Qualitative fusion, sublimation, vapor pressure up to critical point — probe labeled regions (pedagogical curves).
About this model
This page shows a qualitative P–T phase diagram for water with pedagogical curves for fusion, sublimation, and vapor-pressure (liquid–vapor) equilibria up to the critical point. Labeled regions mark solid, liquid, and vapor; you can probe points to see which phase field they occupy. The curves are schematic teaching guides, not high-precision IAPWS steam-table data: metastable extensions, ice polymorphs beyond ordinary ice Ih, and dissolved-solute shifts are omitted. Moving across the fusion or sublimation line illustrates coexistence; approaching the critical point shows where liquid and vapor cease to be distinct. Use it to build intuition for triple-point topology and the negative slope of water’s melting curve.
Who it's for: General chemistry and introductory physical chemistry students learning phase equilibria of water.
Key terms
Phase diagram
Triple point
Critical point
Vapor pressure
Fusion curve
Sublimation
How it works
Qualitative P–T diagram for H₂O: ice–liquid fusion line with negative slope, sublimation, vapor pressure up to the critical point (≈647 K, 22.064 MPa). Boundaries are stylized; move the probe to label the region.
Key equations
Clapeyron: dP/dT = ΔS/ΔV — water expands on freezing (ΔV < 0) ⇒ negative dP/dT along fusion.
Frequently asked questions
Why does water’s melting curve slope the ‘wrong’ way?
Ordinary ice is less dense than liquid water, so increasing pressure favors the liquid and lowers the melting temperature — a negative dP/dT along fusion from the Clapeyron equation. Most substances show a positive slope; water is the famous exception in intro courses.
What happens at the critical point?
Liquid and vapor densities become equal and the meniscus disappears; above the critical temperature you cannot liquefy the fluid by pressure alone. The simulator’s critical endpoint is schematic — real steam tables give precise T_c and P_c.
Can I read exact boiling points from this chart?
Only qualitatively. The vapor-pressure curve indicates the trend of boiling temperature with pressure, but the drawn line is pedagogical. For engineering numbers use steam tables or a validated equation of state.