Boyle's Law states that for a fixed amount of gas at constant temperature, what happens to pressure when volume decreases?

• A. Pressure increases
• B. Pressure decreases
• C. Pressure remains constant
• D. Pressure equals zero

Answer: (A)

When a fixed amount of gas is kept at a constant temperature, the product of pressure and volume stays the same (isothermal ideal gas behavior). If you decrease the volume, there are fewer spaces for the gas molecules, so they collide with the container walls more frequently and with greater force on average. That increased collision rate raises the pressure. In practical terms, shrinking the volume makes pressure go up; for example, cutting the volume roughly in half makes the pressure rise roughly twice, assuming ideal gas behavior. The other possibilities—pressure decreasing, staying the same, or dropping to zero—don’t align with the way gas molecules crowd together when confined to a smaller space.