Industrial nitric acid (HNO3): Concentrated nitric acid often appears pale to light yellow in color. This coloration is primarily due to the presence of which dissolved nitrogen oxide species?

Introduction / Context:
Nitric acid used in laboratories and industry may appear water-clear when very pure, but it frequently develops a pale to light yellow coloration during storage. This visible tint is a classic indicator of dissolved nitrogen oxides formed by slow decomposition or contamination. Recognizing the responsible species is useful in quality control and safety handling of oxidizing acids.

Given Data / Assumptions:

• Chemical: HNO3 (typically 65–70% w/w commercial grade).
• Observed feature: pale/light yellow color in the liquid phase.
• Environment: storage in light/heat may accelerate decomposition.

Concept / Approach:
Concentrated HNO3 slowly decomposes to nitrogen dioxide (NO2), oxygen, and water. NO2 is brownish-red as a gas but imparts a yellow to amber coloration when dissolved in nitric acid, even at modest concentrations. Other nitrogen compounds listed (NO, NH3, N2O, hydrazinium species) either do not persist in strongly oxidizing acidic media or do not produce the characteristic yellow hue under normal storage conditions.

Step-by-Step Solution:

Verification / Alternative check:
Freshly distilled, fuming nitric acid is nearly colorless but rapidly yellows on exposure to light/air as NO2 dissolves; degassing under vacuum or cooling can reduce the tint, supporting the NO2 explanation.

Why Other Options Are Wrong:

• NO: colorless and not persistent in strong oxidizer.
• NH3: protonated/consumed; does not cause yellow color.
• N2H5 species: incompatible with strong oxidizing acid.
• N2O: colorless, low solubility; no yellow tint.

Common Pitfalls:
Confusing the brown gas above warm acid (NO2) with the liquid's pale tint; both arise from the same oxide but differ in appearance due to phase and concentration.

Final Answer:
NO2 (nitrogen dioxide).