Gas molecules in the solution (present in dissolved state) and those in the gaseous state are in dynamic equilibrium with each other. Though the concentrations of gas in the dissolved and undissolved state remains constant there is a constant exchange of gas molecules at the surface. The constant exchange is such that the number of molecules entering dissolved state is same as that of the molecules leaving dissolved state. In the piston cylinder set up with solvent and gas present in the cylinder, if the piston is moved upwards then the volume of the gas increases. In the previous article, we discussed how increased volume results in more collisions at the surface of liquid.
The reverse is the case here and the number of collisions at the gas-liquid interface decreases. Consequently the number of molecules entering the liquid decreases as a result of increased volume. Since the number of molecules leaving liquid surface is dependent on the amount of gas dissolved, it remains unaffected (initially). As a result, equilibrium is lost and there are more molecules leaving liquid than those entering it. Hence the gas bubbles out.
The same scenario can be extrapolated to the case of deep sea diving. As we have learnt during our Physics tuition classes, the deeper you go into the sea, the higher the pressure. When deep sea diver comes out to the surface in a very short span of time (following an exposure to high pressure in the deeper regions of the sea) the pressure is taken off the blood vessels suddenly. This sudden decompression is similar to the piston and cylinder case discussed above. As a result in the blood vessels dissolved gas comes out of the blood rapidly causing the formation of bubbles. This bubbling is the reason behind all the symptoms experienced during decompression sickness.
The solution to this issue is relatively simple. Imagine what happens if the piston is moved up slowly in installments instead of a sudden upward movement. At every installment, the dissolved and undissolved gas molecules reach equilibrium and since only small change is made in the volume the gas doesn’t suddenly escape the liquid. Thus bubbling can be avoided. The same can be applied to diver’s case. If the diver moves up slowly waiting for sometime at every installment of this upward movement, the gas molecules have enough time to slowly escape blood and are eventually released through lungs. Thus bubbling and the associated symptoms can be avoided. The whole explanation tells us whether the diving is assisted, by artificially carried oxygen, or unassisted the effects remain the same.