A water soluble compound 'X' of sodium on reacting with dil.`H_2SO_4` released a colourless gas accompanied by brisk effervescence. When the gas was passed through water, the solution obtained turned blue litmus solution red. On bubling the gas through lime water, it initially became milky and the milkness disappeared when the gas was passed in excess. Identify the substance 'X' and write the chemical equation for the reactions involved.