Hydrogen is a colorless gas, and its density is lower than that of air. Under standard conditions, the mass of 1 liter of hydrogen is 0.0899g, which is also much lighter than air). Because hydrogen is insoluble in water, it can be collected by drainage and gas collection. In addition, at the pressure of 10 1 kPa and the temperature of -252.87℃, hydrogen can be converted into a colorless liquid. At -259. 1℃, it becomes a snow-like solid. At room temperature, the nature of hydrogen is very stable and it is not easy to react with other substances. But when the conditions change (such as ignition, heating, using catalyst, etc. ), the situation is different. For example, after hydrogen is adsorbed by metals such as palladium or platinum, it has strong activity (especially palladium adsorption). Palladium metal has the strongest adsorption effect on hydrogen. When the volume fraction of air is 4%-75%, it may cause an explosion when it meets a fire source.