Rubidium reacting with water (6 of 6). Rubidium (Rb) is an highly-reactive alkali metal element that reacts violently with water to produce the strong base rubidium hydroxide, also releasing hydrogen gas (see other images in this sequence.) 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. This image is a still taken from a high speed film shot at 2000 frames per second.