Question

question 22 of 30
with carbon dioxide, what phase change takes place when the temperature
decreases from -40°c to -80°c at 2 atm?

carbon dioxide phase diagram
phase diagram with pressure (atm) on y - axis, temperature (°c) on x - axis, regions labeled solid, liquid, gas, with melting point and boiling point arrows

options:
○ a. a gas changes to a liquid.
○ b. a solid changes to a liquid.
○ c. a liquid changes to a solid.
○ d. a gas changes to a solid.

Explanation:

Step1: Analyze initial state

At 2 atm and -40°C, check the phase diagram. The pressure (2 atm) and temperature (-40°C) place the substance in the liquid region (since liquid region is between solid and gas, and 2 atm is above the vapor - pressure curve at -40°C? Wait, no, looking at the phase diagram: the x - axis is temperature (°C) from -100 to 20, y - axis is pressure (atm) from 0 to 20. The liquid region is between the solid - liquid boundary (melting point line) and the liquid - gas boundary (boiling point/vapor pressure line). At 2 atm, let's see the temperature: -40°C. Let's check the phases: solid is on the left (lower temperature, higher pressure? Wait, no, the solid region is the left - most (blue), liquid is middle (light blue), gas is right (yellow). The vapor pressure curve (liquid - gas boundary) at 2 atm: let's see the temperature. Wait, when pressure is 2 atm, what's the phase at -40°C? Let's see the liquid region: the liquid region is above the vapor pressure curve. At -40°C, 2 atm: is 2 atm above the vapor pressure at -40°C? The vapor pressure curve (the curve between liquid and gas) – at lower temperatures, the vapor pressure is lower. At -40°C, the vapor pressure of CO₂: from the diagram, the liquid region is between the solid - liquid line (melting point) and liquid - gas line (boiling point). So at 2 atm and -40°C, the substance is liquid (since 2 atm is above the vapor pressure at -40°C, so liquid).

Step2: Analyze final state

Now, temperature decreases to -80°C at 2 atm. So we move left along the horizontal line (constant pressure, 2 atm) from -40°C to -80°C. Now, at -80°C and 2 atm, we cross into the solid region (the blue region). So the phase change is from liquid to solid (freezing, or solidification).

Now let's check the options:

• Option A: Gas to liquid – initial state is liquid, not gas. Eliminate.
• Option B: Solid to liquid – we are going from higher temp (less solid - like) to lower temp (more solid - like), not solid to liquid. Eliminate.
• Option C: Liquid to solid – this matches our analysis.
• Option D: Gas to solid – initial state is liquid, not gas. Eliminate.

Answer:

C. A liquid changes to a solid.