(2) Oxygen can support combustion, and its chemical formula is O2.
(3) Hydrogen is reducible and its chemical formula is H2.
(4) Calcium carbonate can be used as calcium supplement, and its chemical formula is CaCO3. ..
(5) Carbonic acid is an acid in soda water, and its chemical formula is H2CO3. ..
(6) According to the expression method of ions, the number of positive and negative charges carried by ions is marked in the upper right corner of the symbol of elements representing ions. 1 will be omitted when the number comes first and the sign comes last. If more than one ion is represented, the corresponding number will be added before the ion symbol, so the sulfate ion can be represented as: SO42-.
So, the answer is: (1) Cao; (2)O2； (3)H2； (4) calcium carbonate; (5)h2co 3； (6)SO42-。