Behavior of gases
Definition of Gas
A gas is a state of matter that has neither a fixed shape nor a fixed volume. Gas molecules move freely in all directions and occupy the entire space available in a container.
Gases are homogeneous fluid, generally have low density and low viscosity compared to solids and liquids. Examples of gases include oxygen, nitrogen, carbon dioxide and hydrogen.
The behavior of gases depends mainly on pressure, volume, temperature and the quantity of gas present. The relationship among these quantities is explained by different gas laws.
Types of Gases
Gases are mainly divided into two types:
- Ideal Gas
- Real Gas
Ideal Gas
An ideal gas is a hypothetical gas that perfectly follows all gas laws under every condition of temperature and pressure.
In an ideal gas:
- The size of gas molecules is negligible.
- There are no intermolecular forces between molecules.
- Molecules move randomly in all directions.
- Collisions between molecules are perfectly elastic.
Real Gas
Real gases are the gases that actually exist in nature. Real gases do not obey gas laws perfectly under all conditions because intermolecular forces and molecular volume cannot be neglected.
At low pressure and high temperature, real gases behave approximately like ideal gases.
Gas Laws
The behavior of gases is explained by several important gas laws.
Boyle’s Law
Boyle’s law states that at constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure.
Formula:
P1V1 = P2V2
When pressure increases, volume decreases, and when pressure decreases, volume increases.
Charles’s Law
Charles’s law states that at constant pressure, the volume of a gas is directly proportional to its absolute temperature.
As temperature increases, the volume of the gas also increases.
Gay-Lussac’s Law
Gay-Lussac’s law states that at constant volume, the pressure of a gas is directly proportional to its absolute temperature.
Avogadro’s Law
Avogadro’s law states that equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules.
Ideal Gas Equation
The combined relationship among pressure, volume, temperature and number of moles of a gas is represented by the ideal gas equation.
Ideal Gas Equation:
PV = nRT
Where:
- P = Pressure of the gas
- V = Volume of the gas
- n = Number of moles
- R = Universal gas constant
- T = Absolute temperature
Factors Affecting the Behavior of Gases
The following factors affect the behavior of gases:
- Temperature
- Pressure
- Volume
- Quantity of gas
Effect of Temperature
When temperature increases, gas molecules move faster and the volume of the gas generally increases. When temperature decreases, molecular motion becomes slower and the volume decreases.
Effect of Pressure
Increasing pressure forces gas molecules closer together, reducing the volume of the gas.
Effect of Quantity of Gas
An increase in the amount of gas increases pressure if the volume remains constant.
Examples of Gas Behavior
- A balloon expands when heated because the gas inside gains kinetic energy.
- A bicycle pump becomes warm when air is compressed.
- LPG cylinders contain gas stored under high pressure.
- Hot air balloons rise because heated air becomes less dense.
Difference Between Ideal Gas and Real Gas
| Ideal Gas | Real Gas |
|---|---|
| Imaginary gas | Exists in nature |
| No intermolecular forces | Intermolecular forces are present |
| Perfectly follows gas laws | Shows deviation from gas laws |
| Molecular volume is negligible | Molecular volume is significant |
Frequently Asked Questions
What is the behavior of gases?
The behavior of gases refers to how gases respond to changes in pressure, temperature, volume and quantity.
What is an ideal gas?
An ideal gas is a hypothetical gas that perfectly obeys all gas laws.
Why do gases expand when heated?
Gases expand when heated because their molecules gain kinetic energy and move farther apart.
Under what conditions do real gases behave ideally?
Real gases behave most ideally at low pressure and high temperature.