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Chemical formula H2. Hydrogen accounts for 17% by atomic percentage in the Earth’s crust (including the atmosphere, water, and lithosphere), and about 1% by mass percentage. Hydrogen is the main component of the solar atmosphere, accounting for 81.75% by atomic percentage. It is the main raw material for nuclear reactions in the sun, and it is also the primary source of energy for all living organisms on Earth. In nature, hydrogen exists primarily in combined forms throughout water, organic compounds, and living organisms. Only a small amount of free hydrogen is present in a few substances such as natural gas. Hydrogen is a colorless and odorless gas. Boiling point—252.87°C, freezing point—259.14°C. Under standard conditions, the density of hydrogen is 0.08987 g/L, making it much lighter than air. Balloons and airships can fly in the air when filled with hydrogen. Hydrogen can be transformed into a liquid by deep freezing and pressurization; at –259.2°C, it turns back into a transparent solid. Low temperatures can be achieved using liquid hydrogen. Hydrogen is slightly soluble in water; at 0°C, 1 volume of water can dissolve only 0.02 volumes of hydrogen. However, hydrogen is easily adsorbed by certain metals. At room temperature, the volume ratio of hydrogen absorbed by fine palladium powder is approximately 1:900. Hydrogen is flammable and releases a large amount of heat at the same time. The temperature of a hydrogen-oxygen flame reaches 2,700°C, and it can be used to cut metal. When hydrogen is mixed with oxygen, an explosion occurs upon exposure to fire when the volume percentage of hydrogen ranges from 7% to 92%. Its volume fraction in air ranges from 4.1% to 75%, and it can also explode; therefore, the purity of hydrogen must be checked before igniting it. It is a strong reducing agent that can directly combine with many metals and non-metals. When heated, it easily takes oxygen away from metal oxides, thereby reducing the metals. There are many ways to produce hydrogen. Metallic zinc is used in the laboratory to be reacted with dilute sulfuric acid (or hydrochloric acid) to produce it. In industry, water vapor is commonly produced by passing it over hot coal; hydrogen and oxygen can also be obtained by electrolyzing water. Hydrogen is a raw material for the synthesis of hydrochloric acid and ammonia; in organic synthesis, it is used in hydrogenation reactions and for the hardening of fats. Liquid hydrogen is a high-energy fuel, while solid hydrogen possesses metallic properties and superconductivity.