Substance added to water, to manufacture sulphuric in contact process is Oleum $(\mathrm{H}_2\mathrm{S}_2\mathrm{O}_7)$ .Explanation :Oleum is diluted by adding calculated amount of water to obtain sulphuric acid of the desired strength. $\mathrm{H}_2\mathrm{S}_2\mathrm{O}_7 + \mathrm{H}_2\mathrm{O} \rightarrow 2\mathrm{H}_2\mathrm{SO}_4$