Characteristics of Gases

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Characteristics of Gases

• Vapor substance ordinarily a liquid or solid
  but in its gaseous phase
• Gas expands to fill its container
• Highly compressible when pressure is applied
  liquids and solids not
• Form homogeneous mixtures

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Gas Particles

• Properties of gases are a result of the fact that
  the particles are very far apart
• In air molecules make up only .1 of the volume
• Distance between the particles is 10x as great as
  the size of the molecules themselves
• In liquid 70 of volume is molecules
• Attractive force can exist only in liquids and
  solids

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Pressure

• Force acting over an area
• P F/ A
• Unit - N/ m2 Pascal (Pa)

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Barometer

• Instrument used to measure pressure

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Standard Atmospheric Pressure

• 760 mm Hg 760 torr 101.325 kPa
• Another unit of pressure
• The atmosphere (atm)
• 1 atm 760 torr

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Convert

• .650 atm ____________mmHg
• 3.5 x 10-4 mm Hg ___________atm
• 735 mm Hg _____________kPa

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Pressure and the number of molecules are directly
related

• More molecules means more collisions.
• Fewer molecules means fewer collisions.
• Gases naturally move from areas of high pressure
  to low pressure because there is empty space to
  move in.

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• If you double the number of molecules

1 atm
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• If you double the number of molecules
• You double the pressure.

2 atm
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• As you remove molecules from a container

4 atm
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• As you remove molecules from a container the
  pressure decreases

2 atm
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• As you remove molecules from a container the
  pressure decreases
• Until the pressure inside equals the pressure
  outside
• Molecules naturally move from high to low pressure

1 atm
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Changing the size of the container

• In a smaller container molecules have less room
  to move.
• Hit the sides of the container more often.
• As volume decreases pressure increases.

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• As the pressure on a gas increases

1 atm
4 Liters
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• As the pressure on a gas increases the volume
  decreases
• Pressure and volume are inversely related

2 atm
2 Liters
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Temperature

• Raising the temperature of a gas increases the
  pressure if the volume is held constant.
• The molecules hit the walls harder.
• The only way to increase the temperature at
  constant pressure is to increase the volume.

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300 K

• If you start with 1 liter of gas at 1 atm
  pressure and 300 K
• and heat it to 600 K one of 2 things happens

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600 K
300 K

• Either the volume will increase to 2 liters at 1
  atm

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600 K
300 K

• Or the pressure will increase to 2 atm.
• Or someplace in between