Nitric acid is colourless when freshly prepared. But, on long standing, it turns to yellow. How do you account for this?

Explanation

**Step-by-Step Solution:**

1. **Understanding Freshly Prepared Nitric Acid**: Freshly prepared nitric acid (HNO3) is a colorless liquid. This is due to its pure composition without any impurities or additional compounds.

2. **Observation of Color Change**: Upon standing for a period, nitric acid changes from colorless to yellow. This change in color indicates a chemical reaction or decomposition occurring over time.

3. **Chemical Reaction Involved**: The yellow color in nitric acid is primarily due to the formation of nitrogen dioxide (NO2). When nitric acid is exposed to light and air, it can decompose. The reaction can be summarized as:
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