ACT Science: Apply Boyle's Law to Predict Pressure-Volume Changes

Boyle's Law: P₁V₁=P₂V₂. When temperature and amount of gas stay constant, pressure and volume are inversely related. If volume decreases, pressure increases. If pressure increases, volume decreases. Example: A gas at 2 atm and 5 L is compressed to 2 L. New pressure: (2 atm)(5 L)=P₂(2 L), so P₂=5 atm. The smaller volume traps the gas particles, increasing pressure. Boyle's Law only applies when temperature is constant (isothermal process).

On ACT Science, passages give you initial conditions (P₁, V₁) and a change (either P₂ or V₂), then ask for the missing variable. Substitute into the equation and solve. It's arithmetic, not conceptual reasoning.

Mistake 1: Forgetting to keep units consistent. If P₁ is in atm but P₂ is in Pa, convert first. Mistake 2: Using Boyle's Law when temperature changes. If temperature isn't constant, use Charles's Law (V/T) or the combined gas law instead. Mistake 3: Assuming larger pressure always means larger volume. That's wrong; they're inversely related. Always check the problem statement: Does it say temperature is constant or does the container have heat transfer?

Verification: After solving, ask yourself: Does the answer make sense? If volume decreased, did pressure increase? If yes, you're likely correct.

Scenario 1: A syringe contains gas at 1 atm and 10 mL. The plunger is pushed, reducing volume to 5 mL. New pressure: (1)(10)=P₂(5), so P₂=2 atm. Scenario 2: A balloon at 0.5 atm expands from 2 L to 8 L (isothermal). New pressure: (0.5)(2)=P₂(8), so P₂=0.125 atm. Scenario 3: Gas at 3 atm and 4 L is allowed to expand until pressure drops to 1.5 atm. New volume: (3)(4)=(1.5)V₂, so V₂=8 L. Solve all three without checking answers; then verify each.

Practice variation: Change the given variable each time. If pressure is given in scenario 1, find it second in scenario 2, third in scenario 3. This builds flexibility.

Boyle's Law is fundamental to chemistry and physics. ACT Science expects you to apply inverse relationships and solve proportions reliably. Expect 1-2 Boyle's Law questions per Science section, and the equation method solves them both instantly.

Spend 10 minutes this week solving 5-6 Boyle's Law problems. By test day, you'll recognize isothermal gas scenarios immediately and solve them in under 90 seconds.