A water insoluble substance 'X' on reacting with dilute `H_(2)SO_(4)` released a colourless and odourless gas accompanied by risk effervescence. When the gas was passed through water, the solution obtained turned blue litmus red. On bubbling the gas through lime water, it initially became milky and the milkiness disappeared when the gas was passed in excess. Identify the substance 'X' and write the chemical equations of the reactions involved.