##  [Boyle's Law](/boyles-law-0) 

 Definition

The relation for a fixed amount of ideal gas at constant temperature that pressure is inversely proportional to volume, commonly expressed as PV = constant for an isothermal process.

 

 

 

 

 

 





## Principle

Principle

Under isothermal conditions (constant T) and fixed n, compressing a gas reduces its volume and increases its pressure so that the product P·V remains constant; the law encodes isothermal mechanical balance in the ideal approximation.

 

 

 

 

 





## Demonstration

Demonstration

Slowly compressing a confined ideal gas in thermal contact with a thermostat keeps T constant; recorded pressure versus volume values satisfy P1V1 = P2V2 within experimental uncertainty for moderate conditions. Practical example: plunger motion in a syringe kept at constant temperature.

 

 

 

 

## Misapplication

Misapplication

Applying PV = const during rapid (adiabatic) compression where temperature changes and the relation PV^γ = const applies, or using the law close to condensation or at high densities where intermolecular forces contribute significantly.

 

 

 

 

 





## Consequence

Consequence

Enables calculation of pressure or volume changes during isothermal processes and is foundational to early thermodynamic thinking about work and heat in gas compression and expansion scenarios.

 

 

 

 

## Reversal

Reversal

The reverse viewpoint is an isothermal expansion: increasing V lowers P to keep PV constant; contrastingly, an adiabatic reversal follows PV^γ = const, highlighting the difference introduced by heat exchange.

 

 

 

 

 





## Boundary

Boundary

Applies to isothermal, quasi-static changes of a fixed amount of a dilute gas where ideal behavior holds; excludes processes with significant heat insulation (adiabatic), chemical reactions changing n, or regimes dominated by non-ideal interactions.

 

 

 

 

 





## Semantic Tension

Semantic Tension

Tension exists between Boyle's isothermal inverse relation and adiabatic relations (e.g., Poisson's law), so one must distinguish process constraints (constant T vs. no heat exchange) to choose the correct law.

 

 

 

 

 





## Synthesis

Synthesis

Boyle's law states that, for a fixed amount of ideal gas held at constant temperature, pressure and volume are inversely related so that their product is constant; it captures isothermal mechanical response within the ideal-gas framework.