Stability of F- Cl- Br- I-? In aqueous state
If we consider $\ce{HX}$ as the common formula for hydrogen halogenides, then there is an equilibrium
$$\ce{HX(aq) + H2O &lt;=&gt; X-(aq) + H3O+(aq)}$$
that can be expressed also as:
$$\ce{acid1 + base2 &lt;=&gt; base1 + acid2}$$
The equilibrium is competition of 2 acids, $\ce{HX}$ ansd $\ce{H3O+}$. $\ce{H3O+}$ is the strongest acid stable in aquaeous environment. Acids we consider as strong acids ($\ce{HCl}$, $\ce{H2SO4}$, $\ce{HNO3}$ ) are much stronger acids than $\ce{H3O+}$. Therefore for $\ce{HCl}$, $\ce{HBr}$, $\ce{HI}$, the equilibrium is practicaly fully pushed toward right:
$$\ce{HX(aq) + H2O -&gt; X-(aq) + H3O+(aq)}$$
On the other hand, $\ce{HF(aq)}$ is a weak acid, weaker than $\ce{H3O+}$, with dissociation constant $\mathrm{p}K_\ce{a}=3.17$, for which the equilibrium is rather on the left side:
$$\ce{HF(aq) + H2O &lt;&lt;=&gt; F-(aq) + H3O+(aq)}$$
Highly concentrated $\ce{HF(aq)}$ is rather a strong acid:
$$\ce{2 HF(aq) + H2O &lt;=&gt;&gt; HF2-(aq) + H3O+(aq)}$$
and liquid HF is very strong acid ( H0 = −11 ) :
$$\ce{3 HF &lt;=&gt;&gt; HF2- + H2F+}$$