Lesson 4 of 13 · 5 min
Real gases, Boyle's law and Charles' law
NCERT §12.3
As the evening cools, the balloons tied to the stall sag a little. When a customer squeezes one, it pushes back harder. Meher sees two gas laws at once.
The lesson in notes
In short
Real gases drift away from ideal behaviour; plots at three temperatures all move towards the ideal line as pressure falls and temperature rises (Fig. 12.1).
The reason: at low pressure or high temperature the molecules are far apart, their interactions hardly matter, and the gas acts like an ideal one.
Boyle's law: fix μ and T, and PV = constant. At constant temperature, the pressure of a given mass of gas is inversely proportional to its volume.
Measured P–V curves for steam at three temperatures follow Boyle's law well at high temperature and low pressure and depart from it elsewhere (Fig. 12.2).
Charles' law: fix P, and V ∝ T. Hold the pressure steady and a gas's volume grows in step with its absolute temperature (T–V curves for CO₂ in Fig. 12.3).
Both laws are special cases of PV = μRT with one more variable held fixed.