There are many laws governing the physics and chemistry of scuba diving. A few of the basic principles are described below.
Pressure: Pressure is defined as the amount of force per unit area. At sea level we are exposed to 14.7 pounds of pressure per square inch, or one atmosphere. Seawater is a lot denser then air and so a diver is subject to increased pressures as he/she dives to great depths. A column of ten metres of seawater exerts a pressure of 1 atmosphere. This means that a diver at 20 metres is exposed to 3 atm of pressure (2 atm for the 20 metres depth, and 1 atm from the sea level atmospheric pressure). Due to this pressure a diver needs to breath pressurized air, as the lungs are not capable of expanding enough to intake air that is not under pressure.
"If the temperature is kept constant, the volume of a given mass of a gas is inversely proportional to the absolute pressure"
For scuba diving this means that as a diver descends the air in the divers cavities decreases in volume as the pressure increases. At a depth of 10 m the same volume of gas will occupy only half the volume it would at sea level.
"The total pressure exerted by a mixture of gases is the sum of the pressures that would be exerted by each of the gases if it alone were present and occupied the total volume"
"The amount of gas that will dissolve in a liquid at a given temperature is almost directly proportional to the partial pressure of that gas"