Phase Changes Intro
Presentation
•
Chemistry
•
10th - 11th Grade
•
Hard
Joseph Anderson
FREE Resource
17 Slides • 8 Questions
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Phase changes Part 1
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Phenomenon: Why does water appear blue?
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Open Ended
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Phenomenon: Why does water appear blue?
Water appears blue because of the motion of water molecules.
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Multiple Choice
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Multiple Choice
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Multiple Choice
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Kinetic Theory
and Motion
The kinetic theory states that atoms and molecules are constantly in motion.
Kinetic energy is the highest in the gas phase.
How do you think this relates to temperature?
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Temperature
The temperature of any substance is a measurement of that substance's kinetic energy, or molecular movement.
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Multiple Choice
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Vibrations
Vibrations within molecules can absorb energy.
In water, the bonds between the oxygen atom and each hydrogen atom can vibrate at just the right frequency to absorb all colors of light except blue.
Because all the colors except blue have been absorbed by the water molecules, the "leftover" blue light makes water look blue.
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Pressure
Pressure comes from gas molecules colliding with the walls of their container.
As the gas molecules move around, they frequently collide with the walls of their container. The force of gas molecules colliding with their container is called pressure.
Pressure is often measured in the
unit atmosphere (atm). Normal air pressure at sea level is 1 atm.
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Phase Diagrams
Phase diagrams show the phase of a substance at a variety of temperatures and pressures.
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Reading a Phase Diagram
Pressure is shown on the vertical (y-axis) of a phase diagram, and temperature is shown on the horizontal (x-axis).
The different sections on the phase diagram show the temperatures and pressures at which a substance will be in each phase. The lines within a phase diagram indicate where one phase ends and the next begins.
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Multiple Choice
When water is at 25°C and has a pressure of 0.5 atm, what phase is it in?
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Melting and Boiling Points on a Phase Diagram
The lines on a phase diagram indicate the melting and boiling points.
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Finding Melting and Boiling Points
When you need to find a melting point or boiling point of a substance at a given pressure, first draw a line across the phase diagram at that specific place. For example, say you want to find the temperature at which water boils at 1.0 atm.
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Multiple Choice
Would the boiling point of water at 0.5 atm be lower, the same, or higher than the boiling point of water at 1 atm?
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Comparing Phase Diagrams
The phase diagram for every substance is unique.
For example, here are the phase diagrams for carbon dioxide (CO2) and water (H2O).
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Water Is the Exception
Water has an unusual phase diagram.
The melting point lines on the carbon dioxide (CO2) and water (H2O) graphs slope in different directions. As the pressure is increased, CO2 goes from gas to liquid to solid. However, as pressure is increased in solid water, it melts.
Almost all substances have melting point lines that slope like CO2.
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Intermolecular Forces and Melting
Intermolecular forces must be overcome in order for melting to occur.
As you add heat to a solid, the kinetic energy of the molecules increases. Eventually, the molecules have enough kinetic energy to overcome the intermolecular forces. It is at this point that the solid melts into a liquid.
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Intermolecular Forces and Boiling Point
The stronger the intermolecular forces in a liquid, the higher the boiling point.
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Multiple Choice
Phase changes Part 1
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