In a laboratory demonstration, small, equal-sized pieces of lithium, sodium, and potassium are added to separate troughs of water. Only potassium reacts vigorously enough to immediately ignite with a lilac flame. Potassium produces a flame because it:
is the most reactive of the three metals
has the highest melting point of the three metals
is the densest metal of the three
is the only one of the three metals that reacts with water
75 exam-style questions on Edexcel GCSE Chemistry 7.1 Group 1, covering 7.1.1 Classifying elements by group in the periodic table and 7.1.2 Properties and reactions of the alkali metals. Each one has a worked solution and a mark scheme showing where the marks go.