X is a metallic oxide which is a part of soda lime that reacts with hydrochloric acid to form a water-soluble salt. X is 

CaO is known as quick lime and is a part of soda lime. the mixture of CaO and NaOH is known as soda lime.

When CaO reacts with hydrochloric acid (HCl), it forms CaCl₂ (calcium chloride) which is soluble in water.

The chemical rection between CaO and HCl is represented as:

${\mathrm{CaO}}_{\left(\mathrm{s}\right)}+2{\mathrm{HCl}}_{\left(\mathrm{aq}\right)}\to {\mathrm{CaCl}}_{2\left(\mathrm{aq}\right)}+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}$