Final answer:
Two substances in contact with no net exchange of internal energy must have the same temperature, as this indicates they have reached a state of thermal equilibrium.
Step-by-step explanation:
If two substances are placed in contact with each other and no net exchange of internal energy occurs between them, the substances must have the same temperature. This is because when two substances or objects at different temperatures come into contact, heat will naturally flow from the warmer substance to the cooler one until they reach a state of thermal equilibrium. At this point, there will be no further net transfer of energy; the substances have equal temperatures.
An important concept related to temperature is thermal equilibrium. Two objects in contact are in thermal equilibrium if, after sufficient time, no net energy is transferred between them. This principle stems from the zeroth law of thermodynamics. When we talk about the temperature of objects, we're really referring to the average kinetic energy of their molecules. So, in thermal equilibrium, both substances have molecules with the same average kinetic energy.