The lightest of all chemical elements, hydrogen has a density about one-fourteenth that of air. It has a number of special chemical and physical properties. For example, hydrogen has the second lowest boiling and freezing points of all elements. The combustion of hydrogen produces large quantities of heat, with water as the only waste product. From an environmental standpoint, this fact makes hydrogen a highly desirable fuel. Many scientists foresee the day when hydrogen will replace fossil fuels as our most important source of energy.