Qualitative Analysis: Practice Questions

Which gas turns acidified aqueous potassium manganate(VII) from purple to colorless?

A student adds aqueous sodium hydroxide to an unknown solution and observes a white precipitate that does not dissolve in excess. When aqueous ammonia is added to a fresh sample of the same solution, no precipitate is observed. Which cation is present in the solution?

A student performs the following tests on a colorless solution:
1. The solution is heated with aqueous sodium hydroxide, and no gas is evolved.
2. Aluminum foil is then added to the same mixture and heated. A gas is evolved that turns damp red litmus paper blue.
Which ion is identified as being present in the original solution?

A student bubbles an unknown gas through a solution of limewater. A white precipitate is observed in the limewater. Which gas is being tested?

A student tests a colorless solution and observes that adding aqueous silver nitrate followed by dilute nitric acid produces a yellow precipitate. Additionally, the solution turns acidified potassium manganate(VII) from purple to colorless. Which ion is present and how does it react with the potassium manganate(VII)?

State the chemical test and the expected observation used to identify the presence of hydrogen gas, \( \text{H}_2 \).

A student adds aqueous sodium hydroxide dropwise until in excess to a solution containing an unknown metal cation. A green precipitate forms that is insoluble in excess. Upon standing in air, the top layer of the precipitate turns reddish-brown. Identify the cation present and explain the chemical process that causes the color change at the surface.

A student is testing for the presence of sulfate ions, \( \text{SO}_{4}^{2-} \), in an unknown aqueous solution. State the reagents used for this test and the observation that indicates a positive result.