Gases and the Absolute Scale of Temperature
The behavior of gases can be described by relationships between pressure, volume, and temperature. These are known as the gas laws.
1. Pressure Law (Temperature–Pressure Relationship)
Statement
For a fixed mass of gas at constant volume:
P ∝ T or P₁ / T₁ = P₂ / T₂
Increasing temperature gives gas particles more kinetic energy. They move faster, colliding more frequently and forcefully with the walls of the container, increasing pressure. Lowering temperature reduces molecular motion and hence lowers the pressure.
Summary of Effect (Constant Volume)
- ↑ Temperature → ↑ Molecular speed → ↑ Pressure
- ↓ Temperature → ↓ Molecular speed → ↓ Pressure
2. Boyle’s Law (Pressure–Volume Relationship)
Statement
For a fixed mass of gas at constant temperature:
P ∝ 1/V or P × V = constant
Explanation
Reducing the volume of a gas pushes particles closer together, increasing the rate of collisions with the container walls. This increases pressure. Doubling the volume halves the pressure, and vice versa.
Worked Example
P₁V₁ = P₂V₂
If P₁ = 2 × 10⁵ Pa, V₁ = 0.35 m³, and P₂ = 5 × 10⁵ Pa:
V₂ = (P₁ × V₁) / P₂ = 0.14 m³.
3. The Absolute (Kelvin) Temperature Scale
Temperature is a measure of the average kinetic energy of particles. Raising temperature increases motion; lowering temperature reduces it. At absolute zero (0 K), particle motion ceases completely.
| Scale | Freezing Point of Water | Boiling Point of Water | Conversion |
|---|---|---|---|
| Celsius (°C) | 0 °C | 100 °C | T (K) = T (°C) + 273 |
| Fahrenheit (°F) | 32 °F | 212 °F | |
| Kelvin (K) | 273 K | 373 K |
Exam Tip
Always convert temperatures to Kelvin when applying gas laws. 0 K (−273 °C) is the lowest attainable temperature where particle kinetic energy is minimum.
4. Pressure Explained by Molecular Motion
- Gas pressure is due to collisions of particles with container walls.
- Each collision exerts a force on the wall.
- More particles or higher temperature → more frequent collisions → higher pressure.
- Pressure = Force / Area.
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