According to Charles' law, the volume of a gas becomes zero at

Analyze the question: Charles' law relates the volume of a fixed mass of gas to its temperature at constant pressure. The question asks at what temperature the volume would theoretically become zero.

Explain the concept: Charles' law states that, at constant pressure, the volume of a fixed mass of gas is directly proportional to its absolute (Kelvin) temperature:

V ∝ T, or V₁/T₁ = V₂/T₂

If you plot volume against temperature (in °C), you get a straight line. Extending (extrapolating) this line backwards, it meets the temperature axis where V = 0. That point is absolute zero.

Solution:

• V is proportional to T (in Kelvin), so V = 0 when T = 0 K.
• Convert to Celsius: °C = K − 273
• 0 K = 0 − 273 = −273°C

Check the other options:

• A. 0°C is 273 K, where the gas still has volume.
• B. −100°C is 173 K, so the volume is still not zero.
• D. −373°C is below absolute zero, which is impossible.

Answer: The volume becomes zero at −273°C, which is option C.

Note: In reality, gases liquefy before reaching this temperature, so the volume never actually becomes zero. The result comes from extrapolation of the ideal gas behaviour.

Exam tip: Whenever you see "volume becomes zero" or "absolute zero" in a Charles' law question, think 0 K = −273°C. Always convert temperatures to Kelvin (add 273) before doing Charles' law calculations.