What NEET Asks
- Conceptual understanding of V-T relationship at constant pressure.
- Direct calculations involving changes in volume or temperature.
- Graphical representation of Charles' Law.
- Often combined with other gas laws or ideal gas equation.
Key Points
- Charles' Law: States that at constant pressure and number of moles, the volume of a given mass of gas is directly proportional to its absolute temperature.
- Mathematical Form: V ∝ T or V/T = constant.
- Combined Form: V₁/T₁ = V₂/T₂.
- Temperature Unit: Always use Kelvin (K). T(K) = t(°C) + 273.15 (or 273 for NEET calculations).
- Graphical Representation: A plot of V vs T(K) is a straight line passing through the origin. A plot of V vs t(°C) is a straight line extrapolating to -273.15°C (absolute zero) on the temperature axis.
- Absolute Zero: The theoretical temperature (-273.15°C or 0 K) at which an ideal gas would have zero volume.
Must-Know Formula / Reaction
V₁/T₁ = V₂/T₂
- V: Volume of the gas (e.g., L, mL)
- T: Absolute temperature (Kelvin, K)
Common Mistakes
- Students often forget to convert Celsius temperature to Kelvin before applying the formula.
- Don't confuse Charles' Law (V-T relationship) with Boyle's Law (P-V relationship).
- Students sometimes incorrectly assume that a V vs t(°C) graph passes through the origin.
Rapid Revision
Charles' Law: V/T = constant (P, n constant). Temperature must be in Kelvin. V vs T(K) is linear through origin. Absolute zero is -273.15°C. Remember: V₁/T₁ = V₂/T₂.