Pressure and Density and Changes in State

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Pressure and Density


  • How heavy something is for its size (how tightly packed the particles are)
  • Density = Mass / Volume
  • The units for density are: g/cm2 or kg/m2
  • (If you do not know the volume of an object, immerse it in water and measure the displacement.)


  • The force that is being applied to something. 
  • Pressure = Force / Area
  • Units of pressure are: Pa (pascals) or N/m2
  • If a force is applied on a large area, the pressure is lower than the pressure of the same force on a smaller area.
  • In gases and liquids the pressure acts equally in all directions and the pressure increases with depth.
  • For liquids and gases: Pressure Difference = Height x Density x g (10)
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Changes of State

Solid --melts-> Liquid --evaporates-> Gas --condenses-> Liquid --solidifies-> Solid


  • Strong forces of attraction between particles with little energy and very small vibrations.
  • Fixed in a regular arrangement.


  • Weaker attractions with more energy and random movements at a low speed.
  • Close but moving particles, arranged irregularly.


  • No attractions between particles.
  • High energy, random movements at high speeds.

Evaporation --> Particles escape from liquid to gas. They have to be moving near to the surface of the liquid at a high speed and in the right direction (enough kinetic energy).

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