The Gaseous State
It is the simplest state of matter. It has following properties.
·          Gases are highly compressible
·          Gases exert pressure equally in all directions
·          Gases have density much lower than the solids and liquids
·          Gases do not have definite volume and the shape. They assume volume and                      shape of the container
·         Gases mix evenly and completely in all proportions without any mechanical                     aid.


The Gas Laws
Boyle’s Law (Pressure – Volume Relationship): Chemist and physicist Robert Boyle published the law in 1662. At constant temperature, the pressure of a fixed amount of gas, is inversely proportional to its volume.
Mathematically,
P α 1/V
      P = k1 × 1/V, where k1 is the proportionality constant.
k1 constant will be dependent on the amount of the gas and temperature of the gas.

Consider a fixed amount of gas at constant temperature T. Let V1 & P1 are its initial volume and pressure respectively. Let the gas undergoes expansion, so that its final volume and pressure becomes V2 and P2.
Then according to Boyle’s law,
P1V1 = P2V2
If we plot graphs between
a). Pressure against Volume:





b). Pressure against 1/Volume:


            

c). PV against Pressure:



These graphs are obtained at constant temperature and are called isotherms.


We know that density = mass/volume
i.e., d = m/V
If we put value of V in this equation from Boyle’s law equation, we get the relationship, d = (m/k) × p
At constant temperature, pressure is directly proportional to the density of a fixed mass of the gas.
Boyle's Law and Human Breathing
Boyle's law may be applied to explain how people breathe and exhale air. When the diaphragm expands and contracts, lung volume increases and decreases, changing the air pressure inside of them. The pressure difference between the interior of the lungs and the external air produces either inhalation or exhalation.