Gas Solubility: How it enables the production of carbonated drinks and its harsh effects on scuba divers

Solubility is the maximum amount a solute can dissolve in a certain amount of solvent. Carbonation occurs when pressurised carbon dioxide is injected and dissolved in water.

The solubility of a gas dissolved in a liquid is directly proportional to the pressure of the gas above the solution. If there is a greater pressure in the space above the solution or on the solution, there will be a greater solubility of the gas dissolved in the solution.

This trend is what the carbonation process relies on in order to obtain fizzy drinks. The process is conducted at high pressure in order to allow for a greater amount of CO₂ to dissolve in the liquid (improving CO₂ solubility). Under normal atmospheric pressures carbon dioxide is only slightly soluble, therefore manufacturers will increase the pressure inside cans or bottles to maximise gas absorption. The greater the pressure in the space above the solution, the greater the fizz. When the drink is opened, the carbon dioxide gas escapes resulting in a decrease in gas pressure above the solution (CO₂ gas is depressurized) resulting in the carbon dioxide gas to form bubbles and come out of the solution providing the drink with its fizz.

This is what results in the ‘POP’ sound when you open a carbonated drink!

An additional factor in affecting gas solubility is the temperature. As seen in the above image, at lower temperatures gas solubility increases. Thus this means that at higher temperatures there is lower gas solubility. This means that on a warm day a fizzy drink will become flat very quickly. Opening a bottle of a fizzy drink decreases the pressure above the solution and allows carbon dioxide gas to form bubbles and come out of the solution. At higher temperatures more bubbles will form and more of the gas will escape into the surroundings resulting in the drink to lose its fizz very quickly.

Have you ever wondered what was happening to Ethan in Mission Impossible - The Final Reckoning when he went scuba diving? If you don’t know what I am talking about, watch this VIDEO from 6 minutes 50 seconds to 7 minutes 30 seconds. 

So what does gas solubility have to do with what Ethan experienced?

Ethan experienced what is known as the bends. This is about gas dissolved in a diver’s blood. Just like many other liquids, blood has gases dissolved in it such as oxygen gas, carbon dioxide gas and nitrogen gas. These gases are all under pressure due to the water pushing on the diver’s body. 

When Ethan was deep under the water, the pressure on his body had increased. This resulted in a greater amount of nitrogen dissolved in the diver’s bloodstream and tissues when the diver breathes. High pressure, high gas solubility (Henry’s Law). The longer the diver remains at depth, the more nitrogen is dissolved in the bloodstream. Since Ethan is in a highly pressurized environment, the excess can only be relieved when he ascends to levels with lower external pressure. 

However a  problem occurs when divers come up to the surface too quickly. As they come up, the pressure of surrounding water decreases which means that the nitrogen gas (solubility in their blood decreases as well). All the extra nitrogen gas that used to be dissolved bubbles out of the blood just like a carbonated drink is opened where pressure is released, bubbles form, and the gas escapes.

Diagram showcasing Ethan's journey

Think of Ethan as a bottle of coke. When the soda bottle is sealed under high pressure, a lot of carbon dioxide gas dissolves in the liquid. When the seal is released the pressure drops quickly and the dissolved gas “wants” to come out, so bubbles form and rush out. However if the cap of the bottle is opened slowly not as much pressure is released and fewer bubbles form. 

This creates a syndrome known as decompression sickness, often referred to as “The Bends”, it is extremely painful and can be deadly as the bubbles of nitrogen gas interfere with the normal functioning of the nervous system. Instead, divers need to return to the surface gradually, so that the pressure decreases slowly, and the nitrogen gas escapes from their blood gradually rather than all at once. 

So can you cure decompression sickness? 

Well if there was no cure then nitrogen bubbles would remain in Ethan’s bloodstream and tissues, blocking blood flow and damaging organs, which could lead to paralysis or death. Thankfully Ethan was placed in a decompression chamber. 

The decompression chamber is a high pressure environment which simulates the feeling of being underwater again. This forces nitrogen bubbles back into the blood and makes them dissolve again (according to Henry’s Law).

After that, the pressure is lowered very slowly and safely. This gives nitrogen time to leave the body through the lungs without forming bubbles. It also provides pure oxygen which helps remove nitrogen faster, improving oxygen delivery to damaged tissues and supports healing. The chamber is the safest and most effective treatment for decompression sickness.