Do strong acids completely dissociate in water?

For all practical purposes, strong acids completely dissociate in water.

That is the definition: A strong acid is an acid that completely dissociates in water.

In theory, every reaction is an equilibrium reaction. For example

HCl + H₂O ⇌ H₃O⁺ + Cl⁻

##K_"a" ="[H₃O⁺][Cl⁻]"/"[HCl]"## = 1.55 × 10⁶

We see that the position of equilibrium lies far to the right.

We can calculate that in 1 mol/L HCl there are more than 1200 H₃O⁺ ions for every molecule of undissociated HCl.

For all practical purposes, HCl is completely dissociated in solution.

Strong acids have a large dissociation constant, so they dissociate completely in water.