Introduction to Gas Laws (Boyle's Law)

Robert Boyle (1627 - 1621)

was an Anglo-Irish natural philosopher, chemist, physicist, alchemist and inventor. Boyle is largely regarded today as the first modern chemist, and therefore one of the founders of modern chemistry, and one of the pioneers of modern experimental scientific method. He had published this law in 1662.

15.2 Gas Laws

Physics • Chapter 15 — Molecular Theory of Gases

Gases have no fixed volume or shape. For a fixed mass of gas, volume can be changed by changing the pressure (and temperature). We describe gases using four variables: pressure (P), volume (V), temperature (T), and mass. Relations between any two variables are found experimentally while the other two are kept constant.

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15.2.1 — Boyle's Law

Boyle's law states that:

Volume V of a given mass of gas is inversely proportional to the pressure P, provided the temperature T remains constant.

In mathematical form:
V ∝ 1 / P (at constant T)
PV = constant
For two states: P₁ V₁ = P₂ V₂ = constant

DO YOU KNOW?

The liquid–vapor equilibrium region ends at the critical point. The pressure and temperature at that point are called critical pressure (Pc) and critical temperature (Tc).

Graphical Representation

(At constant temperature) Boyle's law can be represented in two common ways:

• P vs V: a downward-sloping hyperbola (inverse relation).
• P vs 1/V: a straight line through the origin (direct proportionality).

Pressure vs Volume — inverse relation. As V increases, P falls such that PV = constant.

Pressure vs 1/Volume — direct proportionality (straight line through origin).

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Key Takeaways

• At constant temperature, PV = constant for a fixed mass of gas.
• If pressure doubles, volume halves: P₁V₁ = P₂V₂.
• P vs V → hyperbola; P vs 1/V → straight line.