Final answer:
To return to the initial temperature of 0 °C from 5 °C, the temperature must decrease by 5 °C, demonstrating thermal equilibrium as per the zeroth law of thermodynamics.
Step-by-step explanation:
If the temperature was initially 0 °C and then underwent a temperature change of 5 °C, the temperature would be 5 °C above the initial temperature. To return to the original temperature of 0 °C, the temperature would need to decrease by 5 °C. Temperature changes in the opposite direction of equal magnitude will bring a system back to its initial temperature, demonstrating a key concept of thermal equilibrium according to the zeroth law of thermodynamics. This law implies that when objects are in thermal contact, they will exchange heat until reaching the same temperature, or thermal equilibrium.