Phase Diagram

A phase diagram (also called a P–T diagram) is a graph of Pressure (P) on the Y-axis versus Temperature (T) on the X-axis. It is one of the most powerful tools in physics and chemistry — it tells you, at any given combination of temperature and pressure, whether a substance will be a solid, liquid, or gas. The diagram is divided into three regions by three boundary curves, and all three curves meet at a single unique point called the triple point.

A phase is a homogeneous, uniform portion of matter with consistent composition and physical properties throughout. The same substance can exist in different phases.

The vaporisation curve is a graph of boiling point vs. pressure. Every point on this curve is a state where liquid water and water vapour coexist in equilibrium.

Key Facts:

• At 1 atm pressure, water boils at exactly 100°C​
• As pressure decreases, the boiling point also decreases​
• As pressure increases, the boiling point also increases

The fusion curve is a graph of the melting (freezing) point vs pressure. Points on this curve represent states where ice and liquid water coexist in equilibrium.

Key Facts:

• At 1 atm, the freezing point of water is 0°C​
• Between 0°C and 100°C at 1 atm, water exists only as liquid​
• Below 0°C → solid (ice); above 100°C → vapour (steam) at 1 atm

Special Property: Negative Slope of Water's Fusion Curve

For most substances (e.g., CO₂), the fusion curve has a positive slope — increasing pressure raises the melting point.​

For water, the fusion curve has a negative slope — increasing pressure slightly lowers the melting point. This occurs because:​​

• Water expands when it freezes (ice is less dense than liquid water — an anomalous property)​
• Applying pressure favours the denser phase (liquid), so ice tends to melt under high pressure

Diagram of an ice skate blade creating a high-pressure zone and an ice-rich slurry for low friction

The sublimation curve is a graph of the sublimation point vs pressure. Points on this curve represent states where ice and water vapour coexist directly — the substance transitions from solid to gas without passing through the liquid phase.

Key Facts for Water:

• Water sublimates only at pressures less than 0.006 atm (below its triple point pressure)​
• At pressures above 0.006 atm, water cannot sublimate directly

The triple point is the unique temperature and pressure at which all three phases of a substance — solid, liquid, and vapour — coexist in equilibrium simultaneously.

• It is where all three curves (l–v, l–s, s–v) meet on the phase diagram​
• It exists at only one specific temperature and one specific pressure for each substance​
• At any other condition, at most two phases can coexist​

1. A phase diagram maps all phases of a substance against pressure and temperature​
2. Three curves — vaporisation (l–v), fusion (l–s), and sublimation (s–v) — divide the diagram into solid, liquid, and vapour regions​
3. Water's fusion curve has a negative slope because water expands on freezing — this is an anomalous property​​
4. CO₂'s fusion curve has a positive slope — normal behaviour for most substances​
5. The triple point is the only condition at which all three phases coexist; for water, it is 273.16 K and 6.11 × 10⁻³ Pa