Alkali metals reacting in water. Footage of (from left to right) three alkali metals lithium (Li), sodium (Na), and potassium (K) reacting in water (H20). The alkali metals are highly reactive metallic elements that float on water and react with it. These exothermic (heat-producing) reactions form the corresponding hydroxide (e.g. LiOH) and hydrogen gas (H2). The reactivity increases from lithium to potassium. The potassium ignites and burns with a purple flame. The piece of sodium fizzes and moves around on the water's surface, as do the smaller pieces of lithium. The smaller pieces of lithium don't react more vigorously than the sodium, despite having a larger surface area. By the end of the clip, nearly all the metals have finished reacting.