Nitric Acid Test - Reddish Brown Fumes
Most of the reactions on this bench need two things brought together. Concentrated nitric acid needs only heat: strongly warmed on its own, it breaks apart into water, oxygen and nitrogen dioxide, the reddish-brown gas responsible for the acid's own yellowish tinge on standing. It is the simplest possible demonstration that a substance can decompose without reacting with anything else — heat alone is the reagent.
4 steps · Nitric Acid · Study of Compounds
What you will use
Apparatus
- Flask
Chemicals
- Concentrated Nitric Acid
The steps
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Pour concentrated nitric acid into the flask.
Faintly yellow, and nothing changes until it is heated.
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Attach a burner under the flask.
The flame is lit; the burner slider now controls how hot the acid gets.
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Heat the acid.
The acid begins to break down as it warms.
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Watch the gas coming off.
Reddish-brown fumes fill the flask — nitrogen dioxide, escaping as the acid decomposes.
What it shows
Concentrated nitric acid decomposes on strong heating alone: 4HNO₃ → 2H₂O + 4NO₂ + O₂. The reddish-brown fumes are nitrogen dioxide; the colourless oxygen escapes alongside it unnoticed. Nothing else was added — heat by itself was enough to break the acid apart, which is also why concentrated nitric acid left standing in sunlight slowly yellows.