What is the Phase Diagram Explorer?
Quick Answer: An interactive P-T (pressure-temperature) phase diagram for water and carbon dioxide. Drag the cursor to any point to see its phase (solid, liquid, gas) and read the pressure and temperature values. Triple and critical points are marked. Learn about phase transitions, sublimation, and the unique behavior of water.
Theory of Phase Diagrams
A phase diagram shows the state of a substance at different pressures and temperatures. It includes phase boundaries (melting/freezing, boiling/condensation, sublimation/deposition) that meet at the triple point where all three phases coexist. The critical point marks the end of the liquid-vapor boundary; above it, the substance is a supercritical fluid. Water's melting curve slopes backward (negative slope) due to ice being less dense than liquid water. For CO2, the triple point is at 5.1 atm, so at 1 atm, solid CO2 (dry ice) sublimes directly to gas. The tool uses realistic data to draw the curves and updates the phase description in real time.
Step-by-Step Examples
Example 1: Explore Water's Triple Point
- Select "Water". Drag the cursor to T = 0.01°C and P = 0.006 atm (near bottom-left). The info panel displays "Triple Point: Solid, Liquid, Gas coexist".
- Move slightly up to see liquid, down to solid, right to gas. The boundaries show the transitions.
Example 2: Dry Ice Sublimation
- Choose "CO2". The triple point is at 5.1 atm. At room pressure (1 atm), drag to T = -78.5°C. The point is in the gas region – solid CO2 cannot exist as liquid at 1 atm.
- Move above 5.1 atm to see the liquid phase appear.
Example 3: Critical Point of Water
- For water, drag to T = 374°C and P = 218 atm. The label shows "Critical Point". Above this, the substance is a supercritical fluid, where liquid and gas phases are indistinguishable.
Frequently Asked Questions
Why does water's melting curve have a negative slope?
Because ice (solid) is less dense than liquid water. Increasing pressure lowers the melting point, so the solid-liquid boundary slopes leftward.
What is the triple point?
The unique temperature and pressure where all three phases (solid, liquid, gas) are in equilibrium. For water, it's at 0.01°C and 0.006 atm.
Why can't liquid CO2 exist at atmospheric pressure?
CO2's triple point pressure is 5.1 atm. Below that, the solid sublimes directly to gas without passing through a liquid phase.
What happens above the critical point?
Above Tc and Pc, the substance becomes a supercritical fluid, which has properties of both a liquid and a gas.
Are these phase diagrams exact?
The curves are based on real physical data for water and CO2, using polynomial fits. They are accurate for educational purposes.