Caesium reacting with water (1 of 5). Caesium (Cs) is an highly-reactive alkali metal element that reacts violently with water to produce the strong base caesium hydroxide, also releasing hydrogen gas. The caesium sinks and (see subsequent images in the sequence) generates hydrogen gas so rapidly that shock waves form in the water. The heat of the reaction boils some of the water into steam. This image is a still taken from a high speed film shot at 2000 frames per second.