Answer:
The number of atoms in the reactants, will be equal to the number of atoms in the products
Step-by-step explanation:
The above statement is known as "The Law of Conservation of the Atom", meaning that atoms do not simply "appear out of nowhere" or "disappear". If an atom is present in the reactant, it has to be present in the product.
Since this question is incomplete, an exact answer cannot be given, but the concept is easy to explain.
When a chemical reaction is written out, the reactants are on the left side of the arrow, and the products will be on the right. The number of atoms in the products must have come from the reactants, hence if the reactant is CaCO₃, then the products cannot have an atom other than Ca, C or O.
In this example the full reaction is: CaCO₃ -> CaO + CO₂
The equation may not always be balanced, however it would be clear that hydrogen (as an example) is not part of this reaction, therefore if the reactant had H₂O, then it would be clear that it is incorrect.