Topic 6 – “Airs” and The Chemical Revolution

Topic 6 – “Airs” and The Chemical Revolution

Today: Some properties of air; overcoming the “horror vacui”

Dr. George Lapennas

Dept. of Biology

Studies of “air” and “airs” lead 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

Aristotle’s 4 earthly elements

Aristotle’s 4 earthly elements – natural motions were up or down to their natural places, where they came to rest

Aristotle’s 4 earthly elements:

- element “fire” is absolutely light

- element “air” is relatively light

- element “water” is relatively heavy

- element “earth” is absolutely heavy

The “horror vacui”

For Aristotle (and for most others until mid 1600’s, including Descartes):

- Space is defined by the matter that occupies it

-“Empty space” is a logical impossibility (“void”; “vacuum”); matter is everywhere

- Nature “abhors a vacuum” (the “horror vacui”) and will do what is necessary to prevent formation of a vacuum

(Contrast: Democritus’ concept of “atoms” moving in the “void”)

Examples of the power of the horror vacui

1. Draw water up a tube (soda straw; water pump; syringe)

Water pump



2. Water does not drain from a vessel unless air can enter to replace it




3. Water “siphon” through a tube

Siphon




3. Water “siphon” through a tube

4. Glass bottle breaks when water in it freezes (water presumed to shrink upon freezing; nature crushes the bottle to prevent formation of a vacuum)

Apparent limitations to the power of the horror vacui

1. Water pumps cannot lift water more than 34 feet

2. Water siphon cannot carry water over a hill more than 34 feet high

3. Behavior of water in a tall, inverted, closed tube

Gasparo Berti (1600-1643)

• water filled tube

• level of water inside tube stayed at 34 ft

• space left above water in tube

• Suggests vacuum existing in the space above the water.

Water height is the same whatever the length of the tube

Wouldn’t nature more strongly abhor a larger void?

Torricelli used mercury instead of water

http://www.zyra.org.uk/mercury3.htm

Torricelli used mercury instead of water:

same pattern, except …

… height of mercury columns were only 2½ feet, or

1/13.6 the height of water column

… 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 were the same.

Does mercury abhor a vacuum less strongly than water does, and it is merely a coincidence that its abhorrence is less by the same factor that its weight per unit volume is greater?

… or is there an underlying explanation?

Torricelli’s alternate hypothesis to the horror vacui:

Perhaps something pushes the water or mercury up the tubes, and could push up the same weight of both liquids?

Torricelli’s hypothesis: Perhaps the weight of the air (atmosphere) is doing the pushing. (Galileo had already weighed air ~ 1/800 as heavy as same volume of water.)

Pascal’s prediction: If so, then there should be less push as one moves up through the atmosphere, because there would be less air above the observer.

Blaise Pascal

In 1648, Pascal sent his brother-in-law Florence Périer up 3000-foot Mt. Puy-de-Dome with bowls, tubes and mercury

The mercury rose to 2 ½ feet … minus 3 inches!

These results proved that Torricelli’s hypothesis was true.



NOT!



NOT!These results supported Torricelli’s hypothesis.

Today we use the “barometer” to measure changes in atmospheric pressure to help predict weather changes.

Weather station atop Mt. Puy-de-Dome

Another test of weight of air hypothesis:

Predict that if a barometer is placed in a chamber and the air pumped out, then the mercury column will not be as high.

von Guericke’s original air pump

Boyle’s/Hooke’s improved pump of 1660

When Boyle and Hooke pumped air out of a chamber containing a barometer, the mercury dropped lower and lower – down to a small fraction of an inch.

This result lent further support to the hypothesis that water and mercury columns were pushed up to their heights by the weight of air, rather than climbing up in attempts to eliminate the vacuum.

von Guericke’s Magdeberg Hemispheres demonstration - 1657

Boyle’s experiments on the “spring” of air

Air resists compression like a spring does.



Explanation?



Explanation?

Boyle: Air consists of tiny particles that are like springs, pressing against each other, and resisting compression.



Explanation?

Boyle: Particles of air 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.



Explanation?

Boyle: Particles of air 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.

Both of these hypotheses ultimately proved incorrect. (Air pressure results from the force of air molecules colliding with surfaces and bouncing off them – exerting force on the surfaces that are equal and opposite to the forces the surfaces are exerting on them.)

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 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.

… but those are stories for other days.

Topic 6 – “Airs” and The Chemical Revolution

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