Answer :
To determine how the phases of water change with temperature at a constant pressure of 0.5 atm, we can use the phase diagram for water. We're starting at a high temperature of 110°C and decreasing the temperature to -10°C.
At 110°C and 0.5 atm, water exists in the gaseous state, because this temperature and pressure point lies in the region of the phase diagram corresponding to the gas phase.
As we decrease the temperature, keeping the pressure constant at 0.5 atm, we will eventually reach the boundary between the gas and liquid phases. This boundary is the line that represents the temperatures and pressures at which water vapor (gas) will condense into liquid water. At 0.5 atm, this phase change occurs at the boiling point of water, which is below 100°C at this pressure.
Continuing to lower the temperature, water will remain in the liquid phase until we reach the freezing point at this pressure. At 0.5 atm, the temperature at which water freezes into ice (solid) will be at the line that represents the temperatures and pressures at which liquid water turns into ice. This temperature is 0°C at 1 atm, and it will be close to 0°C but could be slightly lower at 0.5 atm; the diagram does not show exact values for 0.5 atm.
Once the temperature passes this freezing point, water will be in the solid phase, and it will remain solid as we further decrease the temperature to -10°C.
The correct sequence of phase changes as the temperature is decreased from 110°C to -10°C at 0.5 atm is from gas to liquid to solid.
The correct answer is C. Water changes from a gas to a liquid to a solid.
