The Nature of Boiling

  1. Boiling occurs when molecules have enough kinetic energy to separate from each other.
  2. Two forces hold molecules and atoms in a condensed state
    1. Pulling forces from the intermolecular attractions.
    2. Pushing forces from the pressure applied on all surfaces of the liquid (from both the container and the gas above the liquid)
  3. Boiling will occur at the point at when the vapor pressure of a substance is equal to the pressure on the substance.
  4. Vapor pressure
    1. It is an inherent property of a substance like density.
    2. Can be measured by sealing a liquid inside a container and measuring the pressure created by the molecules which naturally evaporate at that temperature.
      Here the water has just been added and no vapor has evaporated. Here the water begins to evaporate. The gaseous molecules are beginning to increase the pressure inside the flask. Eventually enough molecules are bouncing around such that the rate of evaporating molecules equals the rate of condensing molecules. This is when you can measure the equilibrium vapor pressure.
    3. An increase in temperature will increase the vapor pressure.
      Vapor Pressure of Water at Various Temperatures
      Temperature (°C)
      Pressure (atm)
      0
      0.0060
      20
      0.0231
      40
      0.0728
      60
      0.1968
      80
      0.4675
      100
      1.0000
      Why?
    4. An increase in intermolecular forces will decrease the vapor pressure. Why?
  5. Below is an interactive graph that displays what happens when you alter the temperature, pressure, and type of substance with varying intermolecular forces. The red line depicts the amount of energy needed by a molecule to break free from the liquid state. See if you can demonstrate two distinctly different circumstances in which you have created the conditions for boiling. Sketch these graphs and describe the boiling environment.

  1. The boiling point occurs when enough molecules have enough energy to break free from their intermolecular forces and the external pressure keeping them together. This can be achieved in several ways.
    1. You can heat the substance so that enough molecules break free.
    2. You can lower the pressure on a substance to make it easier for the molecules to break free with the energy they already posses.
    3. Some combination of either i or ii.

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