Properties of Gases

General Characteristics of Gases

Gases are one of the fundamental states of matter, distinguished by their unique physical properties. Unlike solids and liquids, gases do not have a fixed shape or volume and are highly responsive to changes in pressure and temperature.

• No Definite Volume or Shape: Gases expand to fill the entire volume of their container and assume its shape.

• Compressibility: The volume occupied by a gas changes significantly with pressure, unlike solids and liquids.

• Temperature Dependence: The volume of a gas changes with temperature.

• Miscibility: Gases are easily mixed unless they chemically react with one another.

• Low Density: Gases are typically much less dense than liquids or solids. Gas densities are often expressed in g/L rather than g/mL.

Comparison Table: States of Matter

Characteristic Properties of Gases

Key properties that distinguish gases from other states of matter include:

• Expands to fill its container

• Assumes both the volume and shape of its container

• Readily flows

• Is compressible

• Diffusion within it occurs rapidly

Example: Air in a room will spread out to fill the entire space, regardless of the room's shape or size.

Gas Pressure

Definition and Formula

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

• Formula:

Where: = pressure (Pa, N/m2) = force (N) = area (m2)

• Force is calculated as: (mass × acceleration)

• Units: 1 Pa = 1 N/m2

Example: If the same force is applied over a smaller area, the pressure increases (e.g., pointed heel vs. wide heel).

Calculating Pressure: Real-World Example

Consider a 56 kg person wearing shoes with different heel areas:

• Pointed heel area: 0.45 cm2

• Wide heel area: 16 cm2

Pressure exerted is higher for the pointed heel due to the smaller area.

Comparing Gas Samples by Pressure

Pressure in a gas sample depends on the number of particles and temperature. For samples with the same volume and temperature, more particles mean higher pressure.

• Sample B: Highest pressure (most particles)

• Sample D: Lowest pressure (fewest particles)

• Sample A: Second highest pressure

• Sample C: Second lowest pressure

Atmospheric Pressure

Definition and Origin

Atmospheric pressure is the force exerted by the weight of air in the atmosphere on a unit area at Earth's surface.

• Formula:

• Standard acceleration due to gravity:

Example: Atmospheric pressure decreases with altitude because there is less air above the surface to exert force.

Measuring Pressure: Unit Conversion

Barometers and Pressure Units

Pressure is measured using instruments such as barometers. Evangelista Torricelli invented the mercury barometer and defined the unit Torr.

• 1 mmHg = 1 Torr

Common Pressure Units Table:

Unit Conversion Example: To convert mmHg to bar, use the relationship:

Mercury Barometer

A mercury barometer measures atmospheric pressure by the height of a mercury column. The pressure exerted by the atmosphere supports the column of mercury, and the height is proportional to the atmospheric pressure.

• Standard atmospheric pressure: 760 mmHg (1 atm)

Example: If the barometric pressure is 760 mmHg, the height of the mercury column will be 760 mm.

Additional info: These notes cover the introductory concepts of gases, their properties, pressure, and measurement units, as presented in the first part of Chapter 5 of a General Chemistry textbook.