Zinc in a single displacement reaction with hydrochloric acid
The zinc replaces the hydrogen in that is reduces (gives electrons to) $\ce{H^+}$, forming $\ce{H2(g)}$ and $\ce{Zn^2+(aq)}$ according to this reaction: $$\ce{Zn(s) + 2H^+(aq) -&gt; Zn^2+ + H2(g)}$$ This reaction relies on the prescence of $\ce{H^+}$.

The hydrogen in acids exists as $\ce{H^+}$. Acids are also ionic compounds that involve hydrogen as the donor of electrons. For example, hydrogen gas and chlorine gas react to make hydrogen chloride: $$\ce{H2 -&gt; 2H^+ + 2e^-}$$ $$\ce{Cl2 + 2e^- -&gt; 2Cl^-}$$ $$\ce{2H^+ + Cl^- -&gt; 2HCl}$$  The reverse of this happens when the hydrogen chloride is dissolved into water.  this forms hydrochloric acid.The compound breaks apart into hydrogen and chlorine ions:$$\ce{H2O + HCl -&gt; H+ + Cl-}$$ Hydrochloric acid completely dissociates, meaning that every $\ce{HCl}$ molecule yields one $\ce{H^+}$ ion.

Essentialy, acids increase the amount of $\ce{H^+}$ in solution. Normally, in pure water, concentrations of $\ce{H^+}$ are very low. The hydrogen in $\ce{H2O}$ is covalently bonded to the oxygen, so it is difficult for the hydrogen to react. In order to liberate a hydrogen ion from $\ce{H2O}$, this must occur: $$\ce{H2O -&gt; H^+ + OH^-}$$
This reaction happens infrequently in water, so concentrations of $\ce{H^+}$ are very low. Therefore, zinc barely replaces hydrogen in water.