$(a)$ Classify the following chemical reactions as exothermic or endothermic:
$(i)$ Electrolysis of water
$(ii)$ Burning of natural gas
$(iii)$ Decomposition of calcium carbonate
$(iv)$ Burning of magnesium ribbon in air.

(N/A) $(i)$ Electrolysis of water is an endothermic reaction because it requires electrical energy to break the bonds of water molecules.
$(ii)$ Burning of natural gas (methane) is an exothermic reaction as it releases a large amount of heat energy during combustion.
$(iii)$ Decomposition of calcium carbonate is an endothermic reaction because it requires heat energy to decompose into calcium oxide and carbon dioxide.
$(iv)$ Burning of magnesium ribbon in air is an exothermic reaction as it releases heat and light energy during the formation of magnesium oxide.