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

What happens to potassium when it reacts with water?

1. Becomes a solid

2. Produces a liquid

3. Forms a gas

4. Produces a lilac flame

The correct answer is: Produces a lilac flame

When potassium reacts with water, it undergoes a vigorous exothermic reaction, producing potassium hydroxide and hydrogen gas. One of the hallmark characteristics of this reaction is the distinctive lilac flame that is produced. This lilac color is due to the emission of light from excited potassium ions as they release energy when returning to their ground state. This specific flame color is a key identifier of potassium in chemical reactions. The other scenarios presented do not accurately depict the behavior of potassium during its reaction with water. Potassium does not become a solid; rather, it remains in a liquid state due to the heat generated during the reaction. While both a liquid and a gas are produced during the reaction, the production of the lilac flame is what most distinctly characterizes the reaction with potassium. Thus, the lilac flame is a unique visual cue that relates specifically to this metal's interaction with water.