10.4 Phase Diagrams

Phase Diagrams

Phase diagrams are graphical tools that show which state of matter (solid, liquid, or gas) a substance will be in at any given combination of temperature and pressure. Think of them as maps: pick a temperature and pressure, find that spot on the diagram, and you can immediately see what phase the substance is in.

Reading a Phase Diagram

The Axes and Regions

The x-axis represents temperature (in Kelvin or °C), and the y-axis represents pressure (in atm or Pa). The diagram is divided into three main regions:

• Solid region — found at low temperatures and high pressures
• Liquid region — found at moderate temperatures and pressures
• Gas region — found at high temperatures and low pressures

To figure out the phase of a substance, locate the point on the diagram that matches its current temperature and pressure. Whichever region that point falls in tells you the phase.

Phase Boundaries

The lines separating the regions are called phase boundaries. Each boundary represents conditions where two phases exist in equilibrium (both are stable at the same time):

• Melting/freezing curve — separates solid and liquid
• Vaporization/condensation curve — separates liquid and gas
• Sublimation/deposition curve — separates solid and gas

When you cross a phase boundary by changing temperature or pressure, a phase transition occurs:

• Melting — solid → liquid (e.g., ice melting)
• Freezing — liquid → solid (e.g., water freezing)
• Vaporization — liquid → gas (e.g., water boiling)
• Condensation — gas → liquid (e.g., steam on a cold window)
• Sublimation — solid → gas without passing through liquid (e.g., dry ice disappearing)
• Deposition — gas → solid without passing through liquid (e.g., frost forming on grass)

Special Points

Two points on every phase diagram deserve extra attention:

Triple point — the single temperature-pressure combination where all three phases coexist in equilibrium. All three phase boundaries meet here. For water, the triple point is at 0.01°C and 0.006 atm.

Critical point — the endpoint of the vaporization/condensation curve. It represents the highest temperature and pressure at which liquid and gas can be distinguished as separate phases. Beyond this point, the substance becomes a supercritical fluid (more on that below).

Supercritical Fluids

What They Are

A supercritical fluid forms when a substance is heated and pressurized beyond its critical temperature and critical pressure. At that point, the boundary between liquid and gas disappears. The substance doesn't behave like a typical liquid or a typical gas; instead, it has properties of both:

• Density close to that of a liquid, giving it strong dissolving ability
• Viscosity close to that of a gas, allowing it to flow easily and penetrate into small spaces
• Low surface tension, so it wets surfaces more effectively than most liquids

One of the most useful features of supercritical fluids is tunable density: by adjusting the temperature or pressure slightly, you can change how much the fluid can dissolve. This gives chemists precise control over extraction and separation processes.

Common Supercritical Fluids

• Carbon dioxide ($CO_2$) — critical point at 31.1°C and 73.8 atm. It's the most widely used supercritical fluid because it's cheap, non-toxic, and evaporates completely when pressure is released, leaving no residue behind.
• Water ($H_2O$) — critical point at 374°C and 218 atm. Used in specialized industrial processes like hydrothermal synthesis and waste oxidation, though the extreme conditions make it harder to work with.

Applications

1. Supercritical fluid extraction (SFE) — Supercritical $CO_2$ is pumped through raw materials to pull out specific compounds. This is how caffeine is removed from coffee beans to make decaf, and how essential oils are extracted from plants without harsh chemical solvents.
2. Supercritical fluid chromatography (SFC) — A separation technique that combines advantages of both gas and liquid chromatography, useful for analyzing compounds that don't work well with either method alone.
3. Supercritical fluid cleaning — Removes contaminants from delicate items like electronic components and historical artifacts without damaging them.
4. Supercritical fluid synthesis — Used to produce nanoparticles and other advanced materials with tightly controlled properties.

Thermodynamic Basis

Phase diagrams are rooted in thermodynamics. At any given temperature and pressure, the phase with the lowest Gibbs free energy is the most stable, and that's the phase you'll find on the diagram. Phase boundaries mark conditions where two phases have equal Gibbs free energy, which is why both phases can coexist there.