Strong acids or bases can dissociate completely when placed in a solution of water. HCl is a strong acid with a pH value between 0 and 2. Pure water has a pH of 7. When placing HCl into solution with water, it will shift the pH by increasing the number of Hydrogen ions (H+) in solution. When HCl dissociates, it will add H+ ions along with Cl- ions to the solution.
Actually, because the compound HCl contains H atoms bound to the nonmetal Chlorine, this type of compound is known as a nonmetal hydride. When placed in water, the HCl acts as a source of H+ ions thus lowering the pH of the solution.
To summarize, when HCl dissolves in water, its components dissociate into H+ ions and Cl- ions when the covalent bond is broken between them. Water molecules are polar with oxygen atoms negatively charged and the hydrogens positively charged. This helps make water an excellent solvent for polar compounds like HCl due to the attraction of opposite charges.