Topic 6 – “Airs” and The Chemical Revolution Today: We Live Under a “Sea of Air” Dr. Joel Benington Dept. of Biology

Aristotle’s Four Earthly Elements

Alchemy & the Nature of Matter Based on ancient ideas of matter Ancient  medieval Experimental Developed chemical techniques Solitary & secret Complex, diverse, confused conclusions

Why Study Air? An element (if you believe Aristotle) Homogeneous—no structure But difficult to contain, work with Does it have weight? Does it fill space? Can there be a vacuum? What causes air pressure? Is air an element or are there many “airs”?

Aristotle: air finds its natural place above earth and water, but below fire The atmosphere

Aristotle: air finds its natural place above earth and water, but below fire The atmosphere So is it heavy or light?

Evidence for Weight of Air Aristotle: a filled bladder weighs more –Implication: the air must have been under pressure Benington: weighed balloons empty and full

The Bonas Balloon Experiment Weight EmptyWeight Full 1.82 grams1.90 grams 2.29 grams2.43 grams 2.04 grams2.13 grams Used cotton fibers to absorb moisture in breath. Filled three balloons to roughly the same pressure.

The Bonas Balloon Experiment Weight EmptyWeight Full 28 grains29 ½ grains 35 ½ grains37 ½ grains 31 ½ grains33 grains Used cotton fibers to absorb moisture in breath. Filled three balloons to roughly the same pressure grains per ounce (weight of one barleycorn)

Evidence for Weight of Air Aristotle: a filled bladder weighs more –Implication: the air must have been under pressure Benington: weighed balloons empty and full Galileo: compressed air in glass bottles –Concluded air is 460 times less dense than water –(Actually, it’s about 800 times less dense)

The “Horror Vacui” For Aristotle (and Descartes and others until): Space defined by matter occupying it “Empty space” is a logical impossibility –Matter is everywhere, no “void” or “vacuum” Nature “abhors a vacuum” –The “horror vacui” –Will do what is necessary to prevent it Contrast: Democritus’ “atoms” moving in the “void”)

The Power of the Horror Vacui 1. Draw water up a tube (soda straw; water pump; syringe)

A water pump uses vacuum to pull water up

The Power of the Horror Vacui 1. Draw water up a tube (soda straw; water pump; syringe) 2. Water does not drain from a vessel unless air can enter to replace it

The Power of the Horror Vacui 1. Draw water up a tube (soda straw; water pump; syringe) 2. Water does not drain from a vessel unless air can enter to replace it 3. Water “siphon” through a tube

Siphon

Apparent Limitations to the Power of the Horror Vacui Water pumps cannot lift water more than 34 feet –Discovered pumping water out of mines Water siphon cannot carry water over a hill more than 34 feet high Water forms 34-foot column in closed tube

Really big thumb!

Gasparo Berti ( ) Water filled tube Level of water inside tube stayed at 34 ft Space left above water in tube Vacuum 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

Same pattern, but only 2 ½ feet high

Torricelli used mercury instead of water Same pattern, but only 2 ½ feet high (1/13.6 height of water column… and mercury is 13.6 times as heavy as water)

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. “We live under a sea of air” 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.

The mercury rose to 2 ½ feet … minus 3 inches! These results proved that Torricelli’s hypothesis was true. NOT!

The mercury rose to 2 ½ feet … minus 3 inches! These results proved that Torricelli’s hypothesis was true. 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

As air pumped out…mercury in barometer dropped lower and lower – down to a small fraction of an inch. Supports 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.

Boyle’s experiments on the “spring” of air Air resists compression like a spring does.

Boyle’s experiments on the “spring” of air Air resists compression like a spring does. Explanation?

Boyle’s experiments on the “spring” of air Air resists compression like a spring does. Explanation? Boyle: Air consists of tiny particles that are like springs, pressing against each other, and resisting compression.

Boyle’s experiments on the “spring” of air Air resists compression like a spring does. 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.

Boyle’s experiments on the “spring” of air Air resists compression like a spring does. 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.)

But is Air an Element? What would persuade you that there are chemically distinct airs? What properties could be used to identify airs? Color? Taste? Density? What materials it comes from? How it reacts with other materials?

Gases in the Atmosphere Gases are one form of matter –Molecules more separated than in liquids and solids Composition of atmosphere –78% nitrogen gas (N 2 ) –21% oxygen gas (O 2 ) –Argon (0.9%), carbon dioxide (0.035%), etc.

Other Common Gases Hydrogen gas (H 2 ) Nitric oxide (NO) Water vapor (H 2 O) Helium (He) Methane (CH 4 )

Why Were Gases Difficult to Study? Hard to keep contained –Escape and mix with the atmosphere –Must be kept under pressure In bladders? Glass jars? They all look alike! –Colorless, (mostly) odorless

Early Reports of Distinct Gases Non-experimental observations –“Unhealthy” air –Marsh gases, mine damps –How were these interpreted? 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”

Robert Boyle’s “Factitious Air” Used air pump to create vacuum

Robert Boyle’s “Factitious Air” Used air pump to create vacuum Produced airs by fermentation of organic material –Cherries, rotting beef, wheat paste, etc.

Robert Boyle’s “Factitious Air” Used air pump to create vacuum Produced airs by fermentation of organic material –Cherries, rotting beef, wheat paste, etc. Called it “artificial air” or “factitious air”

Robert Boyle’s “Factitious Air” Used air pump to create vacuum Produced airs by fermentation of organic material –Cherries, rotting beef, wheat paste, etc. Called it “artificial air” or “factitious air” Transferred using evacuated containers

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!