Bleach and HCl.....
Chlorine gas reacts with water to produce solutions of HOCl and HCl. The latter as H+ and Cl- ions since HCl is a strong acid and completely ionizes. HOCl on the other hand is a weak acid and only marginally ionizes. Bleach can be prepared by neutralizing the acid with sodium hydroxide.
Cl2(g) + H2O(l) <==> HOCl(aq) + HCl(aq)
HOCl(aq) + NaOH(aq) --> NaOCl(aq) + HOH(l)
HCl(aq) + NaOH(aq) --> NaCl(aq) + HOH(l)
Bleach is made commercially by reacting chlorine gas with a solution of NaOH. The reaction is the combination of the three equation above.
Cl2(g) + 2NaOH(aq) --> NaOCl(aq) + NaCl(aq)
The solution of bleach consists of Na+ ions, OCl- ions and HOCl(aq), and Cl- ions along with some OH- ions. Bleach is basic.
When bleach is combined with HCl, the acid can neutralize the solution and cause a shift in the equilibrium, thus converting some of the OCl- and Cl- ions back into Cl2 gas (which could be a bad thing). After neutralizing the bleach solution with HCl, we have the following equilibrium. (Na+ is a spectator ion.)
Cl2(g) + H2O(l) <==> HOCl(aq) + Cl- + H+
Using LeChatelier's principle, we can see how the addition of HCl (H+ + Cl-) can shift the equilibrium to the left producing chlorine gas.
Therefore, don't acidify bleach with any acid (particularly hydrochloric acid).