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