Pure hydrogen gas, H2, is almost purely water insoluble, and like all other gases, its solubility decreases as the temperature of the water increases. At zero degrees Celsius, the solubility of hydrogen is 0.0019 grams per kilogram of water, which drops to below 0.0012 g/kg at 60 degrees.
Hydrogen gas is produced by reacting almost any metal with a strong acid, such as hydrochloric acid. As hydrogen gas is mostly insoluble, the bubbles of hydrogen gas that rise to the top of the solution are easily recovered through water displacement. However, this hydrogen is not pure, as it also contains small amounts of water vapor and air, which must then be removed through purification.