Which of the following methods could a scientist use to identify the anion present in an unknown substance?

• A. Measure the pH of the solution to determine the anion.
• B. Heat the substance and check for the evolution of hydrogen gas.
• C. Test the substance with a glowing splint to see if it relights.
• D. Add a few drops of silver nitrate solution and observe if a precipitate forms, indicating the presence of halide ions.

Answer: (D)

Verified step by step guidance

1. Understand that identifying an anion in an unknown substance often involves observing its chemical reactions, especially precipitation reactions with specific reagents.
2. Recognize that measuring the pH of a solution primarily gives information about the acidity or basicity, which is influenced by the cation and anion but does not directly identify the anion.
3. Know that heating the substance to check for hydrogen gas evolution is a test more relevant to identifying metals or metal hydrides, not anions.
4. Recall that testing with a glowing splint is used to detect oxygen gas, which is unrelated to identifying anions in a solution.
5. Use the reaction with silver nitrate solution: silver ions (Ag\^{+}) react with halide ions (Cl\^{-}, Br\^{-}, I\^{-}) to form characteristic precipitates (AgCl, AgBr, AgI), which helps identify the presence of halide anions.