Final answer:
A gas under enough pressure will condense into a liquid and revert to a gaseous form when the pressure is reduced. Carbon dioxide is a practical example of a gas that becomes a liquid under high pressure. Phase diagrams help visualize these changes at different temperatures and pressures. So, the correct option is 1.
Step-by-step explanation:
When a gas is under enough pressure, it will condense, transitioning into a liquid form. This happens because the molecules of the gas are forced closer together, increasing the intermolecular forces and allowing the substance to change into a liquid state. Conversely, when the pressure is reduced, the condensed gas will revert back into its gaseous form as the intermolecular forces weaken and the molecules move further apart. For instance, substances like carbon dioxide become a liquid under high pressure and return to a gaseous state when that pressure is released. This is evident in carbonated beverages where carbon dioxide is dissolved under high pressure and forms bubbles when the bottle is opened, releasing the pressure. Moreover, the physical properties of a liquid, such as vapor pressure, indicate the presence of its molecules in a gas state above the liquid at a given temperature. Understanding the behavior of matter under various pressures and temperatures is crucial to grasping concepts such as phase changes, which is illustrated in phase diagrams like those showing the condensation and vaporization at different points around a substance's triple point, where solid, liquid, and gas phases coexist.