11.2 Phase Diagrams | General Chemistry

Summary

This lesson explores phase diagrams, covering essential concepts such as the lines of equilibrium, triple point, and critical point, along with the six key phase transitions: fusion, freezing, vaporization, condensation, sublimation, and deposition. Special focus is given to the unique behaviors of CO₂ and water. CO₂ sublimes at atmospheric pressure, while water’s phase diagram features a negative slope between solid and liquid phases due to the expansion of ice, which is crucial for life on Earth. Engaging examples like ice hockey and the importance of water’s behavior in freezing lakes make the content both educational and practical.

Takeaways

• 😀 Phase diagrams plot pressure vs. temperature to show the different phases of matter (solid, liquid, gas).
• 😀 Lines of equilibrium on a phase diagram represent the points where two phases are in equilibrium, like solid-liquid, liquid-gas, and solid-gas.
• 😀 The triple point is where all three phases (solid, liquid, and gas) coexist in equilibrium.
• 😀 Six common phase transitions are: fusion (solid to liquid), crystallization (liquid to solid), vaporization (liquid to gas), condensation (gas to liquid), sublimation (solid to gas), and deposition (gas to solid).
• 😀 The critical point is the temperature and pressure beyond which liquid-gas phase transitions do not occur, creating a supercritical fluid.
• 😀 Supercritical fluids occur above the critical point, where liquid and gas phases merge and exhibit unique properties.
• 😀 The normal melting and boiling points are defined at one atmosphere of pressure, which is important for identifying phase changes at standard conditions.
• 😀 Carbon dioxide (CO2) is unique because its one-atmosphere pressure is below the triple point, meaning CO2 sublimates from solid to gas without forming a liquid at atmospheric pressure.
• 😀 Water’s phase diagram is unique because the solid-liquid equilibrium line has a negative slope, unlike most substances, due to ice being less dense than liquid water.
• 😀 The negative slope of water’s solid-liquid line is crucial for life on Earth, allowing ice to float on water, insulate aquatic life, and prevent complete freezing of oceans and lakes in winter.

Q & A

What is a phase diagram?

A phase diagram is a graphical representation showing the relationship between pressure and temperature for a substance, illustrating the different phases (solid, liquid, gas) and the conditions under which they exist.

What are the lines of equilibrium in a phase diagram?

The lines of equilibrium in a phase diagram represent the boundaries where two phases coexist in equilibrium. For example, the line between solid and liquid is the fusion line, while the line between liquid and gas is the vaporization line.

What is the triple point in a phase diagram?

The triple point is the unique point on a phase diagram where all three phases (solid, liquid, and gas) coexist in equilibrium at a specific temperature and pressure.

What are the six phase transitions, and what do they mean?

The six phase transitions are: fusion (solid to liquid), crystallization or freezing (liquid to solid), vaporization (liquid to gas), condensation (gas to liquid), sublimation (solid to gas), and deposition (gas to solid). These describe the changes in state as temperature or pressure changes.

What is the critical point in a phase diagram?

The critical point is the end of the liquid-gas equilibrium line. Beyond this point, the substance becomes a supercritical fluid, a state that exhibits properties of both liquids and gases, and no distinct phase transition occurs between liquid and gas.

How does pressure affect phase transitions at the critical point?

At the critical point, increasing the pressure or temperature does not lead to the substance transitioning between liquid and gas. Instead, it enters a supercritical fluid state, where the substance behaves like both a liquid and a gas, with no distinct phase change.

Why does carbon dioxide sublimate rather than melt at one atmosphere of pressure?

Carbon dioxide sublimes at one atmosphere because its phase diagram places the one atmosphere pressure below the triple point, meaning liquid CO₂ cannot exist at standard atmospheric pressure. Instead, solid CO₂ (dry ice) transitions directly to a gas.

What makes water’s phase diagram unique compared to typical substances?

Water’s phase diagram is unique because the solid-liquid equilibrium line has a negative slope, meaning that water expands as it freezes, making ice less dense than liquid water. This property is due to hydrogen bonding, which causes water to have unusual behaviors compared to other substances.

How does water’s negative slope in the phase diagram affect aquatic life?

The negative slope in water's phase diagram causes ice to float on liquid water. This is crucial because it allows lakes and oceans to freeze from the top down, insulating the water below and allowing aquatic life to survive in winter.

How does the pressure from ice hockey skates affect ice and make it slippery?

The pressure from an ice hockey player’s skate blade causes the ice to melt slightly, creating a thin layer of liquid water on the surface. This liquid layer reduces friction, making the ice slippery and allowing the player to skate faster.