What is the chemical equation when aluminium sulphate(iv) added to aqueous sodium carbonates

the observation for the reaction is effervescence occurs and white precipitate formed. please explain me the reaction take place.

Al2(SO4)3 is slightly acidic (acidity depending on concentration) because it can be regarded as the salt of a weak base and strong acid.
Hence it decomposes Na2CO3 producing the effervescence - CO2.
So the Al2(CO3)3 nominally produced:

Al2(SO4)3 + 3Na2CO3 ---> 3Na2SO4 + Al2(CO3)3

actually forms a preciptate of Al(OH)3:

Al2(CO3)3 + 3H2O ---> 2Al(OH)3 + 3CO2