(1) By dissolution of acid anhydride.
CO2(g) + H2CO3(g)
NB: Acid anhydrides are non-metallic oxides, which dissolve in water to form acid. E.G. CO2, SO2 etc
(2) By combination of constituent elements
H2(g) + H2O(L) 2HCL(g)
Note: The HCL readily dissolves in water to form acid HCL(ag)
(3) By the use of stronger acids with salts of weaker ones so as to liberate the weaker acids.
2Nacl(ag) + H2SO4(ag) Na2SO4(ag) + 2HCL(ag)