1. Unit 2: Gases and the Atmosphere
Chemistry 30S
Unit 2: Gases and the Atmosphere

2. Learning Outcomes
C Examine the historical development of the measurement of pressure. Examples: the contributions of Galileo Galilei, Evangelista Torricelli, Otto von Guericke, Blaise Pascal, Christiaan Huygens, John Dalton, Joseph Louis Gay-Lussac, Amadeo Avogadro…
C Describe the various units used to measure pressure. Include: atmospheres (atm), kilopascals (kPa), millimetres of mercury (mmHg), millibars (mb)

3. Defining Gas Pressure Recall:
According to Kinetic-Molecular Theory gas particles are constantly moving and their collisions are completely elastic
As they move they collide with the sides of the container
The force per unit area due to these collisions = GAS PRESSURE

4. History of Measuring Pressure
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Galileo Galilei (1564 – 1642)
Developed the suction pump
He used air to draw underground water up a column, similar to how a syringe draws water
He was perplexed as to why there was a limit to what height the water could be raised
That limit was 32 feet or about 11 metres.

5. History of Measuring Pressure
In 1643, Evangelista Torricelli ( ) developed the first barometer
He carried on Galileo’s work by determining the limit to the height with which Galileo’s pump could draw water was due to atmospheric pressure
He inverted a closed-end tube filled with mercury into a pan of mercury at sea level.

6. History of Measuring Pressure
The height of the column of mercury in the tube (in mmHg) is equal to the atmospheric pressure acting on the mercury in the pan.
He determined that the height of mercury supported by atmospheric pressure at sea level is 760 mm or 76 cm.
He did the same experiment with water first, but found that the glass tube he had to use was too long and fragile
Therefore, Standard Pressure = 760 mm Hg

7. History of Measuring Pressure
Between 1643 and 1645 Otto von Guericke ( ) made a pump that could create a vacuum so strong that a team of sixteen horses could not pull two metal hemispheres apart.
Otto von Guericke reasoned that the hemispheres were held together by the mechanical force of the atmospheric pressure rather than the vacuum.
The same experiment can be demonstrated by pushing two plungers together.

8. History of Measuring Pressure
In 1648, Blaise Pascal ( ) used Torricelli’s “barometer” and traveled up and down a mountain in southern France.
He discovered that the pressure of the atmosphere increased as he moved down the mountain.
Sometime later the SI unit of pressure, the ‘Pascal’, was named after him.
He is also well known for his work in mathematics and inventing the first calculator.

9. History of Measuring Pressure
In 1661 Christiaan Huygens ( ) developed the manometer to study the elastic forces in gases.
He also developed some of the first practical vacuum pumps. Huygens, however, is better known for his work in mathematics and astronomy.

10. History of Measuring Pressure
In 1801, a couple years before publishing his atomic theory, John Dalton ( ) stated that in a mixture of gases the total pressure is equal to the sum of the pressure of each gas, if it were in a container alone.
The pressure exerted by each gas is called its partial pressure.
This is known as Dalton’s Law of Partial Pressures.
Another way of stating this relationship is the total pressure of gases in a container is the sum of the pressures of each gas.
Ptotal = PgasA + PgasB + PgasC…

11. History of Measuring Pressure
In 1808 Joseph Louis Gay-Lussac ( ) observed the law of combining volumes.
He noticed that, for example, two volumes of hydrogen combined with one volume of oxygen to form two volumes of water.
One volume of multiple gases = Double the volume of combined gas
He is also well known for his passion for hot air ballooning.

12. History of Measuring Gas Pressure
Amadeo Avogadro ( ), after studying the work of Gay-Lussac and others, published what is known as Avogadro’s Hypothesis in 1811.
His hypothesis stated that a sample of any gas at the same temperature and pressure will contain the same number of particles.
His hypothesis also suggests that the more gas particles, the greater the pressure.
Unfortunately, because Avogadro did not do his own experiments, his hypothesis was ignored for about 50 years.

13. Units of Pressure KiloPascal (kPa) Millibar Atmosphere (atm)
Newton of pressure per square metre of area
1 Pascal is quite small, so we report in kilopascals
SI unit of pressure
Millibar
Meteorological unit of atmospheric pressure
One bar is equal to standard atmospheric pressure or 1 atmosphere.
Atmosphere (atm)
Derived from standard atmospheric pressure at sea level
1 atmosphere is equal to 760 mmHg, or kPa

14. Units of Pressure Mm of mercury Pounds per Square Inch (psi)
Not a common unit used today outside the laboratory, however, many barometers found in the home use both mm of mercury as well as another unit like kilopascals
Pounds per Square Inch (psi)
An imperial unit of pressure, common in everyday life
E.g. 1 kPa = PSI