Charles Law

The volume of a fixed mass of gas is inversely proportional to its pressure at constant temperature.

The volume of a fixed mass of gas is directly proportional to its temperature at constant pressure.

v ∝ t
v = k t
v = k t

As you can see, the line, when extended backwards crosses the x-axis at -273 C⁰. There is a special temperature when all gases have zero volume, called Absolute Temperature.
So, 0 T = -273 C
+ 273 => 273 T = 0C
+ t => (273 + t) T = t C
T = 273 + t, where t is the temperature in Celsius.

If the temperature and volume of a fixed mass of gas take the values of v₁ T₁and V₂ T₂ respectively, where T is absolute temperature,

V₁ / T₁ = V₂ / T₂

E.g.

The volume of a fixed mass of air is volume is 8 cm³ at 27 C⁰. Its temperature is raised to 127 C⁰, while keeping the pressure constant. Find the volume.
p₁ = 20, v₁ = 8, p₂ = 40
According to Charles’s law,
V₁ / T₁ = V₂ / T₂
8 / (273 + 27) = v₂ / (273 + 127)
v₂ = 10.6cm³

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