# Boyle's Law of pressure and volume

Boyle's Law states that:
Pressure is inversely propotional to Volume. But if we take an example of a balloon. As we fill air in it it's pressure increases but it's volume also increases.

Can anyone EXPLAIN?

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## 2 Answers

Balloon is not an ideal system to study pressure-volume relationship. Because, on expansion, the elastic skin also expands and there needs to be an additional pressure build-up on the inner side to counter that force, very similar to the extra pressure in a spherical bubble. (where the surface tension acts exactly similar to the elastic balloon skin)

But in your arguments, you are forgetting an important point. Blowing a balloon also involves increasing the amount of air in it. Boyle's law holds for only a fixed volume of gas. If you change the number of moles, Boyle's law no longer stays valid.

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Let's start with the general gas equation: $$P1V1=nRT1$$ Now to get to Boyle's law, we assume that both T(temperature) and n (the number of moles of the substance involved in our experiment) are both constant. Now we can rewrite the above equation as $$P1V1=constant$$ Of course if we change P or V in our system their product must still produce the same constant, or $$P1V1=P2V2$$ So we can't blow additional air into our balloon (adding more moles of material and violating one of our initial assumptions) and still expect Boyle's law to hold. In a fixed system like the one we just described where Boyle's Law holds, if we reduce the volume of our balloon (V2) by 1/2 then the pressure P2 must increase by a factor of 2 in order to maintain the value of the constant.

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