Phase Diagram Calculator

A phase diagram is a graphical representation of the conditions of temperature and pressure under which a substance exists as a solid, liquid, gas, or supercritical fluid. Phase diagrams are among the most important tools in thermodynamics and physical chemistry because they summarize all the phase behavior of a substance in a single chart. The boundaries between regions on the phase diagram represent phase transitions — conditions where two phases coexist in equilibrium. Understanding phase diagrams is essential for predicting how a substance will behave under different conditions, from everyday situations (will ice melt at room temperature?) to extreme environments (what happens to water at the center of the Earth?).

Every pure substance has a unique phase diagram with several key features. The triple point is the specific temperature and pressure where all three common phases (solid, liquid, gas) coexist simultaneously in equilibrium. For water, the triple point is at 273.16 K (0.01°C) and 611.73 Pa (0.006 atm). The critical point marks the temperature and pressure above which the distinction between liquid and gas disappears, creating a supercritical fluid. Water's critical point is at 647.1 K (373.9°C) and 22.064 MPa (218 atm). The normal boiling point and normal melting point are the phase transition temperatures at standard atmospheric pressure (1 atm = 101.325 kPa).

This calculator determines the phase of a substance at specified temperature and pressure conditions by applying the Clausius-Clapeyron equation to estimate vapor pressures and comparing them to the input conditions. It supports six common substances: water, carbon dioxide, ammonia, ethanol, oxygen, and nitrogen.

The phase boundaries on a phase diagram are described by the Clausius-Clapeyron equation, which relates vapor pressure to temperature along the liquid-gas boundary.

This calculator determines the phase of a substance at specified conditions:

1. Select a Substance: Choose from water (H₂O), carbon dioxide (CO₂), ammonia (NH₃), ethanol (C₂H₅OH), oxygen (O₂), or nitrogen (N₂).
2. Set Temperature: Enter the temperature in Kelvin. The calculator also shows the equivalent values in °C and °F for convenience.
3. Set Pressure: Enter the pressure in Pascals (Pa). The calculator shows equivalents in atm and kPa.
4. Use Quick Conditions: Click preset buttons for STP (298 K, 101325 Pa), 0°C at 1 atm, or 100°C at 1 atm for common reference states.
5. View Results: The calculator identifies the phase, provides details about the phase determination, and shows key thermodynamic properties including the triple point, critical point, reduced temperature and pressure, and estimated vapor pressure.

The reduced temperature (Tr = T/Tc) and reduced pressure (Pr = P/Pc) are dimensionless values that allow comparison across different substances using the principle of corresponding states.

The calculator provides comprehensive information about the substance's phase behavior:

Phase Identification: The primary result identifies the phase at the specified conditions: Solid, Liquid, Gas/Vapor, Supercritical Fluid, or a coexistence region. The phase details explain why that phase is present (e.g., "Above vapor pressure curve — liquid phase stable").

Triple Point: The temperature and pressure where all three phases coexist. Below the triple point pressure, the substance cannot exist as a liquid — it sublimes directly from solid to gas. This is why dry ice (solid CO₂) sublimes at room pressure rather than melting.

Critical Point: The temperature and pressure above which the liquid-gas distinction disappears. Above the critical point, the substance is a supercritical fluid with properties of both liquids (density) and gases (diffusivity). Supercritical CO₂ is widely used as a green solvent for decaffeination and extraction processes.

Reduced Properties: The reduced temperature and pressure (Tr and Pr) are dimensionless ratios that normalize the conditions relative to the critical point. According to the principle of corresponding states, all substances at the same Tr and Pr have similar thermodynamic properties, allowing predictions for one substance based on data from another.

Vapor Pressure: The estimated vapor pressure at the specified temperature, calculated using the Clausius-Clapeyron equation. This value helps determine whether the substance is above or below the liquid-gas equilibrium boundary.

Phase diagrams and phase behavior calculations have critical applications across science and engineering:

Supercritical Fluid Technology: Supercritical CO₂ (Tc = 31.1°C, Pc = 73.8 bar) is used as a green solvent for decaffeinating coffee, extracting essential oils, dry cleaning, and pharmaceutical processing. Operating above the critical point eliminates the need for organic solvents, making the process more environmentally friendly and producing cleaner products.

Cryogenics and Refrigeration: The phase behavior of nitrogen (boiling point 77.36 K = -195.79°C) and helium makes them essential cryogens. Liquid nitrogen is used for preserving biological samples,冷却 superconducting magnets, and flash-freezing food. Understanding phase diagrams is essential for designing and operating cryogenic storage and transport systems.

Geology and Planetary Science: The phase diagram of water explains phenomena like glacier flow (pressure melting), volcanic eruptions (degassing of magma), and the possible existence of liquid water on other planets. The phase diagrams of minerals at extreme pressures explain the structure of the Earth's mantle and core.

Industrial Process Design: Chemical engineers use phase diagrams to design distillation columns, crystallization processes, and gas separation systems. The vapor-liquid equilibrium data extracted from phase diagrams determines the number of theoretical stages needed for separation and the energy requirements of the process.