HCl + KMnO4 + Hg → H2O + KCl + MnO2 + HgCl2
8 HCl (aq) + 2 KMnO4 (aq) + 3 Hg (l) → 4 H2O (l) + 2 KCl (aq) + 2 MnO2 (s) + 3 HgCl2 (aq)
This is an oxidation-reduction (redox) reaction:
2 MnVII + 6 e– → 2 MnIV (reduction)
3 Hg0 – 6 e– → 3 HgII (oxidation)
KMnO4 is an oxidizing agent, Hg is a reducing agent.