CaCO3 contains two ions Ca2+ and CO3^2-
H2O contains H+ and OH- ions
Positive ion combines with the negative ion
so Ca2+ and OH-combines to form Ca(OH)2 and
H+and CO3^2- combines to form H2CO3
[note: criss crossing the charge numbers we get the chemical formula of the compounds]
so the equation will be
CaCO3+2H2O===>Ca(OH)2 +H2CO3 or
CaCO3+2H2O===> Ca(OH)2+CO2+H2O
note : H2CO3 usually produces CO2 gas and H2O liquid