A small piece of metallic sodium is placed in a dish of water containing phenolphthalein on an overhead projector stage. The sodium reacts vigorously, melts, and sputters on the surface of the water as a sphere of continuously decreasing size until it has completely reacted. The moving sodium leaves a pink trail in the water. Go to Top Set Alkali metals react with water.
Sodium reacts very vigorously.
Potassium and rubidium react still more vigorously. Cesium reacts explosively. This reaction is very exothermic. Sometimes the hydrogen produced catches fire with oxygen in the air. Using warm water promotes a vigorous reaction.