1. 10.1 Characteristics of gases
Gas is one of the three states of matter. It is characterized by it's low density. The other states of matter are liquid and solid.
Only a few elements exist as gases under ordinary conditions:
- The noble gases: He, Ne, Ar, Kr and Xe (monoatomic)
- H2, N2, O2, F2 and Cl2 (diatomic gases)
Gas differ significantly from solids and liquids:
A gas expands spontaneously to fill its container
The volume of a gas equals the volume of the container in which it is held
Gases also are highly compressible

3. Pressure
Pressure is a force exerted by the substance per unit area on another substance.
Gases exert a pressure on any surface with which they are in contact
Force
Area
Pressure = F A
P =

4. Atmospheric pressure and the barometer
Atmospheric pressure is the pressure exerted by earth’s atmosphere
Value of atmospheric pressure depends on location, temperature and weather condition
A barometer measures atmospheric pressure by balance the weight of mercury in a glass tube against the weight of air in the atmosphere.
76 cm
Standard atmospheric pressure (1 atm) is equal to the pressure that supports a column of mercury exactly 760 mm (or 76 cm) height at 0C at sea level.
1 atm = 760 mmHg = 760 torr
= 76 cmHg
= Pa
= kPa

5. Example: The atmospheric pressure at a certain location is 732 mmHg
Example: The atmospheric pressure at a certain location is 732 mmHg. Convert the pressure in atm and Pa?
Pressure (in atm)= 732 mmHg 
1 atm
760 mmHg
= atm
Pressure (in Pa)= 732 mmHg  Pa
760 mmHg
= Pa
= kPa

6. A manometer is used to measure the pressure of an enclosed gas.
Their operation is similar to the barometer, and they usually contain mercury
A mercury manometer, used to measure gas pressures near atmospheric pressure.

7. Example: The level of mercury in an open-end arm of the manometer has a height of mm, and the mercury in the arm that is in contact with the gas has a height of mm. What is the pressure of the gas (a) in atmospheres, (b) in kPa
[atmospheric pressure is torr]
Pressure of gas, Pgas = Patm + h
(a)
= torr + (136.4 torr – torr)
= torr
(797.3 torr) 
1 atm
760 torr
Pgas =
= atm
(b)
P in kPa
=(1.049 atm)
kPa
1 atm
= kPa