What is the gas liberated when alkaline formaldehyde solution is treated with $H_2 O_2$?

To an aqueous solution of $AgNO_3$,some $NaOH_{(aq)}$ is added until a brown precipitate is obtained. To this,$H_2O_2$ is added dropwise. The precipitate turns black with the evolution of $O_2$. The black precipitate is:

$H_2O_2$ is manufactured these days by

Which of the following can produce $H_2O_2$?

Addition of $H_{2}SO_{4}$ to $BaO_{2}$ produces

$KO_2 + H_2O \to KOH + O_2 +$ product $(X)$. Choose the correct statement about product $(X)$.