Name: water phase changes on a pressure chart
Uploaded: 2025-05-22T09:05:49.000Z
Duration: 1 min 19 s
Channel: JoVE

K12 Standards

Florida Department of Education

High School

Physical Science

SC.912.P.12

SC.912.P.12.11

Phase Diagram

In nature, matter exists in three phases: solid, liquid, and gas. The transition between phases requires phase equilibrium between two phases and occurs at a specific temperature for a given pressure.
These conditions can be represented graphically with pressure and temperature as the axes, known as a phase diagram.
A phase diagram represents the phases of a substance that coexist in thermodynamic equilibrium and undergo phase transition under different conditions, such as temperature, pressure, or volume.
Water exists in the liquid phase at room temperature and atmospheric pressure. At temperature 0oC or below, the phase changes from liquid to solid, whereas when the temperature is above 100oC, it changes to a gas phase.
The transition between solid and gas is called sublimation, while the transitions from solid to liquid and from liquid to gas are known as fusion and vaporization, respectively.
The point when all three phases coexist is known as the triple point, and the point when the properties of two phases become indistinguishable is known as a critical point.

phase diagram

The phase of a given substance depends on the pressure and temperature. Thus, plots of pressure versus temperature showing the phase in each region provide considerable insights into the thermal properties of substances. Such plots are known as phase diagrams. For instance, in the phase diagram for water (Figure 1), the solid curve boundaries between the phases indicate phase transitions (i.e., temperatures and pressures at which the phases coexist).
<img alt="Phase diagram showing triple and critical points; solid, liquid, gas regions. Key thermodynamics." src="/files/ftp_upload/13673/13673_Figure_1v2.png" style="text-align: center; height: 300px;" data-altupdatedat="1754904545000" data-alt="Phase diagram showing solid, liquid, gas regions and critical point in thermodynamics." />
As the pressure increases, the boiling temperature of water rises gradually to 374 &#176;C at a pressure of 218 atm. This can be demonstrated in a pressure cooker, which cooks food faster than an open pot, because the water can exist as a liquid at temperatures greater than 100 &#176;C without all boiling away. The boiling point curve ends at a certain point called the critical point, the temperature above which the liquid and gas phases cannot be distinguished; the substance is called a supercritical fluid. At a sufficiently high pressure above the critical point, a gas has the density of a liquid but does not condense. The pressure at this critical point is known as the critical pressure. Carbon dioxide, for example, is supercritical at all temperatures above 31.0 &#176;C, the point at which all three phases exist in equilibrium. For water, the triple point occurs at 273.16 K (0.01 &#176;C) and 611.2 Pa; this is a more accurate calibration temperature than the melting point of water at 1.00 atm, or 273.15 K (0.0 &#176;C).

The phase of a given substance depends on the pressure and temperature. Thus, plots of pressure versus temperature showing the phase in each region ...

Education

JoVE Core

Physics

Unit 1

The Kinetic Theory of Gases

KEY TERMS AND CONCEPTS

water phase changes on a pressure chart

Water phase changes can be mapped on a pressure and temperature chart called a phase diagram. These plots show which phase a substance is in at different conditions. They also reveal where phase transitions happen, because the boundary lines mark the temperatures and pressures where two phases coexist.

The water phase diagram shows how pressure affects boiling. As pressure rises, the boiling temperature increases gradually until it reaches 374 °C at 218 atm. A pressure cooker works this way. It lets water stay liquid above 100 °C, so food cooks faster than it would in an open pot.

The boiling-point curve ends at the critical point. Above this temperature, the liquid and gas phases can no longer be told apart. The substance is then called a supercritical fluid. At pressures above the critical point, a gas can have the density of a liquid without condensing.

The pressure at the critical point is called the critical pressure. Carbon dioxide becomes supercritical at all temperatures above 31.0 °C. That is also the point where solid, liquid, and gas exist together in equilibrium.

Water also has a triple point. It occurs at 273.16 K, or 0.01 °C, and 611.2 Pa. This point is useful in science because it is a more accurate calibration temperature than the melting point of water at 1.00 atm, which is 273.15 K, or 0.0 °C.

Water Phase Changes on a Pressure Chart

equation of state

Gas Behavior in the Equation of State

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ideal gas equation

Gas Laws Behind the Ideal Equation

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van der waals equation

How Real Gases Deviate from Ideal Behavior

Real gases often deviate from the ideal gas law, especially at low temperatures and high pressures. The van der Waals equation improves the ideal gas model by adding two corrections that account for these real-world effects. One correction accounts for attractive forces between molecules. These forces become stronger at higher densities and lower the measured pressure, so the equation adds a pressure term based on the square of the molar density and a positive constant a. The second...

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Liquid-Gas Changes on pV Curves

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Gas Pressure and Molecular Motion

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Molecular Speed Curves and Vapor Pressure

Molecular speed curves show how the speeds of gas particles are spread out at a given temperature. For a gas with many molecules, this spread is predictable even though each molecule moves randomly in direction and speed. The pattern is called the Maxwell-Boltzmann distribution. Liquids have a similar speed distribution, but it is not exactly the same as in gases. This difference helps explain boiling and vapor pressure. A molecule must reach at least a certain speed, v a, to escape from the...

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maxwell-boltzmann-distribution-problem-solving

Maxwell-Boltzmann Gas Speed Ratio

The Maxwell-Boltzmann distribution describes how molecular speeds are spread in a gas. Even though individual molecules move in random directions, the whole gas has a predictable speed pattern. This distribution is written as f(v), where v is molecular speed. The distribution function is used to find the expected number of particles with speeds between v1 and v2. Because N is dimensionless, f(v) has units of seconds per meter. The relationship can also be written in differential form for speed...

Water phase changes can be mapped on a pressure and temperature chart called a phase diagram. These plots show which phase a substance is in at different conditions. They also reveal where phase transitions happen, because the boundary lines mark the temperatures and pressures where two phases coexist. The water phase diagram shows how pressure affects boiling. As pressure rises, the boiling temperature increases gradually until it reaches 374 °C at 218 atm. A pressure cooker works this way.

mean free path and mean free time

Gas collisions: path length and time

Gas collisions in a cylinder help explain the mean free path and mean free time. Gas molecules move in random motion. They collide with each other and change speed and direction. The mean free path is the average path length between collisions. The mean free path depends on how many molecules are packed into a volume. It varies inversely with density. When more molecules are present, collisions happen more often, so the mean free path becomes shorter. It also depends on molecule size. Larger...

heat capacity: problem-solving

Heat Capacity of Gases by Degrees of Freedom

Heat capacity of gases depends on the gas type and the motion of its molecules. It is the amount of heat energy needed to raise the temperature of a unit mass of gas by one degree Celsius. This thermodynamic property is important in science and industry. To solve heat capacity problems, first identify the gas. A gas's molecular structure affects its heat capacity because the molecules have different degrees of freedom. These values can be found from tables or from empirical equations. Next,...

dalton's law of partial pressure

Gas Mixtures and Their Partial Pressures

Gas mixtures have a total pressure that comes from the partial pressures of each gas. A partial pressure is the pressure one gas would make if it filled the whole container by itself. This idea helps explain how gases behave when they are mixed together. When two or more gases share a container, their molecules move randomly and collide with one another. If the gases are at the same temperature, they reach thermal equilibrium. Their molecules then have the same average kinetic energy. In that...

escape velocities of gases

Why Lighter Gases Escape Earth First

Earth’s escape velocity is the speed an object needs to leave gravity behind. Near the top of the atmosphere, about 100 km above Earth, that speed is 11.1 km/s. Gas molecules can escape only if their speeds become high enough to reach that limit. The chance of escape depends on gas temperature and molecular speed. According to kinetic theory, the average kinetic energy of gas molecules is proportional to temperature. As temperature rises, the rms speed, or root-mean-square speed, also rises.

key terms and concepts

Explore how water changes phase with pressure and temperature, including boiling point shifts, the critical point, supercritical fluids, and the triple point.

Water Phase Changes on a Pressure Chart | JoVE K12

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Water Phase Changes on a Pressure Chart

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