2H+ +2e- ---> H2 so the oxidation state of H in HCl was +1 which has changed to 0 in H2 this change took place because Ca changes its oxidation state from 0 to +2 and those electrons are accepted by H+ to make H2 Solution 2H+ +2e- ---> H2 so the oxidation state of H in HCl was +1 which has changed to 0 in H2 this change took place because Ca changes its oxidation state from 0 to +2 and those electrons are accepted by H+ to make H2.