When an acid dissolves in water, its molecules ionise and form hydrogen ions, H+(aq). Hydrogen chloride gas dissolves in water to form hydrochloric acid, which contains H+ ions and chloride ions: HCl → H+ + Cl−. The hydrogen ions are free in the solution. They are not still attached to the rest of the molecule.
It is the hydrogen ions that make a solution acidic. The more hydrogen ions there are in a solution, the more acidic it is. All acids, such as sulfuric acid and nitric acid, form hydrogen ions in water.
An alkali is a base that dissolves in water. A solution of an alkali contains hydroxide ions, OH−(aq). Sodium hydroxide dissolves to give sodium ions and hydroxide ions: NaOH → Na+ + OH−. It is the hydroxide ions that make a solution alkaline.