Types of reactions -- identification

[Cheesycheese213]

Homework Statement

What is the reaction occurring in
H₂O + SO₃ -> H₂SO₄

2. The attempt at a solution
Sorry, if this is dumb, I was confused about this question because I think its synthesis but I thought it only worked with elements? (Element A + Element B = Compound AB)
But then with this, I have water and SO₃ which are already compounds becoming a bigger compound?
I don't think it really fits with the other types of reactions (single displacement, double displacement, decomposition and combustion all end up with something like A + B?), but then if it is synthesis does that mean that all compounds can also do this?
Thanks!

 

[Borek]

Yes, it is synthesis. You combine two things producing a third.

 

[snehal]

It is synthesis(direct combination) reaction.

 

[epenguin]

Different terms may describe different aspects, so you may more often encounter this described as 'hydration', the reverse reaction 'dehydration'.