Let's break down the two statements regarding the behavior of a liquid and its vapor in a closed system, particularly focusing on the concepts of equilibrium and gas laws.
Examining Statement 1
Statement 1 claims that if the volume of the vapor in a cylinder is decreased by moving the piston inward, the vapor pressure will remain unchanged. This statement is generally true under specific conditions. When a liquid is in equilibrium with its vapor, the vapor pressure is determined by the temperature and the nature of the liquid, not the volume of the vapor itself.
Understanding Vapor Pressure
Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid at a given temperature. When you compress the vapor by moving the piston, you are increasing the pressure momentarily, but since the system is at constant temperature and in equilibrium, the liquid will adjust to maintain the vapor pressure. The excess vapor will condense back into the liquid until the equilibrium is reestablished.
- Constant Temperature: The temperature remains constant, which means the vapor pressure is also constant for a given liquid.
- Equilibrium: The system will reach a new equilibrium state where the vapor pressure remains unchanged despite the change in volume.
Analyzing Statement 2
Statement 2 asserts that vapor in equilibrium with its liquid does not obey Boyle's Law. This statement is also accurate. Boyle's Law states that for a given mass of gas at constant temperature, the pressure and volume are inversely related. However, this law applies to ideal gases, not to vapors in equilibrium with their liquids.
Why Vapors Don't Follow Boyle's Law
In the case of a vapor in equilibrium with its liquid, the behavior is influenced by the presence of the liquid phase. When the volume of the vapor decreases, the vapor molecules do not behave like an ideal gas because they are in a dynamic equilibrium with the liquid. Instead of simply compressing, the vapor molecules will condense into the liquid phase, which alters the pressure-volume relationship.
- Phase Change: The transition between liquid and vapor means that as you compress the vapor, some of it will turn back into liquid rather than just increasing pressure.
- Non-Ideal Behavior: Real gases and vapors exhibit non-ideal behavior, especially near their condensation point, where intermolecular forces become significant.
Conclusion
Both statements reflect important principles of thermodynamics and phase equilibrium. In summary, while the vapor pressure remains constant when the volume is decreased in a closed system at constant temperature, the vapor does not obey Boyle's Law due to the complexities of phase changes and the interactions between the liquid and vapor phases. Understanding these concepts is crucial for grasping the behavior of substances in different states of matter.
