High-speed footage of caesium reacting explosively as it falls into cold water. Caesium (Cs) is an highly-reactive alkali metal element that reacts with water to produce the strong base caesium hydroxide, also releasing hydrogen gas. The caesium sinks and generates hydrogen gas so rapidly that shock waves form in the water, and the heat of the reaction boils some of the water into steam. Multiple explosions and shocks are seen at this speed. The indicator phenolphthalein was added to the water to highlight the reaction, as it turns from colourless to pink in the presence of the alkaline caesium hydroxide. Filmed at 2000 frames per second (slowed down 80 times). This clip is also available with the chemical equation for the reaction overlaid. See clip K004 3409.