Section: Science

63) Change of phase is a process whereby matter changes form (solid, liquid, gas).Which one of the following constitutes a phase change? 

1. burning of wood
2. digestion of food
3. exhaling
4. melting of candle wax
5. photosynthesis

Since the wax melts because the temperature of the wax is raised, once the heat source is removed, the wax will begin to lose heat, assuming that ambient temperature is low enough. Eventually, the wax will go through another phase change and become solid. So, the short answer is: burning is irreversible because it involves a chemical reaction, while melting is reversible because it's just a phase change.