High-speed footage of rubidium reacting explosively as it falls into cold water. Rubidium (Rb) is an highly-reactive alkali metal element that reacts with water to produce the strong base rubidium hydroxide, also releasing hydrogen gas. 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 rubidium hydroxide. Filmed at 2000 frames per second (slowed down 80 times).