Lithium reacting with water. Lithium (Li) is a highly reactive metallic element. It is light enough to float on water, with which it reacts violently to produce hydrogen (H2) gas and lithium hydroxide (LiOH). Bubbles of H2 can be seen around the piece of metal. The presence of LiOH is indicated by the chemical phenolphthalein, which turns purple as the hydroxide is formed.