CHEM1003 Lecture Notes - Lecture 25: Miscibility, International System Of Units, Torr
Lecture 25
Thursday, 12 May 2016
3:20 pm
Gases (1)
Characteristics of Gases:
• Highly compressible
o When a gas is confined to a container of variable volume an external
force can compress the gas
• Thermally expandable
o When a gas at constant pressure is heated the volume increases
• Low viscosity
o Flow more freely than solids
• Low densities
o Measured in g/L while solids and liquids are in grams/mL
• Very miscible
o Mix with other gases in various portions where solids and solids have
rules of miscibility
Properties of Gases:
• The characteristics of gases arise because molecules of a gas are far apart
• 4 properties used to describe gases
o Pressure, P (units: Pa, atm, mm/Hg)
o Temperature, T (units: oC, K where 0oC = 273.15K)
o Volume, V (units: L)
o Quantity of gas, n (units: moles)
• The pressure of a gas is caused by the force of the gas molecules hitting the
sides of the container
Gases and Gas Pressure:
• Particles, molecules or atoms moving in constant chaotic motion
• Molecules may collide with each other (losing some kinetic energy) or hit walls
• If a gas is a mixture molecules may collide with one another
• Gas pressure - the force per unit area exerted against a surface
• Pressure (P) = force/area
• NOTE: volume of particles themselves is very small compared to volume
occupied by gas
• SI units = newton/meter2 = 1 pascal (Pa)
• 1 standard atmosphere = 101 325 Pa
• 1 standard atmosphere = 1 atm = 760 mm/Hg = 760 torr
Barometer:
• Device used to measure atmospheric pressure
• Mercury flows out of the tube until the pressure of the column of mercury
standing on the surface of the mercury in the dish is equal to the pressure of
the air on the rest of the surface of the mercury in the dish
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