States of Matter 1.14: Phase Diagrams Wiva k12 Chemistry

Phase changes are accompanied by energy changes.

If you hold an ice cube in your hand, it will melt. Melting is a change of state—a phase change—from a solid to a liquid. Melting requires energy in the form of heat. If you collect the water from that ice cube in a pan and let it sit at room temperature, it will eventually evaporate; it will change from liquid to gas. These changes involve adding or absorbing thermal energy. By contrast, if you cool water vapor, you will condense the water vapor into liquid water. If you place water in a freezer, it will turn to ice. Both of these changes involve removing or releasing thermal energy. Notice that changes of state are accompanied by changes in energy. Molecules in gases are in constant motion and have kinetic energy (KE). However, molecules also have intermolecular forces of attraction (IF), which are strongest over short distances.

Could there be water on Mars and Venus?

Liquid water is important for life on a planet. Also, carbon dioxide is an important gas for the atmosphere of a planet. Given the the following conditions for Mars, Earth, and Venus, and the phase diagrams in the Compare Diagrams activity, in what phases can water and carbon dioxide exist on these three planets? -Mars: average surface temperature = 223 K (-50°C) and pressure = 0.63 kPa (0.006 atm) -Earth: average surface temperature = 288 K (15°C) and pressure = 100 kPa (1 atm) -Venus: average surface temperature = 733 K (460°C) and pressure = 9,210 kPa (90.7 atm)

Phase diagrams help you visualize the relationship between the states of a substance.

Matter exists mainly in three states: solid, liquid, or gas. The state depends on the balance between the kinetic energy of the particles and their intermolecular forces. Pressure and temperature can influence these changes of state, which can be depicted in phase diagrams. In this lesson, you will learn how to interpret phase diagrams, obtain useful information from them, and make predictions by using them.

Interpret phase diagrams to determine state.

Phase diagrams allow you to predict what state a substance will be in at a given temperature and pressure. For example, in what phase is substance X at a pressure of 100 kPa and 10°C? 1.) Follow a vertical line up from 10°C on the x-axis. 2.) Follow a horizontal line from 100 kPa on the y-axis. 3.) Check the area where they intersect. (In this case, substance X is a solid.) In what phase is substance X at 50°C and 400 kPa? Under these conditions, substance X is a liquid. If the intersection occurs on a curve, then the substance is in equilibrium between two phases. For example, at 100 kPa and about 28°C, substance X is both a solid and a gas.

Phase changes are often affected by changes in temperature and/or pressure.

Remember that the phase of a pure substance is a balance between KE and IF. As you saw, you can change KE by adding or removing thermal energy. What about the IF? Can you change them, too? Yes, you can. IF are strongest when molecules are close together. Therefore, you can increase the effects of IF by bringing molecules close together. You do this by increasing the pressure. At high pressures, molecules are close together, and IF > KE. By contrast, at low pressures, molecules are far apart, so IF < KE.

Phase diagrams depict states of matter.

States of matter are balances between the intermolecular forces (IF) and kinetic energy (KE) of molecules. Changes in temperature cause phase changes by altering the KE, while changes in pressure cause phase changes by changing the IF. Phase diagrams display the temperatures and pressures at which a pure substance is a solid, liquid, or gas. Phase diagrams have fusion curves, vaporization curves, and sublimation curves. Triple point is the temperature and pressure where all three states of matter coexist. Critical point is the temperature and pressure above which liquids and gases cannot be distinguished.

What happens to molecules during phase changes?

The state of matter is a balance between IF and KE. What happens as energy is added to change from ice to water vapor? -Ice: IF >> KE. Water molecules vibrate within the crystal but don't move around. As energy is added, KE increases. -Melting: KE increases, so KE approaches IF. The crystal breaks apart, and the molecules begin to move around. -Liquid: IF ≥ KE. Molecules move around but stay close together. KE begins to exceed IF. -Vaporization: KE increases, so KE > IF. Molecules move around faster and escape the liquid into vapor phase above the liquid. -Gas: KE >> IF. Molecules move rapidly and are far apart.

Now review some model problems.

Turn to Problem Set 60, Phase Diagrams, in the Chemistry: Problems and Solutions book. Review the sample problems and complete problems 3-5. When you have finished, check your answers in the Solution Key.

States of matter depend on the internal energy of atoms and molecules.

Under most conditions found on earth, matter can exist in three states: solid, liquid, or gas. When energy is added or removed, matter can change from one state to another. Such a change of state is called a phase change. Phase changes are classified as follows: -liquid/gas: vaporization and condensation -solid/liquid: melting and freezing -solid/gas: sublimation and deposition

The phase diagram of water reflects water's unique properties.

Water has an interesting phase diagram in that it is the only substance with a negatively sloped fusion curve. That is, when you increase the pressure at 0°C, the melting point decreases. Generally, when you increase the pressure on a reaction in equilibrium, such as the solid-to-liquid phase change, the reaction will proceed to the state that takes up less volume. For most substances, the solid phase takes up less volume than the liquid phase, but water expands upon freezing. As you increase the pressure, the melting point will decrease. Therefore, at high pressures, the only way to freeze water is to reduce the temperature. Water's phase diagram shows unique features, such as the negatively sloped line between the solid and liquid phases. Note that the axes are not to scale.

Interpret phase diagrams to determine temperature or pressure.

We tend to think that as you heat a substance, it moves from solid to liquid to gas; however, that movement depends on the existing pressure. For example, as you increase the temperature from 10°C to 80°C at 300 kPa, substance X goes from solid to liquid to gas. But if you do the same at 100 kPa, substance X changes from solid to gas. Pressure changes can cause phase changes even at a constant temperature. For example, at 50°C, when pressure is reduced from 500 kPa to 50 kPa, substance X boils to a gas. Interestingly, if the same reduction in pressure is made at 25°C, liquid substance X first freezes solid, and then sublimes to a gas.

Phase diagrams depict the effects of temperature and pressure on phase changes.

You can visualize the effects of temperature and pressure by using phase diagrams. Phase diagrams are graphs of the relationship between the pressure and the temperature of a gas; these diagrams show the solid, liquid, and gas phases. A phase diagram has three curves, representing the temperatures and corresponding pressures for the transitions from one state to another. -fusion curve (blue): solid to liquid change -vaporization curve (green): liquid to gas change -sublimation curve (red): solid to gas change The phases are denoted as areas of the graph between the lines. The three curves intersect at the triple point, the temperature and pressure at which all three phases coexist. The vaporization curve ends at a critical point; above this temperature and pressure, you cannot distinguish between a liquid and gas state.

Critical Point

a point in a phase diagram at which the vaporization curve ends and one cannot distinguish between a liquid and gas state

Phase

a state of matter: solid, liquid, gas, plasma

Which curve or point of a phase diagram would indicate the melting point at various temperatures and pressures? a.) triple point b.) fusion curve c.) sublimation curve d.) critical point

b.) fusion curve

According to this phase diagram of water, what happens when you increase the pressure from 50 kPa to 150 kPa on an ice cube at 0°C? a.) The ice cube sublimes. b.) The ice cube remains solid. c.) The ice cube melts.

c.) The ice cube melts.

Triple Point

conditions of temperature and pressure at which a substance´s solid, liquid, and gas are in equilibrium

Refer to the phase diagram for substance X in Problem Set 60: Phase Diagrams, in the Chemistry: Problems and Solutions book. In the phase diagram for substance X, what is the triple point of substance X? a.) 0°C, 0 atm b.) 22°C, 4 atm c.) 60°C, 5 atm d.) 29°C, 2.2 atm

d.) 29°C, 2.2 atm

For a given sample of carbon dioxide (CO2) you increase the temperature from 80°C to 0°C and decrease the pressure from 30 atm to 5 atm. On a phase diagram this causes a crossing of both the solid-liquid boundary and the liquid-gas boundary. Which of the following best describes what happens? a.) The solid sublimes to gas. b.) The gas condenses to liquid and then freezes solid. c.) The gas deposits directly into a solid. d.) The solid melts to liquid and then vaporizes to gas.

d.) The solid melts to liquid and then vaporizes to gas.

Which of the following occurs when the temperature and pressure are below the triple point of a substance? a.) The substance melts to a liquid. b.) Only the solid form of the substance can exist. c.) Only the gaseous form of the substance can exist. d.) The substance cannot exist in the liquid form.

d.) The substance cannot exist in the liquid form.

Sublimation

the change of state from a solid directly to a gas