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Boyle’s Law Fill a cup with water and while under water, turn it upside down and pick it up. This action tries to increase the volume inside the cup. Pressure drops and outside pressure will push in allowing you to pick up water against gravity. Don’t raise the cup over the water line
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Boyle’s lawVolume of a gas is inversely
proportional to the pressure on a gas.V 1/PTherefore VP = k (some constant
for each gas)ViPi = VfPf
i is initial f is final
Boyle’s LawIf you have 145 mL of a gas at 67
kPa. What volume will it occupy at 127 kPa?
VP = VP145 mL (67kPa) = V (127 kPa)V = 76 mL
Boyle’s LawFill a cup with water and while under
water, turn it upside down and pick it up.
This action tries to increase the volume inside the cup.
Pressure drops and outside pressure will push in allowing you to pick up water against gravity.
Don’t raise the cup over the water line
Water in a cup
Water
Cup (with a little bit of airin it)
Gravity wants to pull the water in the cup down.However if it fell it would leave a big space of nothing (vacuum)Air pressure pushes to stop the vacuum from forming by pushing on the surface of the liquid. Not allowing the water level to rise.
The water will fall once the air pressure equals the pressure (weight divided by area) of the water being lifted.
Which wouldmake the water level rise.
BarometerPressure can be measured with a barometer.Which works just like the cup but with
mercury.
Why use mercury?Mercury is dense and will fill a
small tubeAbout 760 mm Hg is needed
under normal conditions.Water would fill a 10 m high tube
to replace mercuryStandard pressure is 760 torr or
29.9 inches Hg
Modern BarometersDigital Barometers use a sensor with a
sealed drum. Top of the drum is flexible.Sealed inside the drum is air at a known
(calibrated) pressure.Higher outside pressure pushes the drum in.Lower outside pressure bows the drum out.
High pressure
low pressuredrum
High pressure
low pressuredrum
High pressure
low pressuredrum
How do substances expand?
Cooling makes substances contract.Especially with gases.Charles’ Law: The volume of a gas is
directly proportional to the temperature of that gas.
V T; V/T = kVi/Ti = Vf/Tf
Charles’ LawIf you have 6.7 L of a gas at 298 K,
what volume will it occupy at 0o C?V/T = V/T6.7 L / (298 K) = V / (273 K)*Temp must be in Kelvin because
zero would make it undefined!V = 6.1 L
Charles’ LawVolume = 0.731 L of a gas at 318 K,
What is the temperature if it occupies 1.34 L?
V/T = V/T0.731 L / (318 K) = 1.34 L / TT = 583 K
Gay-Lussac’s LawThe pressure of a gas is directly
proportional to its temperature.T/P = T/PThis is why an aerosol can or a tire feels
cooler when air is released.It is also how a diesel engine ignites the
fuel. It compresses it until it ignites.
Combined Gas LawThis is made by combining Charles’ and
Boyle’s Law.Vi Pi / Ti = Vf Pf / Tf
Temperature must be in Kelvin (so it can never be 0)
Volume and pressure can be in any unit as long as it is the same on both sides.
A problemIf a gas occupies 34 L at 1.2 atm
and 290 K, what volume will it occupy at 1.1 atm and 280 K?
Vi Pi / Ti = Vf Pf / Tf
34L (1.2atm) / 290 K = V (1.1atm)/280K
V = 36 L
A problemIf a gas occupies 24 mL at 115 kPa
and 13o C, what volume will it occupy at 101 kPa and 0o C?
24 mL (115kPa)/286 K = V (101kPa)/273 K
V = 26 mL (don’t forget to convert temp to K)
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