# Pressure and Density and Changes in State

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## Pressure and Density

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.)

Pressure:

• 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

Solids:

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

Liquids:

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

Gases:

• 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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