For example, hydrogen is widely used to fill balloons; The combustion of hydrogen in oxygen releases a lot of heat, and the temperature of its flame-hydrogen-oxygen flame reaches 3000℃, which can be used for welding or cutting metals. Liquid hydrogen can be used as high-energy fuel for rockets or missiles. As a fuel, hydrogen has the characteristics of rich resources, high combustion calorific value and less pollution. In the future, if there is a major breakthrough in the technology of hydrogen production from solar energy and water, hydrogen will become an important new fuel. Hydrogen is also widely used in metallurgical and chemical industries.