Well it is solvated by several water molecules. And as the conjugate base of a strong acid it delivers electrical neutrality to the solution.
This is taken from an older answer.......
#HCl(g)# is a source of hydronium ion, #H_3O^+# in aqueous solution.........
We may take a tank of #HCl(g)#, and we can bleed it in to water to give an AQUEOUS solution that we could represent as #HCl(aq)# OR #H_3O^+# and #Cl^−#.
#HCl(g) stackrel(H_2O)rarrunderbrace(H_3O^(+))_("hydronium ion") + Cl^-#
In each case this is a REPRESENTATION of what occurs in solution. If we bleed in enuff gas, we achieve saturation at a concentration of approx. #10.6*mol*L^-1# with respect to hydrochloric acid. Sometimes when you use conc. hydrochloric acid in a lab (and of course you are wearing spectacles to protect your mincers, and a lab coat to protect your clothes), you inadvertently get a whiff of the #HCl# vapour, and you know it!
As far as anyone knows, the actual acidium ion in solution is
#H_5O_2^+# or #H_7O_3^+#, i.e. a cluster of 2 or 3 or 4 water molecules with an EXTRA #H^+# tacked on. We represent it in solution (without loss of generality) as #H_3O^+#, the #"hydronium ion"#, which is clearly the conjugate acid of #H_2O#.
Note that the #H^+# is quite mobile, and passes, tunnels if you like, the extra #H^+# from cluster to cluster. If you have ever played rugby, I have always liked to compare to this to when the forwards form a maul, and can pass it from hand to hand to the back of the maul while the maul is still formed. Of course, tunnelling, proton transfer, is more likely in a cluster of water molecules, so the analogy might not be particularly apt in that there is definite transfer of a ball in a maul, but a charge in a water cluster. For this reason both #H^+# and #HO^-# have substantial mobility in aqueous solution.
#H^(+) + e^(-) rarr 1/2H_2(g)#
With an electron source, a metal, the metal may be oxidized......
Depending at which level you are at (and I don't know!, which is part of the problem in answering questions on this site), you might not have to know the details at this level of sophistication. The level I have addressed here is probably 1st/2nd year undergrad.........
