ACT Science Pressure and Volume: Apply Boyle's Law to Gas Behavior

Boyle's Law: For a fixed amount of gas at constant temperature, P₁V₁=P₂V₂ (pressure times volume is constant). Example: A balloon at sea level has pressure 1 atm and volume 1 L. At high altitude where pressure is 0.5 atm, the volume becomes 2 L (P₁V₁=1×1=1; P₂V₂=0.5×2=1). Pressure and volume are inversely related: when one increases, the other decreases, and their product stays constant. This relationship holds as long as temperature and amount of gas stay constant. Questions ask you to predict volume or pressure changes. Process: (1) Write the law as P₁V₁=P₂V₂. (2) Identify known values. (3) Solve for the unknown.

Another example: Compress a gas from 10 L at 1 atm to 5 L. New pressure: P₂=P₁V₁/V₂=(1×10)/5=2 atm. The volume halved, so pressure doubled.

Mistake 1: Thinking pressure and volume are directly related (both increase together). They're inverse: higher pressure means lower volume. Mistake 2: Forgetting to keep temperature constant. Boyle's Law only applies at constant temperature. If temperature changes, you need a different gas law. Mistake 3: Forgetting units must match. If P₁ is in atm, P₂ must be in atm (or convert both). Same for volume. Always verify units before applying the law.

During practice, write out the law, identify all known values with units, then solve. This step-by-step approach prevents errors.

Problem 1: Gas at 2 atm, 3 L. Pressure increases to 6 atm. New volume? P₁V₁=P₂V₂: 2×3=6×V₂, V₂=1 L. Problem 2: Gas at 1.5 atm, 4 L. Volume decreases to 2 L. New pressure? 1.5×4=P₂×2, P₂=3 atm. Problem 3: Gas at 10 atm, 1 L. Volume triples. New pressure? 10×1=P₂×3, P₂≈3.33 atm. Problem 4: Gas at 0.5 atm, 8 L. Pressure halves. New volume? 0.5×8=0.25×V₂, V₂=16 L. Problem 5: Gas at 4 atm, 2 L. Pressure quadruples. New volume? 4×2=16×V₂, V₂=0.5 L. Solve all five using P₁V₁=P₂V₂, showing substitution and algebra.

Find five Boyle's Law problems from a practice test. For each, apply the law, verify units, and solve. By the fifth problem, gas law calculations will be automatic.

Boyle's Law questions appear on some ACT Science sections. They test whether you understand inverse relationships. Students who apply Boyle's Law systematically pick up 1 point on the science section because the law is universal and the math is mechanical.

Drill Boyle's Law daily this week. Each day, solve five problems using P₁V₁=P₂V₂. By test day, you should apply the law and solve any pressure-volume problem within 60 seconds.