Question

Why does concentrated nitric acid often have a yellow color even though pure \(\mathrm{HNO}_{3}\) is colorless?

Short answer

Answer: The yellow coloration in concentrated nitric acid is due to the dissolution of nitrogen dioxide gas, which is formed through the decomposition of nitric acid under certain conditions, such as elevated temperature, exposure to light, or in the presence of impurities or active metals.

Step-by-step solution

Understanding the properties of nitric acid

Nitric acid (\(\mathrm{HNO}_{3}\)) is a strong acid that is commonly used in laboratories and industries. It is a colorless liquid at its pure form, but concentrated nitric acid can often exhibit a yellow coloration, which needs to be explained.

Identifying the possible reason for the yellow color

The yellow coloration in concentrated nitric acid can be attributed to the presence of nitrogen dioxide (\(\mathrm{NO}_{2}\)) gas dissolved in it. Nitrogen dioxide is a reddish-brown gas that can dissolve in nitric acid, imparting the yellow color.

Understanding the formation of nitrogen dioxide

Nitrogen dioxide can be formed through the decomposition of nitric acid when it is in concentrated form. This decomposition can be caused by various factors, such as elevated temperature, exposure to light, or the presence of impurities or active metals. The decomposition reaction can be represented by the following equation: $$ 4\mathrm{HNO}_{3} \longrightarrow 2\mathrm{H}_{2}\mathrm{O} + 4\mathrm{NO}_{2} + \mathrm{O}_{2} $$

Concluding the explanation for the yellow color

The yellow color observed in concentrated nitric acid is due to the dissolution of nitrogen dioxide gas, which is formed through the decomposition of nitric acid. This decomposition can occur under certain conditions, such as elevated temperature, exposure to light, or in the presence of impurities or active metals.