Which of the following correctly describes a key difference between the phase diagrams of water and carbon dioxide?

• A. Both water and carbon dioxide have a solid-liquid equilibrium line with a positive slope.
• B. The solid-liquid equilibrium line for water has a negative slope, while for carbon dioxide it has a positive slope.
• C. Water has a triple point at a higher pressure than carbon dioxide.
• D. Carbon dioxide can exist as a liquid at atmospheric pressure, while water cannot.

Answer: (B)

Verified step by step guidance

1. Understand what a phase diagram represents: it shows the states of matter (solid, liquid, gas) of a substance at different temperatures and pressures, and the lines represent equilibrium between phases.
2. Focus on the solid-liquid equilibrium line (also called the melting curve) for both water and carbon dioxide. This line shows how the melting point changes with pressure.
3. Recall that for most substances, including carbon dioxide, the solid-liquid equilibrium line has a positive slope, meaning increasing pressure raises the melting point.
4. Recognize that water is unusual because its solid-liquid equilibrium line has a negative slope. This means increasing pressure lowers the melting point of ice, which is why ice melts under pressure.
5. Compare the triple points and phase behavior at atmospheric pressure: water's triple point occurs at a higher pressure than carbon dioxide's, and carbon dioxide sublimates at atmospheric pressure (does not exist as a liquid), unlike water.