The volume of a fixed mass of a gas varies inversely as the pressure on it provided the temperature is constant. This statement is

1. Analyze the question The statement says that for a fixed mass of gas at constant temperature, volume is inversely proportional to pressure. We need to name the law that says this.

2. Explain the concept Each gas law holds one quantity constant:

• Boyle's law: temperature constant, so P ∝ 1/V
• Charles' law: pressure constant, so V ∝ T (in kelvin)
• Pressure law: volume constant, so P ∝ T (in kelvin)
• General gas law: combines all three, PV/T = constant

3. Step-by-step solution

• The condition given is "temperature is constant", which matches Boyle's law.
• The relationship is "volume varies inversely as pressure", which means V ∝ 1/P, or PV = constant.
• This can also be written as P₁V₁ = P₂V₂.
• Charles' law and the pressure law involve temperature changing, so they are ruled out.
• The general gas law involves all three variables changing together, so it is also ruled out.

4. Answer The statement is Boyle's law, which is option C.

5. Exam tip Remember the order B-C-P with the constant quantity:

• Boyle → Temperature constant
• Charles → Pressure constant
• Pressure law → Volume constant

If the question says "inversely", think Boyle, because it is the only one of the three where one quantity goes up as the other goes down.