Hydrogen

Electrolysis of water: $2H_2O\longrightarrow 2H_2+O_2$ Water produces hydrogen and oxygen

Fuel cell: $2H_2$ $+$ $O_2$ $\longrightarrow$ $2H_2O$ Hydrogen and oxygen produce water

Explosion: $2H_2$ $+$ $O_2$ $\longrightarrow$ $2H_2O$ Hydrogen and oxygen produce water

Reduction of copper oxide: $2H_2$ $+$ $CuO$ $\longrightarrow$ $Cu$ $+$ $H_2O$ Hydrogen and copper(II)oxide produce copper and water

Reaction with chlorine: $H_2$ $+$ $Cl_2$ $\longrightarrow$ $2HCl$ Hydrogen and chlorine produce hydrogen chloride (gas)