Phase Changes
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Phase Changes : Opener: Up until now, we’ve talked about the three phases of matter. However, from our own personal experience we know that solids can become liquids and that liquids can become gases. How does this work ?. Solid  Liquid transformations:

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Phase changes opener

  • Phase Changes:

    Opener:

  • Up until now, we’ve talked about the three phases of matter. However, from our own personal experience we know that solids can become liquids and that liquids can become gases. How does this work?


Phase changes opener

  • Solid  Liquid transformations:

  • Melting is when a solid becomes a liquid. The reverse of this process is called freezing.

  • Melting and freezing are the reverse of one another and happen at the same temperature.


Phase changes opener

  • Why things melt:

    • Solids melt when the amount of energy that’s available (because we’ve heated them) is greater than the amount of energy that’s holding them together.

      • Covalent compounds melt at low temperatures because the amount of energy that holds the particles together through intermolecular forces is very small.

      • Ionic compounds melt at high temperatures because the lattice energy that holds the ions together is very high.


Phase changes opener

  • Why things freeze:

    • Liquids freeze when enough energy has been taken away from the liquid that the particles are no longer able to stay separate – the intermolecular forces (or lattice energy, in the case of ionic compounds) now force them to combine into a solid.


Phase changes opener

  • Liquid  Gas transformations:

  • In a liquid, the particles move into the gas phase when they get enough energy to break free of the intermolecular forces that hold them together.

  • Evaporation is the process in which only a very few of the molecules in a liquid have enough energy to vaporize.

    • The pressure of the molecules that have become a gas is called the vapor pressure of the liquid.

    • The higher the temperature of the liquid, the higher the vapor pressure (because more of the molecules have gotten enough energy to become a gas)

      • This explains why a hot shower steams up the bathroom while a cold shower doesn’t.


Phase changes opener

  • When the vapor pressure of the liquid becomes equal to the atmospheric pressure, this means that the average energy of all the particles has become greater than the amount of energy needed to enter the vapor phase.

    • Vaporization/boiling: When the molecules in a liquid have gotten enough energy to break free of the intermolecular forces that hold them together.

      Boiling point: The temperature at which this happens.

  • Condensation: When enough energy has been removed from a gas that intermolecular forces again hold them together as a liquid.

    • Condensation and vaporization are the reverse process of one another and happen at the same temperature.


Phase changes opener

  • Solid Gas transformations:

  • Sublimation is when things go directly from the solid phase to the gas phase. This happens with dry ice (when the white gas comes off of the block).

    • This is why ice cubes in the freezer get smaller over time.

    • This is how things are freeze-dried.

  • Deposition is when things go from the gas phase directly to the solid phase.

    • This is why frost sometimes builds up on the sides of plastic bags or on the sides of a freezer.


Phase changes opener

  • Phase diagrams: How we figure out what’s going on

  • As with many things in chemistry, we like to take all of this information and put it into a chart that makes it possible to see what happens at a glance.

  • Phase diagrams: Tables that show you what phase changes occur at different temperatures and pressure.

    • Obviously, phase changes take place when the temperature changes – that’s how we normally boil water and melt ice.

    • However, pressure is also important – remember how things boil if the vapor pressure = the atmospheric pressure? Well, if we decrease the pressure inside a container, things boil at lower temperatures.

      • Pressure changes have a similar effect on other phase changes.


Phase changes opener

  • Important features of phase diagrams:

  • Lines: Along the lines that separate the phases, both phases are equal to stably coexist. That’s why you can put a glass of ice water in the refrigerator and find both the ice and liquid water there after a few days.

  • Normal freezing point: The temperature at which a substance freezes/melts at a pressure of 1 atm.

  • Normal boiling point: The temperature at which a substance boils/condenses at a pressure of 1 atm.


Phase changes opener

  • Triple point: The conditions of pressure and temperature at which all three phases of matter can stably coexist. For water this is 0.006 atm and 0.010 C, which makes it impossible to observe without special equipment.

  • Critical point: The temperature above which water cannot exist as a liquid. Above this temperature water exists as something between a liquid and a gas.


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