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Phase Diagrams

How can you tell which state a substance is in at any temperature and pressure?

IntermediateStates of Matter & GasesLast reviewed 5 October 2026
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What is it?

A phase diagram is a map that shows which state (phase) of a substance is stable at each combination of temperature (horizontal axis) and pressure (vertical axis).

  • The three areas are the regions where the substance is solid, liquid or gas.
  • The lines between them show the conditions where two phases exist together in equilibrium. Crossing a line means a phase change: the solid–liquid line gives the melting point at each pressure, the liquid–gas line the boiling point, and the solid–gas line the sublimation point.
  • At the triple point, all three phases coexist.
  • The liquid–gas line ends at the critical point. Above it, liquid and gas can no longer be told apart: the substance is a supercritical fluid.

Key idea

To read a phase diagram, find the point for the given temperature and pressure and see which region it lies in. The normal melting and boiling points are where the horizontal line at 1 atm crosses the solid–liquid and liquid–gas lines.

Why does it matter?

  • Dry ice. Solid CO₂ turns straight into gas at room pressure, with no liquid: the phase diagram explains why.
  • Freeze-drying and decaffeination. Freeze-drying works below water’s triple point, where ice sublimes. Supercritical CO₂ is used to remove caffeine from coffee.
  • Weather and planets. Phase diagrams explain why liquid water cannot exist on the surface of Mars, where the pressure is below water’s triple point.

How does it work?

1. Reading the diagram

For a point at a given temperature and pressure:

  • left (low temperature) and high pressure: solid;
  • middle: liquid;
  • right (high temperature) and low pressure: gas.

Moving horizontally (changing temperature at constant pressure) traces a heating or cooling curve. Moving vertically (changing pressure at constant temperature) shows what compression or expansion does.

2. Water: an unusual slope

For almost every substance the solid–liquid line slopes to the right (more pressure raises the melting point), because the solid is denser than the liquid. Water’s line slopes to the left: ice is less dense than liquid water, so squeezing ice favours the liquid and lowers the melting point slightly.

WaterCarbon dioxide
Triple point0.01 °C, 0.00604 atm−56.6 °C, 5.11 atm
Critical point374 °C, 218 atm31.0 °C, 72.8 atm
At 1 atmmelts at 0 °C, boils at 100 °Csublimes at −78.5 °C
Solid–liquid lineslopes leftslopes right

3. Carbon dioxide: no liquid at 1 atm

The triple-point pressure of CO₂ (5.11 atm) is above 1 atm. At atmospheric pressure the horizontal line misses the liquid region completely, so solid CO₂ goes straight to gas. Liquid CO₂ only exists above 5.11 atm, for example inside a fire extinguisher.

Think of it like this

A phase diagram is like a map with three countries: Solid, Liquid and Gas. The borders are the lines, where two countries meet. The triple point is the one spot where all three borders meet. The critical point is where the border between Liquid and Gas simply fades away.

More precisely

The slope of each line is given by the Clapeyron equation: it depends on the enthalpy and the volume change of the transition. Because ice has a larger volume than liquid water, the volume change on melting is negative and the slope is negative. The liquid–gas line is the vapour-pressure curve: each point gives the vapour pressure of the liquid at that temperature. Real phase diagrams of water are much more complicated at very high pressures, with many different forms of ice.

Visualise it

Schematic phase diagram of water, pressure against temperature. Solid region at the upper left, liquid region at the top middle, gas region at the lower right. The solid–liquid line slopes slightly to the left. The three lines meet at the triple point, 0.01 degrees Celsius and 0.006 atm. The liquid–gas line ends at the critical point, 374 degrees and 218 atm. A dashed line at 1 atm crosses the melting line at 0 degrees and the boiling line at 100 degrees.
Phase diagram of water (schematic, not to scale).

Worked example

Worked example: Reading the CO₂ data

Question: In which state is carbon dioxide at (a) 1 atm and 25 °C (b) 60 atm and 0 °C (c) 100 atm and 40 °C?

  1. (a) Above its sublimation point at 1 atm (−78.5 °C): gas.
  2. (b) Above the triple-point pressure, and 60 atm is above the vapour pressure of liquid CO₂ at 0 °C (about 34 atm): liquid.
  3. (c) Above both 31.0 °C and 72.8 atm, the critical point: supercritical fluid.

Worked example: Why dry ice sublimes

Question: Explain why solid CO₂ at 1 atm turns directly into gas, while ice melts.

At 1 atm, CO₂ is below its triple-point pressure (5.11 atm), so the 1 atm line passes from the solid region straight into the gas region: sublimation. For water, 1 atm is far above the triple-point pressure (0.00604 atm), so the 1 atm line crosses the liquid region: ice melts, then boils.

Worked example: Converting triple-point pressures

Question: Express the triple-point pressures of water (611.7 Pa) and CO₂ (5.11 atm) in kPa.

  1. Water:

    611.7 Pa×1 kPa1000 Pa=0.6117 kPa\small\begin{aligned} &611.7\ \cancel{\text{Pa}} \times \frac{1\ \text{kPa}}{1000\ \cancel{\text{Pa}}} \\[4pt] &= 0.6117\ \text{kPa} \end{aligned}

  2. CO₂:

    5.11 atm×101.325 kPa1 atm=518 kPa\small\begin{aligned} &5.11\ \cancel{\text{atm}} \times \frac{101.325\ \text{kPa}}{1\ \cancel{\text{atm}}} \\[4pt] &= 518\ \text{kPa} \end{aligned}

  3. CO₂’s triple point is about 850 times higher in pressure than water’s.

Worked example: Pressure and ice

Question: Ice at −1 °C is squeezed very hard. Using water’s phase diagram, what could happen?

Because water’s solid–liquid line slopes to the left, a vertical move upwards (higher pressure) from a point just left of the line can cross into the liquid region. Enough pressure can melt ice slightly below 0 °C.

Common mistake

Common mistake: Swapping the axes

Phase diagrams almost always have temperature on the x-axis and pressure on the y-axis. Read the axis labels before deciding which way “higher” is.

Common mistake: Thinking the triple point is a normal melting point

The triple point is at one specific pressure (0.00604 atm for water), not at 1 atm. The normal melting point (0 °C) and the triple point (0.01 °C) are close for water, but they are different points.

Common mistake: Assuming every substance has a liquid at 1 atm

CO₂ has no liquid at 1 atm, because its triple point lies above 1 atm.

Notation note

  • “Normal” melting and boiling points are measured at exactly 1 atm.
  • The pressure axis is often drawn on a stretched or logarithmic scale, so diagrams are usually not to scale.

Remember this

Remember this

  • Axes: temperature (x) and pressure (y); areas = single phases; lines = two phases in equilibrium.
  • Triple point: all three phases coexist. Critical point: end of the liquid–gas line; beyond it, a supercritical fluid.
  • Normal melting and boiling points lie on the 1 atm line.
  • Water’s solid–liquid line slopes left because ice is less dense than liquid water.
  • CO₂ sublimes at 1 atm because its triple-point pressure (5.11 atm) is above 1 atm.

Test yourself

Check your understanding before moving on.

Flashcards

10 cards

  1. Question
    What does a phase diagram show?
    Answer

    Which phase of a substance is stable at each temperature (x-axis) and pressure (y-axis).

  2. Question
    What do the lines on a phase diagram represent?
    Answer

    Conditions where two phases coexist in equilibrium (melting, boiling or sublimation points).

  3. Question
    What is the triple point?
    Answer

    The single temperature and pressure at which solid, liquid and gas coexist.

  4. Question
    What is the critical point?
    Answer

    The end of the liquid–gas line; beyond it the substance is a supercritical fluid.

  5. Question
    Where are the normal melting and boiling points on a phase diagram?
    Answer

    Where the 1 atm line crosses the solid–liquid and liquid–gas lines.

  6. Question
    Why does water's solid–liquid line slope to the left?
    Answer

    Ice is less dense than liquid water, so higher pressure favours the liquid and lowers the melting point.

  7. Question
    Give the triple point of water.
    Answer

    0.01 °C and 0.00604 atm (611.7 Pa).

  8. Question
    Give the triple point of CO₂.
    Answer

    −56.6 °C and 5.11 atm.

  9. Question
    Why does dry ice sublime at 1 atm?
    Answer

    Its triple-point pressure (5.11 atm) is above 1 atm, so liquid CO₂ cannot exist at 1 atm.

  10. Question
    In which state is CO₂ at 100 atm and 40 °C?
    Answer

    Supercritical fluid (above 72.8 atm and 31.0 °C).

Quiz

7 questions

  1. Question 1EasyOn a phase diagram, what is found at the triple point?
    Show answer

    Answer: Solid, liquid and gas in equilibrium

    The triple point is where the three lines meet: all three phases coexist.

  2. Question 2EasyWhat lies beyond the critical point?
    Show answer

    Answer: A supercritical fluid

    Above the critical temperature and pressure, liquid and gas can no longer be distinguished: the substance is a supercritical fluid.

  3. Question 3EasyWhat state is water in at 1 atm and 50 °C?
    Show answer

    Answer: Liquid

    At 1 atm water melts at 0 °C and boils at 100 °C, so at 50 °C it is liquid.

  4. Question 4MediumWhy does solid CO₂ sublime at atmospheric pressure?
    Show answer

    Answer: Its triple-point pressure (5.11 atm) is above 1 atm

    Liquid CO₂ only exists above 5.11 atm. At 1 atm the solid region borders the gas region directly.

  5. Question 5MediumWater's solid–liquid line slopes to the left. What does this mean?
    Show answer

    Answer: Increasing pressure lowers the melting point

    Because ice is less dense than liquid water, squeezing ice favours the liquid, so the melting point falls slightly as pressure rises.

  6. Question 6HardIn which state is CO₂ at 60 atm and 0 °C? (Triple point −56.6 °C, 5.11 atm; vapour pressure of liquid CO₂ at 0 °C ≈ 34 atm)
    Show answer

    Answer: Liquid

    The pressure is above the vapour pressure at 0 °C, and both T and P are below the critical point (31.0 °C, 72.8 atm), so CO₂ is liquid, as in a fire extinguisher.

  7. Question 7MediumMoving straight up on a phase diagram (at constant temperature) means:
    Show answer

    Answer: increasing the pressure at constant temperature

    Pressure is on the y-axis, so a vertical move upward is compression at constant temperature.

Notes and downloads

  • Worksheet

    Phase Diagrams Worksheet

    8 questions on reading the phase diagrams of water and carbon dioxide, triple and critical points, sublimation and unit conversions. Answer key included.

    IntermediateFree

References

  1. Brown, T. L.; LeMay, H. E., Jr.; Bursten, B. E.; Murphy, C. J.; Woodward, P. M.; Stoltzfus, M. W. Chemistry: The Central Science, 15th ed.; Pearson, 2022.

Practise this topic with flashcards and a quiz at chemistryclarity.com/chemistry/phase-diagrams/

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