In this case, the described chemical reaction is a redox reaction in fact, since the oxidation states of both magnesium and copper change as shown due to the displacement:
Therefore:
1. Since copper is the cation in the copper (II) nitrate, the (II) means that its charge is 2+ ().
2. Since copper is alone, it means no electrons are being neither shared not given, its charge is 0 ().
Anhydrous sodium carbonate is stable to heat and does not decompose even when it is heated to redness. This is because sodium carbonate salt on heating with acids react to release carbon dioxide.