Final answer:
To obtain a liquid phase for CO₂, one needs to decrease temperature at high pressure, as this will allow CO₂ to transition from a gaseous to a liquid state, according to its phase diagram.
Step-by-step explanation:
To obtain a liquid phase for CO₂, it is important to consider the substance's phase diagram which indicates the conditions of temperature and pressure under which a substance exists as a solid, liquid, or gas. For carbon dioxide, the key to achieving a liquid state is to increase pressure at low temperatures rather than high temperatures. This is due to the fact that at 1 atmosphere of pressure, CO₂ will not liquify; it sublimes directly from solid to gas. However, at pressures above the triple point, which for CO₂ requires a relatively high pressure, the substance can exist as a liquid at certain temperatures. Based on the given options and the behavior of CO₂:
- Increasing temperature at low pressure would not yield a liquid phase because carbon dioxide sublimes or becomes gaseous in these conditions.
- Increasing temperature at high pressure could potentially create a supercritical fluid if above the critical point but is not conducive to liquid formation in normal ranges.
- Decreasing temperature at low pressure would lead to sublimation or deposition into a solid state rather than liquid formation.
- The correct approach to get CO₂ in a liquid state is to decrease temperature at high pressure (option b), which correlates with the phase diagram's depiction for CO₂ transitioning from a gas to a liquid as pressure is increased above the triple point, provided the temperature is not below the substance's freezing point.