Rubidium reacting with oxygen in air. This is a very soft metal. Its melting point is 39 degrees Celsius about 16 degrees warmer than room temperature, so it is no surprise that it is so soft. Notice the graying of the exposed rubidium as it reacts with oxygen in the atmosphere. This piece of rubidium already has some oxide or other rubidium compounds on its surface. A piece of rubidium which has not been exposed to air is a beautiful silvery color. If the rubidium were exposed to pure oxygen it would immediately catch fire. It along with all of the alkali metals must be stored in liquids that do not contain oxygen.