The correct answer is D. Coloured impurities.
Nitric acid is a colorless liquid, but commercial nitric acid is often yellow or brown due to the presence of dissolved impurities. These impurities can be organic compounds, such as tar, or inorganic compounds, such as iron salts. The color of the nitric acid can vary depending on the concentration of the impurities.
Oxygen, nitrous oxide, and nitrogen dioxide are all gases that are not soluble in nitric acid. Therefore, they do not contribute to the color of commercial nitric acid.
Here is a brief explanation of each option:
- Option A: Oxygen is a gas that is not soluble in nitric acid. Therefore, it does not contribute to the color of commercial nitric acid.
- Option B: Nitrous oxide is a gas that is not soluble in nitric acid. Therefore, it does not contribute to the color of commercial nitric acid.
- Option C: Nitrogen dioxide is a gas that is not soluble in nitric acid. Therefore, it does not contribute to the color of commercial nitric acid.
- Option D: Coloured impurities are the most likely cause of the color of commercial nitric acid. These impurities can be organic compounds, such as tar, or inorganic compounds, such as iron salts. The color of the nitric acid can vary depending on the concentration of the impurities.