Phase Diagrams for Pure Substances (2024)

  1. Last updated
  2. Save as PDF
  • Page ID
    3864
  • \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}}}\) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

    This page explains how to interpret the phase diagrams for simple pure substances - including a look at the special cases of the phase diagrams of water and carbon dioxide.

    The Basic Phase Diagram

    At its simplest, a phase can be just another term for solid, liquid or gas. If you have some ice floating in water, you have a solid phase present and a liquid phase. If there is air above the mixture, then that is another phase. But the term can be used more generally than this. For example, oil floating on water also consists of two phases - in this case, two liquid phases. If the oil and water are contained in a bucket, then the solid bucket is yet another phase. In fact, there might be more than one solid phase if the handle is attached separately to the bucket rather than molded as a part of the bucket.

    You can recognize the presence of the different phases because there is an obvious boundary between them - a boundary between the solid ice and the liquid water, for example, or the boundary between the two liquids. A phase diagram lets you work out exactly what phases are present at any given temperature and pressure. In the cases we'll be looking at on this page, the phases will simply be the solid, liquid or vapor (gas) states of a pure substance. This is the phase diagram for a typical pure substance.

    Phase Diagrams for Pure Substances (2)

    These diagrams (including this one) are nearly always drawn highly distorted in order to see what is going on more easily. There are usually two major distortions. We'll discuss these when they become relevant. If you look at the diagram, you will see that there are three lines, three areas marked "solid", "liquid" and "vapor", and two special points marked "C" and "T".

    The Three Areas

    These are easy! Suppose you have a pure substance at three different sets of conditions of temperature and pressure corresponding to 1, 2 and 3 in the next diagram.

    Phase Diagrams for Pure Substances (3)

    Under the set of conditions at 1 in the diagram, the substance would be a solid because it falls into that area of the phase diagram. At 2, it would be a liquid; and at 3, it would be a vapor (a gas).

    Phase Transitions

    Moving from solid to liquid by changing the temperature

    Suppose you had a solid and increased the temperature while keeping the pressure constant - as shown in the next diagram. As the temperature increases to the point where it crosses the line, the solid will turn to liquid. In other words, it melts.

    Phase Diagrams for Pure Substances (4)

    If you repeated this at a higher fixed pressure, the melting temperature would be higher because the line between the solid and liquid areas slopes slightly forward.

    Phase Diagrams for Pure Substances (5)

    So what actually is this line separating the solid and liquid areas of the diagram?

    It simply shows the effect of pressure on melting point. Anywhere on this line, there is an equilibrium between solid and liquid. You can apply Le Chatelier's Principle to this equilibrium just as if it was a chemical equilibrium. If you increase the pressure, the equilibrium will move in such a way as to counter the change you have just made.

    Phase Diagrams for Pure Substances (6)

    If it converted from liquid to solid, the pressure would tend to decrease again because the solid takes up slightly less space for most substances. That means that increasing the pressure on the equilibrium mixture of solid and liquid at its original melting point will convert the mixture back into the solid again. In other words, it will no longer melt at this temperature.

    To make it melt at this higher pressure, you will have to increase the temperature a bit. Raising the pressure raises the melting point of most solids. That's why the melting point line slopes forward for most substances.

    Moving from solid to liquid by changing the pressure

    You can also play around with this by looking at what happens if you decrease the pressure on a solid at constant temperature.

    Phase Diagrams for Pure Substances (7)

    Moving from liquid to vapor

    In the same sort of way, you can do this either by changing the temperature or the pressure.

    Phase Diagrams for Pure Substances (8)

    The liquid will change to a vapor - it boils - when it crosses the boundary line between the two areas. If it is temperature that you are varying, you can easily read off the boiling temperature from the phase diagram. In the diagram above, it is the temperature where the red arrow crosses the boundary line.

    So, again, what is the significance of this line separating the two areas? Anywhere along this line, there will be an equilibrium between the liquid and the vapor. The line is most easily seen as the effect of pressure on the boiling point of the liquid. As the pressure increases, so the boiling point increases.

    The critical point

    You will have noticed that this liquid-vapor equilibrium curve has a top limit (labeled as C in the phase diagram), which is known as the critical point. The temperature and pressure corresponding to this are known as the critical temperature and critical pressure. If you increase the pressure on a gas (vapor) at a temperature lower than the critical temperature, you will eventually cross the liquid-vapor equilibrium line and the vapor will condense to give a liquid.

    Phase Diagrams for Pure Substances (9)

    This works fine as long as the gas is below the critical temperature. What, though, if your temperature was above the critical temperature? There wouldn't be any line to cross! That is because, above the critical temperature, it is impossible to condense a gas into a liquid just by increasing the pressure. All you get is a highly compressed gas. The particles have too much energy for the intermolecular attractions to hold them together as a liquid. The critical temperature obviously varies from substance to substance and depends on the strength of the attractions between the particles. The stronger the intermolecular attractions, the higher the critical temperature.

    Moving from solid to vapor

    There's just one more line to look at on the phase diagram. This is the line in the bottom left-hand corner between the solid and vapor areas. That line represents solid-vapor equilibrium. If the conditions of temperature and pressure fell exactly on that line, there would be solid and vapor in equilibrium with each other - the solid would be subliming. (Sublimation is the change directly from solid to vapor or vice versa without going through the liquid phase.)

    Once again, you can cross that line by either increasing the temperature of the solid, or decreasing the pressure. The diagram shows the effect of increasing the temperature of a solid at a (probably very low) constant pressure. The pressure obviously has to be low enough that a liquid can't form - in other words, it has to happen below the point labelled as T.

    Phase Diagrams for Pure Substances (10)

    You could read the sublimation temperature off the diagram. It will be the temperature at which the line is crossed.

    The Triple Point

    Point T on the diagram is called the triple point. If you think about the three lines which meet at that point, they represent conditions of:

    • solid-liquid equilibrium
    • liquid-vapor equilibrium
    • solid-vapor equilibrium

    Where all three lines meet, you must have a unique combination of temperature and pressure where all three phases are in equilibrium together. That's why it is called a triple point.

    If you controlled the conditions of temperature and pressure in order to land on this point, you would see an equilibrium which involved the solid melting and subliming, and the liquid in contact with it boiling to produce a vapor - and all the reverse changes happening as well. If you held the temperature and pressure at those values, and kept the system closed so that nothing escaped, that's how it would stay.

    Normal melting and boiling points

    The normal melting and boiling points are those when the pressure is 1 atmosphere. These can be found from the phase diagram by drawing a line across at 1 atmosphere pressure.

    Phase Diagrams for Pure Substances (11)

    Example \(\PageIndex{1}\): Phase Diagram for Water

    There is only one difference between this and the phase diagram that we've looked at up to now. The solid-liquid equilibrium line (the melting point line) slopes backwards rather than forwards.

    Phase Diagrams for Pure Substances (12)

    In the case of water, the melting point gets lower at higher pressures. Why?

    Phase Diagrams for Pure Substances (13)

    If you have this equilibrium and increase the pressure on it, according to Le Chatelier's Principle the equilibrium will move to reduce the pressure again. That means that it will move to the side with the smaller volume. Liquid water is produced. To make the liquid water freeze again at this higher pressure, you will have to reduce the temperature. Higher pressures mean lower melting (freezing) points.

    Now lets put some numbers on the diagram to show the exact positions of the critical point and triple point for water.

    Phase Diagrams for Pure Substances (14)

    Notice that the triple point for water occurs at a very low pressure. Notice also that the critical temperature is 374°C. It would be impossible to convert water from a gas to a liquid by compressing it above this temperature. The normal melting and boiling points of water are found in exactly the same way as we have already discussed - by seeing where the 1 atmosphere pressure line crosses the solid-liquid and then the liquid-vapor equilibrium lines.

    Just one final example of using this diagram. Imagine lowering the pressure on liquid water along the line in the diagram below.

    Phase Diagrams for Pure Substances (15)

    The phase diagram shows that the water would first freeze to form ice as it crossed into the solid area. When the pressure fell low enough, the ice would then sublime to give water vapor. In other words, the change is from liquid to solid to vapor.

    Example \(\PageIndex{2}\): Phase Diagram for Carbon Dioxide

    The only thing special about this phase diagram is the position of the triple point which is well above atmospheric pressure. It is impossible to get any liquid carbon dioxide at pressures less than 5.11 atmospheres.

    Phase Diagrams for Pure Substances (16)

    That means that at 1 atmosphere pressure, carbon dioxide will sublime at a temperature of -78°C. This is the reason that solid carbon dioxide is often known as "dry ice". You can't get liquid carbon dioxide under normal conditions - only the solid or the vapor.

    Contributors and Attributions

    Phase Diagrams for Pure Substances (2024)

    FAQs

    What is phase diagram of pure substance? ›

    A phase diagram lets you work out exactly what phases are present at any given temperature and pressure. In the cases we'll be looking at on this page, the phases will simply be the solid, liquid or vapor (gas) states of a pure substance. This is the phase diagram for a typical pure substance.

    Do pure substances have 3 visible phases? ›

    Phases of a Pure Substance

    There are three principal phases solid, liquid, and gas. A phase: is defined as having a distinct molecular arrangement that is hom*ogenous throughout and separated from others (if any) by easily identifiable boundary surfaces.

    What is the phase rule for a pure substance? ›

    For pure substances C = 1 so that F = 3 − P. In a single phase (P = 1) condition of a pure component system, two variables (F = 2), such as temperature and pressure, can be chosen independently to be any pair of values consistent with the phase.

    Which is not a valid phase diagram for a pure substance? ›

    (1) The 4th diagram is not a valid phase diagram for a pure substance. This is because there is a point where four phases exist together. This cannot be true for a pure substance. where, F is the degrees of freedom, C is the number of components and P is the number of phases.

    What is an example of a pure substance? ›

    Examples of Pure Substances

    A few of them include gold, copper, oxygen, chlorine, diamond, etc. Compounds such as water, salt or crystals, baking soda amongst others are also grouped as pure substances.

    What is the summary of phase diagram? ›

    Phase diagrams plot pressure (typically in atmospheres) versus temperature (typically in degrees Celsius or Kelvin). The labels on the graph represent the stable states of a system in equilibrium. The lines represent the combinations of pressures and temperatures at which two phases can exist in equilibrium.

    Does a pure substance only have one phase? ›

    By definition, a pure substance or a hom*ogeneous mixture consists of a single phase. A heterogeneous mixture consists of two or more phases. When oil and water are combined, they do not mix evenly, but instead form two separate layers. Each of the layers is called a phase.

    What substance can exist all 3 phases of matter? ›

    And water is the substance that best represents them, because it is the only one that exists naturally in the three states.

    What are the 3 different phases a substance can exist as? ›

    The three fundamental phases of matter are solid, liquid, and gas (vapour), but others are considered to exist, including crystalline, colloid, glassy, amorphous, and plasma phases.

    What is a state of pure substance? ›

    A pure substance refers to a matter that has a hom*ogeneous and definite chemical composition. A pure substance may exist in a single phase, such as liquid water, ice, and CO2 gas.

    What is the standard state of a pure substance? ›

    The standard state for liquids and solids is simply the state of the pure substance subjected to a total pressure of 105 Pa (or 1 bar).

    What three phases of a pure substance coexist in equilibrium? ›

    In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium.

    What does it mean by phase of a substance? ›

    Those portions of a system that are physically distinct and mechanically separable from other portions of the system are called phases. states of matter. Phases within a system exist in a gaseous, liquid, or solid state. Solids are characterized by strong atomic bonding and high viscosity, resulting in a rigid shape.

    Is a pure substance one phase? ›

    A phase is any part of a sample that has a uniform composition and properties. By definition, a pure substance or a hom*ogeneous mixture consists of a single phase. A heterogeneous mixture consists of two or more phases.

    What is the phase of a substance? ›

    A phase is a distinct and hom*ogeneous state of a system with no visible boundary separating it into parts. Water, H2O, is such a common substance that its gas (steam), liquid (water), and solid (ice) phases are widely known.

    What are the phases of pure metal? ›

    PHASE DIAGRAM OF PURE METAL

    A schematic phase diagram of a pure metal indicating the pressure temperature domain where it can exist as solid, liquid or gas. The line indicates boundary between two phases. On any point on the line two phases are equally stable. There is a point where three lines meet.

    Top Articles
    Latest Posts
    Article information

    Author: Manual Maggio

    Last Updated:

    Views: 6334

    Rating: 4.9 / 5 (49 voted)

    Reviews: 80% of readers found this page helpful

    Author information

    Name: Manual Maggio

    Birthday: 1998-01-20

    Address: 359 Kelvin Stream, Lake Eldonview, MT 33517-1242

    Phone: +577037762465

    Job: Product Hospitality Supervisor

    Hobby: Gardening, Web surfing, Video gaming, Amateur radio, Flag Football, Reading, Table tennis

    Introduction: My name is Manual Maggio, I am a thankful, tender, adventurous, delightful, fantastic, proud, graceful person who loves writing and wants to share my knowledge and understanding with you.