Robert Boyle, in 1662, gave this law. It relates the pressure and volume of ideal gases at constant temperature and mass. It states
“For a fixed mass of a gas, the volume is inversely proportional to the pressure at a constant temperature.”
Thus law can also be stated as:
“For a fixed mass of a gas, the product of pressure and volume remains constant at a constant temperature.”
It means volume changes with change in pressure but product PV remains the same.
Let the volume of a gas is V1 at pressure P1. Then
P1V1 = Kb ……………… (3)
If pressure changes to P2 then the volume will change to V2 such that the product remains the same
P2V2 = Kb ……………… (4)
Comparing equations (3) and (4), we can write
P1V1 = P2V2 ……………… (5)
Experimental Verification of Boyle’s law: Consider a gas present in a cylinder fitted with a movable and weightless piston. A monometer is attached to the cylinder to measure pressure. Let the initial volume of gas be 22.4 dm3 and the pressure on it is 1 atmosphere (as the piston has no weight on it and only air exerts pressure on it). Now place one weight on piston, pressure will become double and it can be observed that volume is reduced to half i.e 11.2 dm3. Similarly by putting more and more weights on the piston pressure goes on increasing while volume goes on decreasing to some extent to which pressure increases. In this way, data can be collected and then by plotting graphs the law can be verified.
The experimental data collected is given in the following table.
(i) Plot of inversely proportional quantities is a hyperbolic curve.
(ii) Plot of directly proportional quantities is a straight line.
(iii) Plot of a constant quantity against a variable quantity, is a horizontal line.
We can verify the law by following three graphs.
Figure III shows a horizontal line which means the product of pressure and volume (PV) is constant. Thus the law is further verified.
PV = Constant
All three plots have verified the law.
Explanation of Boyle’s Law on the basis of KMT:
Consider a cylinder fitted with a fixed piston. Let the cylinder contains the ‘n’ number of molecules at temperature ‘T’, volume is ‘V’ and pressure exerted by molecules is ‘P’.
Further, consider that piston is moved to a place where volume is reduced to half. As temperature remained the same so av. K.E remains the same and in turn, velocity remains the same. But now molecules have to travel half the distance, so the number of collisions becomes double and in turn pressure becomes double. It means when volume reduces to half, the pressure becomes double i.e.
V ∝ 1/P
And this is the statement of Boyle’s law.