1. What Gas Laws Explain
Gas laws describe how pressure, volume, and temperature of a gas are related. By observing how gases respond to changes, simple mathematical relations were discovered long before kinetic theory existed.
The three main gas laws are Boyle’s law, Charles’ law, and Gay-Lussac’s law.
2. Boyle’s Law (Pressure–Volume Relationship)
Boyle’s law states that at constant temperature, the pressure of a gas is inversely proportional to its volume.
P \propto \dfrac{1}{V} \quad \text{(at constant T)}
Or:
P V = \text{constant}
2.1. Meaning
If you decrease the volume of a gas, the molecules have less space and collide more frequently with the walls, increasing pressure.
2.2. Example
Pressing a bicycle pump reduces the volume of air inside it, so the pressure increases, helping inflate the tyre.
3. Charles’ Law (Volume–Temperature Relationship)
Charles’ law states that at constant pressure, the volume of a gas is directly proportional to its absolute temperature.
V \propto T \quad \text{(at constant P)}
Or:
\dfrac{V}{T} = \text{constant}
3.1. Meaning
When temperature increases, molecules move faster, pushing the walls outward. So volume increases.
3.2. Example
A balloon expands when heated because the air inside expands with rising temperature.
4. Gay-Lussac’s Law (Pressure–Temperature Relationship)
Gay-Lussac’s law states that at constant volume, the pressure of a gas is directly proportional to its absolute temperature.
P \propto T \quad \text{(at constant V)}
Or:
\dfrac{P}{T} = \text{constant}
4.1. Meaning
If the gas is heated while the volume is fixed, the molecules hit the walls more frequently and with more force. This increases pressure.
4.2. Example
A sealed pressure cooker builds up pressure when heated because the volume is fixed and temperature rises.
5. Combined Gas Law
All three gas laws can be combined into one relation:
\dfrac{P V}{T} = \text{constant}
This is useful when none of the variables are held constant.
5.1. Interpretation
Changing temperature, pressure, or volume will affect the other quantities in a predictable way.
6. Graphical Forms of Gas Laws
The gas laws are easier to understand when visualized using graphs.
6.1. Boyle’s Law Graph
Plotting P vs. V gives a downward curve (hyperbola). Plotting P vs. 1/V gives a straight line.
6.2. Charles’ Law Graph
Plotting V vs. T gives a straight line passing through a negative temperature on the Celsius scale, pointing toward absolute zero.
6.3. Gay-Lussac’s Law Graph
Plotting P vs. T gives a straight line that also reaches zero at absolute zero if extended backwards.
7. Everyday Uses of Gas Laws
- Hot air balloons rise because warm air expands (Charles’ law).
- Spray cans explain Boyle’s law—reducing volume increases pressure.
- Tyre pressure increases on long drives due to Gay-Lussac’s law.