Question 1

What is Boyle's Law?

Accepted Answer

Boyle's Law says that for a fixed amount of gas held at constant temperature, the pressure and volume are inversely proportional: P₁V₁ = P₂V₂. Squeeze the gas into a smaller volume and the pressure rises in exact proportion; let it expand and the pressure falls. The product of pressure and volume stays constant, which is why halving the volume doubles the pressure.

Question 2

How do you calculate the new pressure or volume?

Accepted Answer

Rearrange P₁V₁ = P₂V₂ for whatever is unknown. New pressure is P₂ = P₁ × V₁ / V₂; new volume is V₂ = P₁ × V₁ / P₂. For example 2 litres of gas at 1 atm compressed to 1 litre gives P₂ = 1 × 2 / 1 = 2 atm. As long as both pressures share a unit and both volumes share a unit, the units cancel and the answer comes out in the unit you used for the known value.

Question 3

Does temperature have to be constant for Boyle's Law?

Accepted Answer

Yes. Boyle's Law only holds at constant temperature and for a fixed amount of gas — it is the isothermal slice of the ideal gas law. If the temperature changes during the process you must use the combined gas law (P₁V₁/T₁ = P₂V₂/T₂) or the full ideal gas law PV = nRT instead, because temperature would otherwise alter the pressure–volume product.

Question 4

What units should I use?

Accepted Answer

Any consistent units work because they cancel. Use the same unit for both pressures — atm, kPa, bar, psi or mmHg — and the same unit for both volumes — litres, millilitres or cubic metres. The result comes back in the same unit as the matching known quantity. There is no need to convert to SI, since Boyle's Law is a ratio relationship, unlike the ideal gas law where the gas constant fixes the units.

Question 5

Why does pressure rise when volume drops?

Accepted Answer

At constant temperature the gas molecules keep the same average speed, so squeezing them into a smaller space makes them strike the container walls more often per unit area — and pressure is force per area. Half the volume means roughly twice the collision rate, hence twice the pressure. This inverse link is exactly what P₁V₁ = P₂V₂ captures, and it underlies how syringes, pumps and the lungs move gas.

Final volume	Final pressure	Note
2 L	1 atm	start
1 L	2 atm	half volume
0.5 L	4 atm	quarter volume

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