International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

What characteristic of potassium is observed during its reaction with water?

1. It dissolves completely

2. It turns blue

3. It emits a loud sound

4. It displays a lilac flame

The correct answer is: It displays a lilac flame

When potassium reacts with water, it produces a characteristic lilac flame. This is due to the excitation of potassium ions in the flame, which emit light at a specific wavelength corresponding to the color lilac when they return to their ground state. This unique coloration serves as a diagnostic feature for the presence of potassium in chemical reactions. In the case of potassium and water, the reaction also generates potassium hydroxide and hydrogen gas, which can lead to effervescence but does not cause the water to turn blue or dissolve completely in any conventional way. Moreover, while the reaction may produce sounds due to the rapid release of hydrogen gas, the prominent and notable feature is the lilac flame produced during the reaction, allowing for easy identification of potassium's presence.