Phase Changes - AP Chemistry
Card 0 of 303
Consider the typical phase diagram of a compound given below.
Which of the following lines or points on the diagram represents a situation in which the rate of vaporization of the compound is equal to its rate of condensation?
Consider the typical phase diagram of a compound given below.
Which of the following lines or points on the diagram represents a situation in which the rate of vaporization of the compound is equal to its rate of condensation?
In this question, we're presented with a phase diagram and are asked to determine where on the graph the rate of vaporization equals the rate of condensation.
First, it's important to realize that when the rate of vaporization and condensation are equal, we have an equilibrium of liquid and gas phases. In other words, for a given temperature and pressure, the rate at which the liquid evaporates into a gas is exactly equal to the rate at which the gas condenses into a liquid.
On a phase diagram, the area of the upper left portion of the diagram represents the solid state. The middle portion of the diagram represents the liquid state. The bottom and right most part of the diagram represents the gas phase.
Furthermore, each line on the diagram represents the specific combination of temperature and pressure in which a given compound will exist in equilibrium between two phases. The point where all three lines intersect, however, represents the triple point. This tells us the temperature and pressure in which the compound will exist in an equilibrium between all three states.
Because we are looking for the equilibrium line that represents equilibrium of vaporization and condensation, we want the line that separates the liquid portion of the diagram from the gas portion. Based on the identification of regions on the diagram discussed above, that would be line C as shown in the diagram. Line A represents equilibrium between solid and gas (sublimation rate = deposition rate). Line B represents equilibrium between solid and liquid (melting rate = freezing rate).
In this question, we're presented with a phase diagram and are asked to determine where on the graph the rate of vaporization equals the rate of condensation.
First, it's important to realize that when the rate of vaporization and condensation are equal, we have an equilibrium of liquid and gas phases. In other words, for a given temperature and pressure, the rate at which the liquid evaporates into a gas is exactly equal to the rate at which the gas condenses into a liquid.
On a phase diagram, the area of the upper left portion of the diagram represents the solid state. The middle portion of the diagram represents the liquid state. The bottom and right most part of the diagram represents the gas phase.
Furthermore, each line on the diagram represents the specific combination of temperature and pressure in which a given compound will exist in equilibrium between two phases. The point where all three lines intersect, however, represents the triple point. This tells us the temperature and pressure in which the compound will exist in an equilibrium between all three states.
Because we are looking for the equilibrium line that represents equilibrium of vaporization and condensation, we want the line that separates the liquid portion of the diagram from the gas portion. Based on the identification of regions on the diagram discussed above, that would be line C as shown in the diagram. Line A represents equilibrium between solid and gas (sublimation rate = deposition rate). Line B represents equilibrium between solid and liquid (melting rate = freezing rate).
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Using the heat curve, define the segment time(s) that the kinetic energy of the substance is increasing.
Using the heat curve, define the segment time(s) that the kinetic energy of the substance is increasing.
Remember, temperature is a measure of the average kinetic energy of molecules. Therefore the kinetic energy increases whenever the temperature is increasing. So, the kinetic energy is increasing during segments 1, 3, and 5.
Remember, temperature is a measure of the average kinetic energy of molecules. Therefore the kinetic energy increases whenever the temperature is increasing. So, the kinetic energy is increasing during segments 1, 3, and 5.
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Using the heating curve, determine which segment(s) relate to an increase in potential energy.
Using the heating curve, determine which segment(s) relate to an increase in potential energy.
Remember, temperature is a measure of the average kinetic energy of molecules. So, the potential energy of the molecules will increase anytime energy is being supplied to the system but the temperature is not increasing. Therefore the potential energy is increasing during segments 2 and 4.
Remember, temperature is a measure of the average kinetic energy of molecules. So, the potential energy of the molecules will increase anytime energy is being supplied to the system but the temperature is not increasing. Therefore the potential energy is increasing during segments 2 and 4.
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What is the definition of the triple point?
What is the definition of the triple point?
Definition of triple point: where solid, liquid, and gas exist in equilibrium
Definition of triple point: where solid, liquid, and gas exist in equilibrium
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What is represented by point D
What is represented by point D
the critical point is where the solid and gas cannot be distinguished
the critical point is where the solid and gas cannot be distinguished
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As condensation forms on a glass of ice water, the temperature of the air surrounding the glass __________.
As condensation forms on a glass of ice water, the temperature of the air surrounding the glass __________.
As a substance condenses from the gas phase to the liquid phase, it loses energy in the form of heat loss. Heat is transferred from the water to the air, resulting in an increase in the temperature of the air.
As a substance condenses from the gas phase to the liquid phase, it loses energy in the form of heat loss. Heat is transferred from the water to the air, resulting in an increase in the temperature of the air.
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How much energy is required to boil 9 moles of liquid water at its boiling point, and what is the temperature of the water vapor product?
How much energy is required to boil 9 moles of liquid water at its boiling point, and what is the temperature of the water vapor product?
The enthalpy of vaporization gives the amount of energy required to evaporate a liquid at its boiling point, in units of energy per mole. The total energy requirement to heat a given amount of steam is found by mulitplying the the number of moles to be vaporized by the energy of vaporization per mole.
The temperature remains constant throughout a phase change, thus the final temperature would still be 100°C.
The enthalpy of vaporization gives the amount of energy required to evaporate a liquid at its boiling point, in units of energy per mole. The total energy requirement to heat a given amount of steam is found by mulitplying the the number of moles to be vaporized by the energy of vaporization per mole.
The temperature remains constant throughout a phase change, thus the final temperature would still be 100°C.
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How much heat must be added to raise a sample of 100g of water at 270K to 280K?
The specific heat capacity of water is 
, and water's heat of fusion is 
.
How much heat must be added to raise a sample of 100g of water at 270K to 280K?
The specific heat capacity of water is
The following fomula gives the heat needed to generate a given temperature change for a substance of known specific heat capacity:
where 
is the heat input in Joules, 
is the mass of the sample in grams, and 
is the specific heat capacity in 
.
However, in the event of a phase change (water melts at 273K), the heat of fusion or vaporization must be added to the total energy cost. The formula becomes:
The following fomula gives the heat needed to generate a given temperature change for a substance of known specific heat capacity:
where
However, in the event of a phase change (water melts at 273K), the heat of fusion or vaporization must be added to the total energy cost. The formula becomes:
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Why does water boil at a lower temperature at high elevation?
Why does water boil at a lower temperature at high elevation?
Increasing temperature means that vapor pressure increases as well. When vapor pressure is equal to the atmospheric pressure, water boils. The atmospheric pressure is lower at high elevation, so water boils at a lower temperature.
Increasing temperature means that vapor pressure increases as well. When vapor pressure is equal to the atmospheric pressure, water boils. The atmospheric pressure is lower at high elevation, so water boils at a lower temperature.
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In the given heating curve, which segment(s) correlate to a mixture of phases?
In the given heating curve, which segment(s) correlate to a mixture of phases?
When kinetic energy is increasing molecules are simply moving faster. However, when the potential energy is increasing molecules are changing phases. Therefore, when the potential energy is increasing is when the molecule is changing phases. Therefore there is a mix of molecules during segments 2 and 4.
When kinetic energy is increasing molecules are simply moving faster. However, when the potential energy is increasing molecules are changing phases. Therefore, when the potential energy is increasing is when the molecule is changing phases. Therefore there is a mix of molecules during segments 2 and 4.
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The heating curve represents a substance in phases solid, liquid, and gas. Which segment represents only the liquid phase?
The heating curve represents a substance in phases solid, liquid, and gas. Which segment represents only the liquid phase?
When the kinetic energy is increasing (the temperature is also increasing) the substance is not going through a phase change. Therefore only the segments that are at an incline will have the substance in just one phase. The flat areas of the graph represent areas in which heat is being added, but there is no corresponding increase in temperature. Rather, this added heat energy is used to break the intermolecular forces between molecules/atoms and drive phase changes.
Finally, because liquids are higher in energy than solids, and lower in energy than gasses the middle slanted line must be the liquid phase. In this case it is labeled as segment 3.
When the kinetic energy is increasing (the temperature is also increasing) the substance is not going through a phase change. Therefore only the segments that are at an incline will have the substance in just one phase. The flat areas of the graph represent areas in which heat is being added, but there is no corresponding increase in temperature. Rather, this added heat energy is used to break the intermolecular forces between molecules/atoms and drive phase changes.
Finally, because liquids are higher in energy than solids, and lower in energy than gasses the middle slanted line must be the liquid phase. In this case it is labeled as segment 3.
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The given heating curve represents a substance in phases solid, liquid, and gas. Which segment represents the substance as it is boiling?
The given heating curve represents a substance in phases solid, liquid, and gas. Which segment represents the substance as it is boiling?
Boiling is a phase change from liquids to gas. Therefore we are looking for a segment that is flat (because the potential energy is increasing) and that is between the liquid and gas phases.
In this case, gas phase is the highest energy phase, and liquids is the next highest. Therefore the substance is boiling during segment 4.
Boiling is a phase change from liquids to gas. Therefore we are looking for a segment that is flat (because the potential energy is increasing) and that is between the liquid and gas phases.
In this case, gas phase is the highest energy phase, and liquids is the next highest. Therefore the substance is boiling during segment 4.
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In the heating curve shown above, at what point do the molecules have the highest kinetic energy?
In the heating curve shown above, at what point do the molecules have the highest kinetic energy?
Remember, temperature is a measure of the average kinetic energy. Therefore the kinetic energy will be the highest when the temperature is the highest.
Remember, temperature is a measure of the average kinetic energy. Therefore the kinetic energy will be the highest when the temperature is the highest.
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In the given heating curve, when is the potential energy the highest?
In the given heating curve, when is the potential energy the highest?
The potential energy increases as the substance goes through phase changes. Therefore the potential energy is highest at the last possible instant of the most energy intensive phase change. In this case it is at the end of segment 4.
The potential energy increases as the substance goes through phase changes. Therefore the potential energy is highest at the last possible instant of the most energy intensive phase change. In this case it is at the end of segment 4.
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The given heating curve represents a substance in phases solid, liquid, and gas. What is the melting point of the substance?
The given heating curve represents a substance in phases solid, liquid, and gas. What is the melting point of the substance?
Because phase changes happen when the kinetic energy (temperature) remains constant melting will occur during a flat portion of the heating curve.
In this case it is the first flat portion because solid is the lowest energy phase represented in this diagram. Therefore the melting point is at 
.
Because phase changes happen when the kinetic energy (temperature) remains constant melting will occur during a flat portion of the heating curve.
In this case it is the first flat portion because solid is the lowest energy phase represented in this diagram. Therefore the melting point is at
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Suppose that a block of ice is exactly at its melting temperature, 
. If heat is added evenly to the block, which of the following does not occur?
Suppose that a block of ice is exactly at its melting temperature,
This question is presenting us with a scenario in which heat is being added to a block of ice at its melting temperature. It's asking us to identify a false statement.
Recall that when anything goes through a phase change, the temperature remains the same until the phase change is complete. Furthermore, since temperature is a measure of the average kinetic energy of a system, this will not change either. Moreover, as energy is added to a system, the bonds between the components of that system will gradually weaken and then break as the entropy of that system increases.
This question is presenting us with a scenario in which heat is being added to a block of ice at its melting temperature. It's asking us to identify a false statement.
Recall that when anything goes through a phase change, the temperature remains the same until the phase change is complete. Furthermore, since temperature is a measure of the average kinetic energy of a system, this will not change either. Moreover, as energy is added to a system, the bonds between the components of that system will gradually weaken and then break as the entropy of that system increases.
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A solution of water is at 0.006atm and 0.01 degrees Celsius. What phase(s) are present in the sample?
A solution of water is at 0.006atm and 0.01 degrees Celsius. What phase(s) are present in the sample?
The point detailed in the question is the triple point of water on the phase diagram. At the triple point all three phases of a chemical coexist; as such the correct answer is solid, liquid, and gas.
The point detailed in the question is the triple point of water on the phase diagram. At the triple point all three phases of a chemical coexist; as such the correct answer is solid, liquid, and gas.
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Liquid water at room temperature is cooled to -75O C
Which of the following descriptions applies to the process outlined above?
I: A physical change occurred
II: A chemical change occurred
III: There was a change in entropy
Liquid water at room temperature is cooled to -75O C
Which of the following descriptions applies to the process outlined above?
I: A physical change occurred
II: A chemical change occurred
III: There was a change in entropy
A physical change is a process where the physical properties of a substance are changed. Here water undergoes a phase change from the more disordered liquid state to the more ordered solid state, which is indeed a physical change. A chemical change describes when a substance undergoes a change in the identity of its constituent molecules. Here although the water has gone from the liquid phase to the solid phase it is still composed only of water molecules so a chemical change has not occured. Entropy describes the degree to which a system is ordered. Here we do have a change in entropy becasue the liquid water was more disordered than the solid water.
A physical change is a process where the physical properties of a substance are changed. Here water undergoes a phase change from the more disordered liquid state to the more ordered solid state, which is indeed a physical change. A chemical change describes when a substance undergoes a change in the identity of its constituent molecules. Here although the water has gone from the liquid phase to the solid phase it is still composed only of water molecules so a chemical change has not occured. Entropy describes the degree to which a system is ordered. Here we do have a change in entropy becasue the liquid water was more disordered than the solid water.
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A large number of molecules begin moving very fast, rarely bumping into one another, and taking up the entire space available to them. As the temperature drops and the kinetic energy of the particles decreases, the particles move more slowly and run into each other more often. They eventually form a lattice structure, only slightly moving. What is this process called?
A large number of molecules begin moving very fast, rarely bumping into one another, and taking up the entire space available to them. As the temperature drops and the kinetic energy of the particles decreases, the particles move more slowly and run into each other more often. They eventually form a lattice structure, only slightly moving. What is this process called?
This is a description of condensation from the gas phase to the liquid phase, then freezing from the liquid phase to the solid phase.
This is a description of condensation from the gas phase to the liquid phase, then freezing from the liquid phase to the solid phase.
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A phase diagram is used to show the phase that a compound will be in depending upon the environmental temperature and pressure.
Carbon dioxide will sublimate from a solid to a gas at room temperature. What would be necessary in order to cause solid carbon dioxide to melt?
A phase diagram is used to show the phase that a compound will be in depending upon the environmental temperature and pressure.
Carbon dioxide will sublimate from a solid to a gas at room temperature. What would be necessary in order to cause solid carbon dioxide to melt?
At a pressure of one atmosphere, carbon dioxide can only exist in the solid phase or the gaseous phase. Solid carbon dioxide will sublimate at standard conditions.
Raising the temperature will simply make the carbon dioxide sublimate at a faster rate. In order to allow the solid carbon dioxide to melt into the liquid phase, the carbon dioxide must be subjected to a greater amount of pressure. This will shift equilibrium away from the gaseous state and toward the liquid state.
At a pressure of one atmosphere, carbon dioxide can only exist in the solid phase or the gaseous phase. Solid carbon dioxide will sublimate at standard conditions.
Raising the temperature will simply make the carbon dioxide sublimate at a faster rate. In order to allow the solid carbon dioxide to melt into the liquid phase, the carbon dioxide must be subjected to a greater amount of pressure. This will shift equilibrium away from the gaseous state and toward the liquid state.
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