When **temperature is constant** (for o-level), when a __fixed mass of gas (fixed number of air molecules)__ is __compressed__ in a closed system (e.g. piston), the __volume V decreases__ and __pressure P increases__, and vice versa.

But when you multiply pressure and volume, PV, it is always a constant.

**PV = constant**

Hence we can always equate the PV of the first scenario = to the PV of the second scenario, provided there is no addition or removal of air molecules from the system.

Hence, you have **P1V1 = P2V2**

The followings are 4 different questions which require this concept to solve. Do revise them.

**Solutions: Option D** (refer to the worked solutions below)

**Solutions: Option D**

**Solutions: A**

4)

**Solutions:**