Topic 6: Airs and The Chemical Revolution

1
Topic 6Airs and The Chemical Revolution
2
Studies of air and airs led to revolution in
the understanding of the nature of elements and
matter
The weight and spring of air
Different kinds of air (airs)
Redefinition of elements
Atoms and their properties
3
Aristotles 4 earthly elements

Ancient Greeks and medieval Europeans ? air one
of four elements from which all things were made
4
Aristotles 4 earthly elements
element fire is absolutely light
element air is relatively light
element water is relatively heavy
element earth is absolutely heavy
5
Does Air have weight?

• Traditionally thought air did not have weight -
  miles of air above us don't weigh down on the air
  at our level.

6
Evidence for Weight of Air
Aristotle a filled bladder weighs more -
Implication the air must have been under pressure
Galileo compressed air in glass bottles -
Concluded air is 460 times less dense than
water (Actually, its about 800 times less dense)

• Miller weighed balloons empty and full

7
The Miller Balloon Experiment
Used cotton fibers to absorb moisture in breath.
Filled five balloons to roughly the same
pressure.
8
The horror vacui
For Aristotle (and for most others until mid
1600s, including Descartes)
- Space defined by matter that occupies it
- Empty space (void vacuum) a logical
impossibility matter is everywhere
- Nature abhors a vacuum (the horror vacui) -
will do whatever necessary to prevent formation
of vacuum
(Contrast Democritus concept of atoms moving
in the void)
9
Examples of the power of the horror vacui
Draw water up a tube
water pump
straw
syringe
10
Examples of the power of the horror vacui
Water does not drain from a vessel unless air can
enter to replace it
11
Examples of the power of the horror vacui
Water siphon through a tube
12
(No Transcript)
13
Examples of the power of the horror vacui
Glass bottle breaks when water in it freezes
WHY?
water shrinks upon freezing
nature crushes the bottle to prevent formation
of a vacuum)
14
Apparent limitations to the power of the horror
vacui
Water pumps cannot lift water more than 35 feet
Water siphon cannot carry water over a hill
more than 35 feet high
Behavior of water in a tall, inverted, closed
tube
15
Gasparo Berti (1600-1643)
water filled tube
level of water inside tube stayed at 35 ft
exact height observed to limit a siphon
space left above water in tube
Suggests vacuum existing in the space above the
water.
16
Galileos Reasoning
power of a vacuum held the water up to a
certain height
force of the vacuum could only hold so much
water, like a rope that can only withstand so
much weight hanging from it
Therefore pump or Bertis tube can only lift
water 35 ft.
17
Aristotelian explanations
(1) water gave off 'spirits' (a sort of tiny
vapor) which filled the space and drove the water
down.

• (2) space filled with aether, which flowed
  through tiny pores in glass tube to fill space
  left by receding water.

18
Torricelli questioned assumption
? proposed air had weight
weight of the air (not attractive force of
vacuum) held (pushed) up column of water.

• water level (35 feet) reflects force of air's
  weight pushing on water in the basin limiting how
  much water can fall from the tube

19
If weight of the air the cause, then column of
liquid substance heavier than water should be
lower

• Early in 1644, Vincenzio Viviana carried out
  Torricelli's experiment using mercury (which was
  known to be 14 times heavier than water)

20
Vincenzio Viviana carried out Torricelli's
experiment using mercury instead of water same
pattern, except
21
height of mercury columns were only 2½ feet
or
1/13.6 the height of water column
Note Mercury is 13.6 times as heavy as the same
volume of water, so the weights of the mercury
and water columns the same.
Does mercury abhor a vacuum less strongly than
water?
And the 13.6 factor a coincidence?
or is there an underlying explanation?
22
Torricellis alternate hypothesis to the horror
vacui seems plausible
Perhaps something pushes the water or mercury up
the tubes, and could push up the same weight of
both liquids?
23

• The mystery of the suction pump was solved - air
  had only enough weight to raise water to a height
  of 35 ft

Torricelli's ideas that the weight of the air
from the atmosphere caused the liquid to stop
falling seemed to be confirmed.
24
Torricelli denied attractive force of vacuum
held up liquid, still believed the space above
the liquid was a vacuum.

• Aristotelian philosophers suggested mercury gave
  off 'spirits' in the same way as water

• vapors of mercury were stronger than the vapors
  of water, and so pushed the column of liquid down
  further.

25
Test of Aristotelian proposition - push of
vapors

• compared more spirituous wine with water

• Aristotelian expectation

wine column lower (more spirituous wine
more push down on column within the tube)
Blaise Pascal (16231662)
26

• Results ? column of wine stood higher

mechanical explanation ? wine weighs less
than water column ? wine pushed up further by
same amount of force from air.
27
Further Studies of barometer
Descartes predicted column of mercury would drop
when taken to higher altitudes due to lower air
pressure.
28
Further Studies of barometer

• hypothesis weight of the air (atmosphere) is
  doing the pushing
• (Galileo had already weighed air 1/460 as heavy
  as same volume of water)

prediction less push as one moves up through the
atmosphere, because less air above the observer
prediction level of liquid in column will be
lower
29
In 1648, Pascal sent his brother-in-law Florence
Périer up 3000-foot Mt. Puy-de-Dome with bowls,
tubes and mercury
30
The mercury rose to 2 ½ feet minus 3 inches!
These results supported Torricellis hypothesis.
31
Puy de Dome experiment
Aristotelian explanations assumed something
other than the weight of the air to be cause of
the barometric observations
could not account for why height would vary
according to altitude.

• Aristotelians lost the explanatory battle to
  account for the observations The mechanical
  philosophy had won out through persistent
  quantitative experiments

32
Boyles/Hookes improved pump of 1660
von Guerickes original air pump
33
Another test of weight of air hypothesis
Predict if barometer placed in chamber and air
pumped out
mercury column will be lower
34
Barometer in vacuum chamber
Boyle and Hooke pumped air out chamber
containing barometer, the mercury dropped lower
and lower down to a small fraction of an inch
result supports hypothesis water and mercury
columns pushed up by weight of air, rather than
climbing up in attempts to eliminate vacuum
35
Robert Boyle (1627 1691)
studies of the spring of the air (elasticity)
led to the formation of what became known as
Boyle's law

• a generalization of the inverse relation between
  pressure and volume of a gas

36
Boyles experiments on the spring of air Air
resists compression like a spring does.
37
Boyles experiments on the spring of air
Explanation?
Boyle Air consists of tiny particles that are
like springs, pressing against each other, and
resisting compression.
Newton Air particles repel each other without
contact, with a force that decreases with distance
38
Spring of air explanation
Both hypotheses ultimately shown incorrect.
Air pressure results from the force of air
molecules colliding with surfaces and bouncing
off them
exert force on the surfaces equal and opposite
to forces the surfaces are exerting on them
39
But is Air an Element?
What would persuade you that there are
chemically distinct airs?
What properties could be used to identify airs?
Density?
Color?
Taste?
What materials it comes from?

• How it reacts with other materials?

40
Why Were Gases Difficult to Study?
Hard to keep contained
Escape and mix with the atmosphere
Must be kept under pressure
In bladders? In glass jars ?

• They all look alike!

Colorless, (mostly) odorless
41
Early Reports of Distinct Gases
Non-experimental observations
Unhealthy air Marsh gases, mine damps
Van Helmont (early 1600s)
Produced gases by combustion of various
materials
Produced what we now call carbon dioxide,
carbon monoxide, sulfur dioxide, chlorine gas

• Called them wild spirits or gas
• From Greek word for chaos

42
Robert Boyles Factitious Air

• Produced airs by fermentation of organic material

Cherries, rotting beef, wheat paste, etc.

• Called it artificial air or factitious air

• Transferred using evacuated containers

43
Is Factitious Air Distinct From Common Air?
Boyle identified distinct chemical properties
of factitious air
Stopped fermentation (again, in air pump)
Killed animals
Put out candles
Problem studies required use of air pump
Expensive piece of equipment!
44
pneumatic troughStephen Hales
45
Further studies of air lead to
the realization that there are many different
kinds of air (airs), not just one.
and to the identification of elements as we
know them today.
and to the theory that matter consists of
atoms, to the understanding of the structure and
properties of atoms, and why they react the way
they do.