选用一种方法或试剂,除去下列物质中少量的杂质(括号内为杂质).
(1)CO(CO2)___________;(2)CO2(CO)__________;(3)CaCl2(CaCO3)_______;(4)CaCO3(CaCl2)________;(5)CaO(CaCO3)_________.
(4,)(5)请写明原因