What is the Difference Between Phase Diagram of Water and Carbon Dioxide

The main difference between phase diagram of water and carbon dioxide is the phase diagram of water includes a distinct liquid phase under atmospheric pressure, while carbon dioxide sublimes directly from solid to gas at similar pressures.

A phase diagram is a graphical representation that illustrates the different phases (solid, liquid, and gas) of a substance under varying conditions of temperature and pressure. Both water and carbon dioxide exhibit interesting phase behaviors, reflected in their respective phase diagrams.

Key Areas Covered

1. What is the Phase Diagram of Water   
      – Definition, Features 
2. What is the Phase Diagram of Carbon Dioxide
      – Definition, Features 
3. Similarities Between Phase Diagram of Water and Carbon Dioxide
      – Outline of Common Features
4. Difference Between Phase Diagram of Water and Carbon Dioxide
      – Comparison of Key Differences
5. FAQ: Phase Diagram of Water and Carbon Dioxide
      – Frequently Asked Questions

Key Terms

Phase Diagram of Water, Phase Diagram of Carbon Dioxide

What is the Phase Diagram of Water

The phase diagram of water is a fundamental representation of the relationships between temperature, pressure, and the different phases of water – solid, liquid, and gas. At standard atmospheric pressure, water exists in its liquid form at temperatures within a wide range, typically 0 to 100 degrees Celsius. As the temperature decreases, water transitions into its solid phase, ice, at 0 degrees Celsius. Conversely, heating ice causes it to transform back into liquid water.

Pressure also plays a crucial role in water’s phase transitions. At higher pressures, the melting point of ice decreases, allowing it to exist in a liquid state at temperatures below 0 degrees Celsius. This phenomenon is observed in everyday life when salt is used to lower water’s freezing point, facilitating liquid brine formation on icy roads.

Figure 1: Phase Diagram of Water

The unique behavior of water in its solid phase is evident in its density anomaly. Unlike most substances, ice is less dense than liquid water, which is why ice floats. This is a result of the hexagonal arrangement of water molecules in the solid state, creating a lattice structure that increases the substance’s volume.

The critical point on the phase diagram represents the conditions at which the distinction between liquid and gas disappears, and there is a continuous transition between the two phases. For water, the critical point occurs at a temperature of 374 degrees Celsius and a pressure of 218 atmospheres.

What is the Phase Diagram of Carbon Dioxide

The phase diagram of carbon dioxide (CO₂) provides valuable insights into the different states this compound can exist in under varying temperature and pressure conditions. CO₂ is unique because it undergoes sublimation directly from a solid to a gas at standard atmospheric pressure.

Carbon dioxide adopts a solid state known as dry ice at low temperatures and pressures. The phase boundary between the solid and gas phases on the diagram represents the sublimation curve. Under normal atmospheric pressure, dry ice sublimates at around -78.5 degrees Celsius.

Figure 2: Phase Diagram of Carbon Dioxide

Increasing pressure while maintaining a constant temperature can lead to the formation of a different solid phase called “high-pressure solid CO2” or “polymorph II.” This phase becomes stable at pressures above 5.1 atmospheres.

The critical point on the phase diagram occurs at a temperature of 31.1 degrees Celsius and a pressure of 73.8 atmospheres. Beyond this critical point, the distinction between the liquid and gas phases becomes indistinct, and CO2 exhibits supercritical fluid behavior. In this state, it possesses properties of both a liquid and a gas, displaying high density and diffusivity.

Similarities Between Phase Diagram of Water and Carbon Dioxide

• Both water and carbon dioxide exhibit phase diagrams that include three phases: solid, liquid, and gas.
• Both substances have a critical point on their phase diagrams.
• They have a triple point on their phase diagrams.

Difference Between Phase Diagram of Water and Carbon Dioxide

Definition

A phase diagram of water is a graphical representation of water’s phases (solid, liquid, gas) under changing temperature and pressure, while a phase diagram of carbon dioxide is a graphical representation showing carbon dioxide’s phases under varying temperature and pressure conditions.

Liquid Phase

The phase diagram of water includes a distinct liquid phase under atmospheric pressure, while carbon dioxide sublimes directly from solid to gas at similar pressures.

Critical Temperature

Water has a higher critical temperature compared to carbon dioxide.

Triple Point

The triple point of water (0.01°C, 611.657 Pa) is at a higher temperature and pressure than the triple point of carbon dioxide (-56.6°C, 5.11 atm).

Boiling Point

The normal boiling point of water is 100°C at standard atmospheric pressure, while the normal boiling point of carbon dioxide is much lower at -78.5°C.

FAQ: Phase Diagram of Water and Carbon Dioxide

How is the phase diagram of water different from others?

The phase diagram of water is unique because its solid-liquid equilibrium line has a negative slope, meaning its solid form (ice) is less dense than its liquid form, which is not typical for most substances.

What are the phases of carbon dioxide under atmospheric pressure?

Usually, carbon dioxide exists as a gas or a solid.

What are the phases of water?

There are three phases of water: solid, liquid, and vapor.

Conclusion

A phase diagram is a graphical representation of different phases (solid, liquid, and gas) of a substance under varying conditions of temperature and pressure. The main difference between phase diagram of water and carbon dioxide is the phase diagram of water includes a distinct liquid phase under atmospheric pressure, while carbon dioxide sublimes directly from solid to gas at similar pressures.

Reference:

1. “Phase Diagram of Water.” Byju’s.
2. “Co2 Phase Diagram.” Chemistry Learner.

Image Courtesy:

1. “Phase diagram of water simplified” By author of the original work: Cmglee – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Carbon dioxide pressure-temperature phase diagram” By Ben FinneyMark Jacobs – Commons, Image: Carbon dioxide pressure-temperature phase diagram.jpg (CC0) via Commons Wikimedia