The basic principle of the hydrogen economy is the transportation and storage of energy in the form of liquid or gaseous hydrogen. Which property of hydrogen may be useful for this purpose? Support your answer with a chemical equation if required.

(N/A) The primary property of hydrogen useful for the hydrogen economy is its ability to be liquefied under high pressure and low temperature. Hydrogen is a gas at room temperature,which makes it difficult to transport in bulk due to its low density. By cooling it and applying high pressure,gaseous $H_2$ can be converted into liquid $H_2$,which occupies a much smaller volume and can be transported easily. The process is a physical change: $H_{2(g)} \xrightarrow[\text{high pressure}]{\text{cooling}} H_{2(l)}$.