It's just H20 but with 3 water molecules
Answer:

Explanation:
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In this case, when solid calcium carbonate, CaCO3 (s), is decomposed by the action of thermal energy (heat), solid calcium oxide, CaO (s) and carbon dioxide gas, CO2 (g) are yielded via the following reaction:

However, since calcium carbonate is solid as well as calcium oxide and carbon dioxide is given off as a gas, we write:

Which also balanced.
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This is a physical phase change it is still water it just separated and became less and less.it stays water though