Balance
Usually it’s a triple beam balance.
A ruler measures length
Answer is: Yes it is balanced. It has the same number of Ca and O on both the reactant side and the product side.
According to principle of mass conservation, number of atoms must be equal on both side of chemical reaction.
There is one calcium atom atom and one oxygen atom on both side of reaction, so chemical reaction is balanced.
Answer: The green copper (II) carbonate
changes to black copper oxide 

Explanation:
Decomposition is defined as the chemical reaction in which a single compound gives two or more simple substances. It requires energy to break the bonds between reactants, thus is an endothermic process.
Thermal decomposition uses heat for decomposition.
The chemical equation for thermal decomposition of copper (II) carbonate is:

The green copper (II) carbonate
changes to black copper oxide 
Explanation:
The substance that is in excess that doesn't get used up as a reactant is called limiting reactant.