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13.3: Changes of State

Created by: CK-12

Lesson Objectives

  • Describe how the phase of a material is affected by changes in the temperature.
  • Describe how the phase of a material is affected by changes in pressure.
  • Draw phase diagrams to relate the pressure, temperature, and the phase of a substance.
  • Describe the energy changes associated with changes of state.

Lesson Vocabulary

  • heating curve: A curve where supplying heat to a solid substance will gradually raise its temperature, and eventually, it will melt.
  • melting point: The temperature of a point where heat is used to break up the attractive forces holding them rigidly in place.
  • boiling point: The temperature of a point where particles start to enter the gas phase.
  • phase diagram: A plot of temperature vs. pressure that indicates the states of matter present at each point.
  • triple point: A point where all three states can exist simultaneously.

Check Your Understanding

  1. Compare and contrast the properties of liquids, solids, and gases.
  2. Which of the following statements about solids and liquids are true? (There may be more than one.)
    1. Solids and liquids are virtually incompressible; their volume is constant.
    2. Solids are typically more dense than liquids.
    3. All liquids have the same density.

Introduction

In the last lesson, we studied the characteristics of liquids and solids at a macroscopic level and at the molecular level. Increasing the temperature of a solid transforms the particles from a rigid arrangement to a fluid (a liquid or gas). Conversely, decreasing the temperature of a liquid or gas slows the particles down, going back from free movement to a fixed arrangement. In this section, we will further explore how temperature and pressure affect the characteristics and behavior of matter. Pressure has a larger effect on gases, which are very compressible, than liquids and solids. However, changes in pressure are still relevant to solids and liquids. For example, Figure above shows ice skates on ice. Because your entire weight is all concentrated on a thin blade, ice skates exert quite a bit of pressure on the ice below them. An interesting property of water is that increasing the pressure on its solid form (ice) will eventually convert it to liquid water. The pressure exerted by ice skates makes small amounts of liquid water on the surface, allowing the skates to glide smoothly.

Heating Curves

Supplying heat to a solid substance will gradually raise its temperature, and eventually, it will melt. Heating the resulting liquid will cause a further increase in temperature until the liquid begins to boil. If we were to graph the temperature of a substance against the amount of heat added, it would look something like the Figure below.

Heating curve depicting phase changes between the solid, liquid, and gas forms of a given substance.

The Figure above is referred to as a heating curve. The most notable feature is that the temperature rise is not steady; there are plateaus during which heat is being added, but the temperature is not increasing. What is happening here? Let's start with the first slope on the left. At this point, the substance is in its solid form. Adding heat causes the particles to move faster. Faster particles means more kinetic energy, which also means a higher temperature. At the first plateau, the vibrations of the particles become energetic enough to break free of the rigid solid form, and the substance converts to a liquid. Heat continues to be added, but instead of increasing the kinetic energy of the particles, it is used to break up the attractive forces holding them rigidly in place. This process is known as melting, and the temperature at which it occurs is the melting point. Because the added heat is used to break up attractive forces instead of adding to the kinetic energy of the particles, the temperature of the material stays constant until the phase change is completed.

Further heating then adds energy to the liquid particles, increasing their speed, kinetic energy, and temperature. This is the second slope on the curve. Once the particles are energetic enough to completely break free of each other, they start to enter the gas phase. Boiling occurs at the second plateau of this curve, and the temperature at this point is referred to as the boiling point. Again, the added heat is being used to break up the interactions between particles instead of increasing their kinetic energy, so no temperature increase is observed until all particles are in the gas phase. Finally, adding even more energy will further speed up the gas particles, increasing the kinetic energy and temperature of the substance.

The reverse process can also be diagrammed, where we start with a gas and gradually remove heat until it condenses to a liquid and then freezes into a solid. For a given amount of a certain substance at a given pressure, heating and cooling curves should be mirror images. The melting point will be equal to the freezing point, and the boiling point will be equal to the condensation point. Additionally, the amount of heat added to completely melt the sample is the same as the amount that must be removed to completely freeze it.

Two other changes of state can occur under at appropriate pressures. The direct conversion of a solid to a gas without becoming a liquid is called sublimation. The reverse process (gas to solid) is known as deposition. Depositions of hot metal vapors are often used in the electronics industry to produce thin films of metal on solid bases. Most substances require reduced pressures (less than one atmosphere) for these processes to occur. At higher pressures, substances would transition through the liquid phase. However, some materials, such as carbon dioxide, will sublime even at standard pressure. Iodine and naphthalene (found in mothballs) are other substances that can sublime at only slightly reduced pressures.

The Figure below summarizes the different processes involved in phase changes.

Summary of phase changes.

Phase Diagrams

Both temperature and pressure have an effect on the phase in which a given substance exists. A plot of temperature vs. pressure that indicates the states of matter present at each point is known as a phase diagram. Figure below shows the phase diagram for water.

The lines on this diagram show the boundaries between the three states of matter for water. Notice that there is one point where all three states can exist simultaneously; this is called the triple point. Also look at the slope of the solid-liquid boundary. For most substances, this line has a positive slope, which means that increasing the pressure of a liquid will eventually form a solid. Water is unusual in that it has a negative slope for this boundary. This has to do with the fact that the liquid form of water is more dense than the solid form (ice). As a result, putting large amounts of pressure on ice at a given temperature will cause it to melt. We already discussed this phenomenon at the beginning of the lesson in the context of ice skating. The phase diagram gives us a more detailed way to look at this occurrence.

Example 13.1

A pressure cooker operates by keeping water in its liquid form at temperatures above its normal boiling point. Can you use the phase diagram to explain this behavior?

Answer:

The pressure cooker maintains a pressure that is above one atmosphere. At this higher pressure, the boundary between the liquid and gaseous forms of water occurs at a higher temperature.

If we look at the phase diagram for carbon dioxide, we can see that its triple point occurs at approximately 5 atm and -56°C. Below this pressure, liquid CO2 cannot exist. At one atmosphere of pressure, only the solid and gas forms are possible, which explains why solid CO2 sublimes instead of melting at standard pressures. Also note that the solid-liquid boundary has a positive slope, which is normal for most substances.

Example 13.2

Why solid CO2 is called dry ice? Use the phase diagram to explain.

Answer:

Solid CO2 exists at temperatures below -78°C at the standard pressure of 1 atm. When solid CO2 is allowed to warm to room temperature at this pressure, the solid changes to a gas without going through the liquid form. Because it does not become a liquid, it is considered "dry."

Lesson Summary

  • Phase transitions occur as heat is added or removed from a substance.
  • The solid to liquid transition is called melting.
  • The liquid to gas transition is called boiling.
  • Sublimation and deposition involve direct transitions between solid and gas without going through the liquid state.
  • A phase diagram gives information about the conditions under which a material can exist as a solid, a liquid, or a gas.

Lesson Review Questions

  1. The following heating curve of an unknown substance shows several phase changes that take place as heat is added. Label each section indicated by a number.
  2. What would happen if you tried to make an ice rink using dry ice? Would you be able to skate? Compare the phase diagrams of water and dry ice to justify your answer.
  3. What would you see if you were to observe water at its triple point?
  4. Referring to the phase diagram of water, what phases can exist at a temperature of 0°C and a pressure of 0.006 atm?
  5. Referring to the phase diagram for CO2, at what temperature must liquid CO2 exist if the pressure is 100 atm?
  6. What temperature must carbon dioxide be in order to remain solid at 7 atmospheres of pressure? Would it melt or sublime at this pressure?

Further Reading / Supplemental Links

Points to Consider

  • Can you think of some substances commonly encountered in their gaseous state?
  • How might you check for the presence of a gas?

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Date Created:

Sep 09, 2013

Last Modified:

Dec 18, 2014
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