Gas Pressure, Gas Laws, and the Kinetic Molecular Theory

Gas Pressure, Gas Laws, and the Kinetic Molecular Theory

Overview of the Physical States of Matter

The physical states of matter—gases, liquids, and solids—differ in their properties and behavior. Understanding these differences is essential for studying the behavior of gases and the laws that govern them.

• Distinguishing Gases from Liquids and Solids:

  • Compressibility: Gas volume changes significantly with pressure, while liquid and solid volumes are not greatly affected by pressure.

  • Thermal Expansion: Gas volume changes significantly with temperature; gases expand when heated and shrink when cooled. The volume change is 50 to 100 times greater for gases than for liquids and solids.

  • Fluidity: Gases flow very freely.

  • Density: Gases have relatively low densities compared to liquids and solids.

  • Miscibility: Gases form a solution in any proportions and are freely miscible with each other.

Gas Pressure and Its Measurement

Pressure is a fundamental property of gases, defined as the force exerted per unit area by gas molecules as they collide with the surfaces of their container.

• Definition of Pressure: Pressure is the force exerted per unit area. Mathematically, $P = \frac{F}{A}$, where $P$ is pressure, $F$ is force, and $A$ is area.

• Atmospheric Pressure: The pressure exerted by atmospheric gases on the Earth's surface. Atmospheric pressure decreases with altitude.

• Units of Pressure:

  • Pascal (Pa): $1\ \text{Pa} = 1\ \text{N/m}^2$

  • Atmosphere (atm): $1\ \text{atm} = 101,325\ \text{Pa} = 760\ \text{torr}$

  • Bar: $1\ \text{bar} = 100,000\ \text{Pa}$

  • Millimeter of mercury (mm Hg): $1\ \text{mm Hg} \approx 1\ \text{torr}$

• Measurement of Pressure: Barometers and manometers are used to measure atmospheric and gas pressures, respectively.

• Effect of Atmospheric Pressure: The difference in pressure inside and outside a container can cause deformation, as shown in the example of a can collapsing when a vacuum is applied inside.

Examples and Applications

• Example: When a vacuum is created inside a sealed can, the pressure outside becomes much greater than the pressure inside, causing the can to collapse. This demonstrates the significant force exerted by atmospheric pressure.

Table: Common Units of Pressure

Unit	Definition or Relation to Other Units	Notes
Pascal (Pa)	$1\ \text{Pa} = 1\ \text{N/m}^2$	SI recommended unit
Kilopascal (kPa)	$1\ \text{kPa} = 1000\ \text{Pa}$	Commonly used in chemistry
Atmosphere (atm)	$1\ \text{atm} = 101,325\ \text{Pa} = 760\ \text{torr}$	Standard atmospheric pressure at sea level
Bar	$1\ \text{bar} = 100,000\ \text{Pa}$	Used in meteorology
Millimeter of mercury (mm Hg)	$1\ \text{mm Hg} \approx 1\ \text{torr}$	Traditional unit, used in medicine

Additional info:

• Further topics such as the gas laws, kinetic molecular theory, and deviations from ideal gas behavior are typically covered in subsequent sections of a general chemistry course.