By Richard E. Bleil, Ph.D.
There are three (main) states; solid, liquid and gas. Unless you’re an old rock star, in which case it’s “gas, gas, gas”. Two additional states include the plasma state (basically, this is what flames are; temperatures are so high that molecules break apart) and the glassy state. The three major states are defined by their volume and shape. Solids have a fixed volume (that is, put a nail in a larger container and its volume remains constant) and fixed shape (that is it will still be nail shaped). Liquids also have a fixed volume, but their shape changes to the container. A gas will expand its volume to fill the container, and their shape will change to that of the container.
Gases are important to us. But, a lot of people do not understand them. What I would like to discuss today is solubility of gases in liquids, which has a bigger impact on us that you might think. Solubility is how much solute will dissolve in a solvent. “Unsaturated” means we can dissolve more solute, “saturated” means as much solute has dissolved as possible, and “supersaturated” means that more solute than possible is dissolved. If that seems impossible, it is. We’ll talk more about this unstable situation in a bit.
First, the solubility of gases actually decreases as temperature increases. This is opposite of many (but not all) solid solutes. If you have ever watched water as it is being heated to boiling, you may have noticed tiny bubbles forming on the sides of the pot forming well before a boil is achieved. These bubbles are air (largely nitrogen and oxygen) forming as the solubility of these gases decreases as temperature increases.
When it is particularly hot, fish will often dwell near the surface of lakes, and will be very lethargic. I remember fishing when I was very young. The fish were right at the surface, and I dropped baited hooks literally right in front of their noses (I wish I could say it was skill, but it was the length of my line and the coincidental distance of the fish). The fish would not move to even begin to nibble. The reason the fish were at the surface is because the amount of oxygen in the lake was decreasing, so they were basically suffocating. What oxygen there was would be closer to the surface. And because they were having trouble breathing, they were also lethargic.
Solubility of gases in liquids also increase with increasing pressure. Anybody sees this every day if they drink carbonated beverages. Any time you open a pop, you hear that “Psshht” sound as the pressure is released. Carbonated beverages were discovered when a chemist noticed that heated water tastes “flat” and flavorless. The chemist correctly deduced this was because of the lack of the gas carbon dioxide, that in water forms carbonic acid. Acids tend to have a sour flavor, so when water is heated, the loss of carbon dioxide means the loss of the flavor of the carbonic acid. Knowing that gases dissolve in higher concentration at higher pressure, the chemist tried putting carbon dioxide at high pressure over water. The result was seltzer water, which was too sour for most palates, but the fizzing was quite pleasurable. So, a lot of sugar was added to counter the sour flavor of the carbonic acid, creating soda-pop.
When a soda-pop is opened, the pressure is released. At this point, the pop is a “supersaturated solution”, meaning that more carbon dioxide is in the soda than possible at normal pressure. It takes time for the gas molecules to find each other, and form bubbles that then rise to the surface and are released from the solution. This is the “fizzing” that is seen, but as the solution loses carbon dioxide, it also loses that sour carbonic acid flavor. Thus, as pop goes flat, it seems to become sweeter. The sour carbonic acid flavor is loss, but the countering sour flavor of carbonic acid is gone.
Scuba divers must be especially careful of this solubility relationship (called “Henry’s Law”). For every ten feet under water the pressure increases by 1 atmosphere. So, at 20 feet, the pressure is 3 atmospheres, or three times higher than regular atmospheric pressure. The air we breathe is about 70% nitrogen, as is the air in the typical scuba air tank. At this higher pressure, nitrogen will begin to dissolve in the body of the scuba diver (which is about 70% water). This is akin to dissolving carbon dioxide in water for pop, but, again, it takes time for this water to dissolve, and to come back out again. As a result, the diver must be careful not to stay in the water for too long, or the amount of nitrogen that can dissolve can become dangerous. Divers are very careful to keep an eye on safety charts that determine the safe amount of time a diver can stay under water for a given depth. The deeper they dive, the longer they can stay under.
If they stay under for too long, their ascension must be slow enough to let the gas leave their system at a safe rate. If they rise too quickly after being under water for too long, as the nitrogen leaves their system, it can form tiny bubbles, which often occurs near the joints. This can cause pain in their joints (or worse). That is, they end up in pain where their bodys bend, hence the term, “the bends”.