When a black metal compound XO is heated with a colourless gas $Y_2$, the metal X and another compound $Y_{2}O$ are formed. Metal X is red- brown in colour which does not react with dilute acids at all. Gas $Y_2$ can be prepared by the action of a dilute acid on any active metal. The compound $Y_{2}O$ is a liquid at room temperature which can turn anhydrous copper sulphate blue. The gas $Y_2$ is: