Green Cr2O3 Powder from the Ammonium Dichromate Volcano is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
One decomposition, two well-known products
The green, fluffy solid product is chromium(III) oxide, Cr2O3 — this is chromium reduced from the dichromate +6 state down to +3, while the nitrogen in the ammonium ion is oxidized from -3 up to 0 (as N2 gas); this is an internal redox reaction (a type of comproportionation-adjacent decomposition) where the oxidizing dichromate half of the molecule and the reducing ammonium half react with each other, needing no external reagent at all. The visually dramatic, bulky green ash and the escaping gas together are exactly what makes this the standard chemistry-classroom volcano demonstration.
Recognising N2 as the gas, and where else it comes from
The gas evolved is nitrogen, N2 — not ammonia (which would require the nitrogen to stay at -3, not be oxidized), not nitrous oxide, and not oxygen. The same N2 gas is produced by several other classic decomposition reactions taught alongside this one, most notably the thermal decomposition of ammonium nitrite (NH4NO2 → N2 + 2H2O), which is in fact the more common laboratory method for preparing small quantities of pure nitrogen gas, precisely because both the oxidizing (nitrite, N+3) and reducing (ammonium, N-3) nitrogen atoms are already present in the one molecule, converging to N2 (N at 0) without needing any external reagent — recognise this shared pattern (an internal redox reaction between an oxidizing and reducing nitrogen-containing group within the same salt) whenever a question links the ammonium dichromate gas to another nitrogen source.
OlympIQ exam lens
- ✓(NH4)2Cr2O7 decomposes on heating to Cr2O3 (green solid) + N2 (gas) + H2O (vapour) — the classic chemical volcano
- ✓Chromium is reduced from +6 (dichromate) to +3 (Cr2O3); nitrogen is oxidized from -3 (ammonium) to 0 (N2) — an internal redox decomposition
- ✓The gas evolved is nitrogen (N2), not ammonia, nitrous oxide, or oxygen
- ✓Heating ammonium nitrite (NH4NO2) is the classic parallel reaction that also gives N2 by the same internal-redox logic
Do not assume the gas from heating ammonium dichromate is ammonia just because the compound contains an ammonium ion. The nitrogen is oxidized (not simply released as NH3) in this internal redox decomposition, giving N2 gas — the same logic and the same gas as heating ammonium nitrite.
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