The property of a substance stays no matter how the substance is used.

Any substance changes to another substance that means the change of the physical property. Like water () has different state which changes as the temperature changes. It remain as liquid in the room temperature, in solid form at or below 0°C and vapor phase on or above 100°C. But in all the stage or phase of the substance the composition of the water i.e.  remains. Thus the chemical property remains fixed when a substance change to other substance.  

it should be p1 v1 = p2 v2

the product of the pressure and volume is also constant