Final answer:
Streams leaving the condenser at equilibrium mean that the system has achieved a state where the rates of condensation and vaporization are equal, leading to a stable condition, typically observed on a graph of pressure versus temperature showing a direct relationship.
Step-by-step explanation:
When we say that 'streams leaving the condenser are at equilibrium,' it implies that the conditions within the condenser are such that the temperature and pressure allow for the simultaneous occurrence of boiling and condensation at equal rates. This state is known as dynamic equilibrium. At this equilibrium, the rate at which molecules in the liquid phase escape to become gas is equal to the rate at which gas molecules condense back into the liquid phase.
A graph of pressure vs. temperature indicates a direct relationship; when a gas is cooled at constant volume, its pressure decreases. This cooling continues until the gas condenses into a liquid, at which point the system reaches equilibrium. Upon reaching this stage, any additional reduction in temperature should not be confused with the 'temperature at which the internal energy of the system approaches zero,' which pertains to the concept of absolute zero, where particle motion theoretically ceases entirely.