Chapter 14 – Gases &
Atmosphere
Mr.Yeung
Observe
• Write your own notes on the following
demonstrations:
– Weight of the newspaper
– Getting Eggy back into the beaker
– Collapsing can
• Historical development of pressure
Air pressure
• Atmospheric pressure (or
air pressure) is the weight
of air resting on the earth's
surface.
• In a pile of books the weight
means that pressure is
greatest at the bottom of
the pile.
• Gravity pulls the
atmosphere towards the
ground, so just as in a pile
of books, the pressure is
greater nearer the surface.
• But air pressure can go
against gravity as well
Weight of the newspaper
• The newspaper was much harder to lift than the printer
paper.
• As the ruler lifted the printer paper, air rush in under the
rising paper and thereby quickly allowed the air pressure
to equalize on all sides.
• Essentually, the weight of the air above the paper had no
effect on the difficulty in lifting the paper.
• As the ruler lifted the newspaper, the edges of the
newspaper remained in contact with the desk. Very little
air was allowed to rush in and equalize the pressure on
the bottom side of the newspaper.
• Since there is less air below the paper the pressure is
less as well. Now the weight of all the air above the
paper now becomes more evident.
History of pressure
•
•
•
•
•
•
Galileo (1564)
Torricelli (1608)
Boyle (1627)
Otto von Guericke (1643)
Blaise Pascal (1648)
Charles’
Back in the day…
• During the late 1500's the mining
industry in Europe had become
very important
• In order to reach the valuable
ore, mines needed to be drained.
• Pumping out the water that
would seep into the shafts was a
terribly difficult and expensive
activity.
• The efficiency of these pumps
was moderate and the best of
designs could not "lift" water
more than 34 feet.
• This limit caused them to place
pumps in series or to use
buckets on chains to haul it up.
All of this at great cost in
materials and energy.
Back in the day
• If water could be pumped higher with less
energy and materials, then profits could be
increased and more ore could be
recovered
Why?
• Why do you think that the water cannot be
pumped up over 34ft?
Galileo
• Galileo was the court mathematician for
the Duke of Tuscany during the early
1600s.
• Developed the suction pump for the mines
• But was perplexed as to why there was a
limit to the height water could be raised
Torricelli
• Was Galileo’s assistant
• Took over Galileo’s position after
he died
• Designed an experiment similar to
the suction of the water in the
mines
• But since 34ft is too long, he used
mercury instead
• Mercury is about 14 times heavier
than water
Torricelli
• Torricelli designed an
experiment in which he
would fill a glass tube 1
meter long and sealed at
one end with mercury.
• Placing his finger over
the opening he would
invert the tube in a bowl
of mercury with the
sealed end up and
measure the resulting
height of the column.
• The mercury would fall to
about 24cm height and
an empty space or
vacuum was created in
the top of the tube.
Torricelli
• So he developed the first barometer
(measures atmospheric pressure)
• He observed the level of the vacuum
changed over time due to the air pressure
• The air pressure from the atmosphere is
pushing down on the mercury dish on the
outside preventing the drainage of the
mercury
“Chemistry does not suck”
• We cannot think of pressure due to the
‘suction’
• Instead it is the pressure of the
atmosphere that is PUSHING it out
• In a syringe, when you are getting the
liquid, it is the pressure that is pushing it
in… not SUCKING it in
• THAT IS A BIG MISCONCEPTION
Answer to the question
• Why the water cannot be lifted 34ft is NOT
due to suction of the pump
• But rather the atmospheric pressure can
only apply enough pressure to PUSH the
water up 34ft
• So what do you think will happen if you
were up a mountain…
– The water will go up less than 34ft due to less
pressure
Further developments
• Otto von Guericke (1643)
– Made a pump that created a vacuum so
strong that a team of 16 hourses could not
pull two metal spheres apart.
– Reasoned that the hemispheres were held
together by mechanical force of the
atmospheric pressure rather than vacuum
Blaise Pascal
• Pascal took Torricelli’s ‘barometer’ and
traveled up and down a mountain
• He saw the pressure increased as he went
down the mountain
• He developed the SI Unit of pressure, the
Pascal, which was named after him
Later…
• Boyle’s law - Pressure and volume are
inversely proportional at a fixed
temperature.
– Pressure increases when volume decreases
• P1V1 = P2V2
• Pressure 1
• Volume 1
Pressure 2
Pressure 2
• Charles’ law
– Increase with temperature, increases
pressure.
So…
• That was the story of pressure!
• “We live submerged at the bottom of an
ocean of elementary air, which is known
by incontestable experiments to have
weight". Torricelli
Summary
• Atmospheric pressure
• Historical build up of atmospheric pressure