Re: How does SO3 a covalent compound dissolve in water to form sulphuric acid?

The explanation for this is the same as that for CO₂ dissolving in water.
This is a very good reference on that.
http://antoine.frostburg.edu/chem/senese/101/solutions/faq/dissolving-gases.shtml

The first thing to remember is that water is NOT ionic. It is a polar covalent compound. SO₂ and SO₃ have polar bonds also, so they and water are more alike than you think. The second point is that of equilibriums and thermo. The reaction of SO₃ is very exothermic (very favorable) and not reversible. Also note, sulfur trioxide is actually a liquid allowing easier dissolution.

Now if you are studying formation of acid rain, that is the reaction of SO₂ with water in the gas state of waer, not liquid.