Chemistry 2007 JAMB Past Questions - Question 4: A solution which on treatment with hydrochloric acid produces a colourless, odourless gas which turns lime water milky is most likely to be a

Correct Answer

C

Explanation

The solution that, upon treatment with hydrochloric acid, produces a colorless, odorless gas which turns lime water milky is most likely a trioxonitrate(V) or nitrate(V) ion. When hydrochloric acid reacts with a nitrate salt, it forms nitrogen dioxide gas (NO2), which is colorless and odorless. However, when the nitrogen dioxide gas comes into contact with moist air, it reacts with oxygen to form nitrogen trioxide (NO3), which then dissolves in water to produce nitric acid (HNO3). Nitric acid reacts with calcium hydroxide (lime water) to form calcium nitrate and water, while the nitrogen dioxide reacts with the water to produce nitrous acid (HNO2), which decomposes into nitric oxide (NO) and water. The nitric oxide gas reacts with oxygen in the air to form nitrogen dioxide, which dissolves in water to produce nitric acid, resulting in the overall reaction:

\[2\text{NO}_2(g) + 2\text{H}_2\text{O}(l) + \text{O}_2(g) \rightarrow 4\text{HNO}_3(aq).\]

The formation of nitric acid is responsible for turning lime water milky due to the formation of a white precipitate of calcium nitrate. Therefore, the correct option is likely to be a trioxonitrate(V) or nitrate(V).