How Temperature Affects A Phase Shift Experiment
A critical point happens when a substance has a high enough temperature and pressure that both liquid and gas can exist at the same time. This state is called a critical point. Now, when a state of matter changes from one state to another is called a phase shift. Phase shifts are extremely important and most commonly observe phase shifts daily. For an example, when water freezes into ice, or the water in the shower turns to steam. To begin to start describing how a phase shift works we must understand factors that influence a phase transition. Variables that effect a phase shift are volume, pressure, temperature, and number of moles or molecules present in a system. For simplicity when analyzing a system it is easiest to assume that the number
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of moles or molecules remains constant, meaning that the there is no change in mass.
Before moving forward it is important to look at the basic characteristics of liquids, gases, and solids. Liquid is a state of matter that consists of free moving particles which can form into the shape of whatever container it is placed in. One important aspect about liquids is that liquid is not compressed easily. Gases can also can form into the shape of the container they are placed into. One important aspect about gases are that they can be compressed semi easily. This means that there is quite a bit of space between each molecule in a gas, and when pressure is applied to these molecules the move closer together. The last state are solids. Solids retain a fixed shaped, unlike liquids and gases, solids tend to consist of rigid particles that do not move around easily.
Now we can start describing temperature, volume and pressure, and how they contribute to phase shift.
To do so we need to take a closer look at each of these variables separately. The first variable I will discuss is temperature. Everyone on earth has accoutered temperatures before. When we walk outside sometimes it is hot or cold out. But what is going on the molecular level that makes it feel hot or cold out? The temperature outside on a hot day can be thought as the speed or velocity of the air molecules. On a hot day the air molecules have a higher velocity then they do on a cold day. This velocity can be considered as the molecules translating, rotating, and vibrating in a container. If the molecules are moving around quickly then they have a large sum of kinetic energy which means that they are warm appear warm. But if energy is removed from the molecules start moving slower and if all energy is removed from the molecules then the system that the molecules exist in will have zero energy. When matter has zero kinetic energy means that the matter has a temperature of absolute zero, or zero kelvin. It is impossible for anything to be colder then absolute zero because molecules cannot have negative speed or velocity. To make an assessment of the kinetic energy of a system is to find the average kinetic energy of a
system. A useful way to describe the average kinetic energy is by using the equipartition theorem. The equipartition theorem states “molecules in thermal equilibrium have the same average energy associated with each independent degree of freedom of their motion” [1]. This means that the average kinetic energy is dependent on temperature and the transitional degrees of freedom of the molecule. To make an approximation of the average kinetic energy it is assumed the velocity is in one dimension.
Where f is the tranistional degree of freedom, k is the Boltzmann’s constant, and R is the gas constant. For the average kinetic energy monatomic molecule have 1 degrees of freedom, and a diatomic molecule has 3 degrees of freedom. The degrees of freedom are represented mathematically as an f.
The next variable that needs to be considered is pressure. Pressure is defined as the force per a unit of area. There are two ways that pressure should be though as. First what if a pressure is enormously large, and the second is what if a pressure is close or equal to zero. If we think back to characteristics of liquids, solids, and gases we will remember that solids and gas are almost impossible to compress. But gases are respectively easier to compress then a solid or liquid is. Therefore it is more common to see pressure influencing the phase shift from gas to a liquid or a solid
Now that we know the basics of liquids, solids, and gases, we can start describing phase shifts and critical points. One way that phase shifts and critical points can be illustrated is through diagrams. Let’s say we have a container of water at liquid state. Next let the volume of the container remain constant. If the volume is constant then there are two variables that can change. The variables that can change are pressure and the temperature. Now how can we describe the relationship between pressure and temperature? This relationship can be described with by using Gibbs Free Energy
Gibbs Free Energy gives us the equation, dG = - S dT + V dP, assuming that the number of molecules or moles does not change. 1 is showing a pressure-temperature diagram, also called a P-T diagram. Assuming that the volume of the system does not change, then we can make conclusions about the state of water. For an example, let’s say that the temperature is 100 degrees Celsius, and the pressure is 1 atmosphere. From this information we can conclude that the state of H2O is in the water phase. But let’s say that the pressure of the system is slightly less than 1 atmosphere and the temperature remains at 100 degrees Celsius, then the H2O will turn to water vapor. This is why water boils at temperatures lower than 100C at high altitudes (recall you derived a pressure vs altitude relationship earlier this semester
Unfortunately P-T diagrams are not the same for different materials. For an example, water has a different freezing point compared to salt water. Different characteristics of materials can cause them to react differently to temperature, pressure, and volume. An example of this is liquid helium. Liquid helium can never become a solid. It can only exist in a gaseous or liquid state.
of moles or molecules remains constant, meaning that the there is no change in mass.
Before moving forward it is important to look at the basic characteristics of liquids, gases, and solids. Liquid is a state of matter that consists of free moving particles which can form into the shape of whatever container it is placed in. One important aspect about liquids is that liquid is not compressed easily. Gases can also can form into the shape of the container they are placed into. One important aspect about gases are that they can be compressed semi easily. This means that there is quite a bit of space between each molecule in a gas, and when pressure is applied to these molecules the move closer together. The last state are solids. Solids retain a fixed shaped, unlike liquids and gases, solids tend to consist of rigid particles that do not move around easily.
Now we can start describing temperature, volume and pressure, and how they contribute to phase shift.
To do so we need to take a closer look at each of these variables separately. The first variable I will discuss is temperature. Everyone on earth has accoutered temperatures before. When we walk outside sometimes it is hot or cold out. But what is going on the molecular level that makes it feel hot or cold out? The temperature outside on a hot day can be thought as the speed or velocity of the air molecules. On a hot day the air molecules have a higher velocity then they do on a cold day. This velocity can be considered as the molecules translating, rotating, and vibrating in a container. If the molecules are moving around quickly then they have a large sum of kinetic energy which means that they are warm appear warm. But if energy is removed from the molecules start moving slower and if all energy is removed from the molecules then the system that the molecules exist in will have zero energy. When matter has zero kinetic energy means that the matter has a temperature of absolute zero, or zero kelvin. It is impossible for anything to be colder then absolute zero because molecules cannot have negative speed or velocity. To make an assessment of the kinetic energy of a system is to find the average kinetic energy of a
system. A useful way to describe the average kinetic energy is by using the equipartition theorem. The equipartition theorem states “molecules in thermal equilibrium have the same average energy associated with each independent degree of freedom of their motion” [1]. This means that the average kinetic energy is dependent on temperature and the transitional degrees of freedom of the molecule. To make an approximation of the average kinetic energy it is assumed the velocity is in one dimension.
Where f is the tranistional degree of freedom, k is the Boltzmann’s constant, and R is the gas constant. For the average kinetic energy monatomic molecule have 1 degrees of freedom, and a diatomic molecule has 3 degrees of freedom. The degrees of freedom are represented mathematically as an f.
The next variable that needs to be considered is pressure. Pressure is defined as the force per a unit of area. There are two ways that pressure should be though as. First what if a pressure is enormously large, and the second is what if a pressure is close or equal to zero. If we think back to characteristics of liquids, solids, and gases we will remember that solids and gas are almost impossible to compress. But gases are respectively easier to compress then a solid or liquid is. Therefore it is more common to see pressure influencing the phase shift from gas to a liquid or a solid
Now that we know the basics of liquids, solids, and gases, we can start describing phase shifts and critical points. One way that phase shifts and critical points can be illustrated is through diagrams. Let’s say we have a container of water at liquid state. Next let the volume of the container remain constant. If the volume is constant then there are two variables that can change. The variables that can change are pressure and the temperature. Now how can we describe the relationship between pressure and temperature? This relationship can be described with by using Gibbs Free Energy
Gibbs Free Energy gives us the equation, dG = - S dT + V dP, assuming that the number of molecules or moles does not change. 1 is showing a pressure-temperature diagram, also called a P-T diagram. Assuming that the volume of the system does not change, then we can make conclusions about the state of water. For an example, let’s say that the temperature is 100 degrees Celsius, and the pressure is 1 atmosphere. From this information we can conclude that the state of H2O is in the water phase. But let’s say that the pressure of the system is slightly less than 1 atmosphere and the temperature remains at 100 degrees Celsius, then the H2O will turn to water vapor. This is why water boils at temperatures lower than 100C at high altitudes (recall you derived a pressure vs altitude relationship earlier this semester
Unfortunately P-T diagrams are not the same for different materials. For an example, water has a different freezing point compared to salt water. Different characteristics of materials can cause them to react differently to temperature, pressure, and volume. An example of this is liquid helium. Liquid helium can never become a solid. It can only exist in a gaseous or liquid state.