Science For Everyone - Gases Dissolved in Liquids

Copyright © Karl Dahlke, 2019

We've all seen solids dissolve in liquids, such as salt in water. The salt molecules slide apart and become part of the liquid. Sodium and chloride ions swim amongst the water molecules like so many fish. If you let all the water evaporate away, or boil it away, the salt is left behind. If you try this at home, e.g. boiling a cup of salt water on the stove, make sure you turn off the stove as soon as the water is gone; heating a dry pan can damage the pan. Yes, you have to watch the pot boil.

Some solids, like sand, do not dissolve in water; the grains just fall to the bottom. Sand is basically tiny rocks, so this isn't a surprise.

It is less intuitive, but some gases dissolve in water as well. A common example is carbon dioxide, which provides the bubbles in our soft drinks. This is sometimes called carbonation - thus a carbonated drink. CO2 naturally dissolves in water, but not enough to create fizz. To make an appealing drink, more CO2 is forced into the water under pressure at the bottling plant. Thus some of the CO2 escapes when the bottle is first opened. Pour the drink slowly and gently and keep it cold to retain its fizz. Eventually all the CO2 escapes into the air, it's just a matter of time. If you haven't finished your drink by then, it is "flat".

Heat and jostling cause the CO2 to bubble out of solution, which is why you want to keep the bottle cold, and don't shake it up! If you do shake it up, or drop it, it's going to spurt all over the place when you open it, because some of the gas has left the water and is now under pressure inside the bottle, as per Boyle's law. So don't open it! Keep it cold and most of that CO2 will force its way back into the water. By the next day it should be safe to open.

Another factor that accelerates degassing is surface area. Pour soda water, or any clear carbonated drink, into a clear glass and note the bubbles on the side of the glass. CO2 doesn't usually bubble out of solution in the middle of the water, most bubbles form in contact with a solid surface. Watch the bubbles form on the side of the glass and climb the walls to the surface, whereupon they pop and escape into the air, as new bubbles form to take their place. If the glass has a chip or scratch, that imperfection seeds a stream of bubbles. Don't scratch a glass on purpose, but if you have a glass with a chip in it, try it and see.

Cold keeps the gas in solution longer, so you want to keep your drink cold, but use ice cubes rather than crushed ice. Crushed ice presents more surface area and seeds more bubbles. Also, crushed ice melts faster and dilutes your drink.

Here is another experiment you can do at home, though it makes a bit of a mess. Pour a carbonated drink slowly and carefully into a glass, then dump in a teaspoon of salt. Each edge and corner of each little grain of salt is a nucleating site for a carbon dioxide bubble. Most of the carbonation bubbles out of solution in one go. The drink spills all over the table, so have paper towel at the ready, or do it in the kitchen sink. You're going to pour it down the drain anyways, as the salt has made it undrinkable.

You can try the same experiment with sand, and CO2 will bubble out of solution, but not as vigorously. Grains of sand are larger, and rounder, than grains of salt, so a teaspoon of sand presents less surface area than a teaspoon of salt. Don't pour this down the sink, as sand will ruin your drain and/or your garbage disposal; toss it out into the back yard.

Fish

Fish survive in the ocean because oxygen dissolves in water. This is O2 the gas, not O as part of H2O. Air is mixed into the water by wind and waves and whitecaps at the surface. There isn't a lot of oxygen in the water, but fish don't need a lot of oxygen to live. They are cold blooded, and they don't have a big hungry brain like we do. Most of the oxygen is at the surface, and that's where fish spend most of their time. Fish breathe out CO2, like you and I, and that dissolves in the water for a short time, but soon dissipates into the air. Therefore, fish breathe atmospheric oxygen, and put carbon dioxide back into the air, just like every land animal, but the process is more indirect.

Do you know anyone who has a fish tank it home? The tank includes a bubbler, or aerator, that runs all the time. There are no waves at the top of the tank to mix air and water; in fact the tank is often covered, to prevent the cat from "playing" with the fish. Without the bubbler, oxygen levels would fall and the fish would die. There is also a heater, a filter, a thermometer, gravel and pretty rocks, aquatic plants, and overhead lights, because what's the point of having fish if you can't see them? Keeping fish at home is not trivial.

Blood Gases

Blood is made of water, and gases dissolve in blood as they would in water. The most important blood gases are oxygen and carbon dioxide. The oxygen that naturally dissolves in water isn't nearly sufficient for our metabolic needs, so red blood cells have a separate mechanism for holding on to oxygen and carrying it to every part of the body. We'll get to that in another chapter. Lungs expose blood to the air, as you breathe oxygen in and CO2 out. If breathing stops, or is inadequate to meet current metabolic needs, then oxygen levels fall, and CO2 rises. The latter triggers alarm bells in your brain, and causes you to resume breathing, or breathe faster. This was described in the previous chapter. This mechanism fails in the open cockpit of a plane at high altitude and low pressure, where life had never been prior to 1915. Above 15,000 feet, oxygen concentrations fall, but carbon dioxide escapes the blood as usual and does not rise, and the pilot may not realize he is in trouble until it is too late. Evolution has not prepared us for this environment. This chapter takes us to another novel environment, the deep sea, where pressure is high. Once again we are not prepared for this experience.

Whales

There is no life in the rarified stratosphere, but there is plenty of life in the deep sea, under extreme pressure. Consider the clams and tube worms that live near the Hydrothermal vents at the ocean floor. Some of these vents are at depths of 3,000 meters, almost 2 miles. Recall that 10 meters, or 30 feet, of water is 1 atmosphere of pressure. thus these animals live and thrive under 300 atm of pressure, 300 times what you and I experience on land. They have no hollow cavities such as swim bladders that would collapse under pressure, or they have lost them through evolution. It's the same hydrostatic pressure inside and out, and they don't seem to mind, however, whenever we bring these animals up for study they invariably die due to the drastic change in pressure. As a result we know very little about these fascinating deep sea creatures.

We know considerably more about the marine mammals who live in the sea. They breathe air, and consequently, spend most of their time at the surface. They are proficient swimmers and divers. The sperm whale is the champion diver, reaching depths of nearly 10,000 feet, or 3,000 meters, while holding his breath for 90 minutes. How long can you hold your breath? Maybe one minute, if you are typical.

A marine mammal may experience 100 atmospheres of pressure or more, and yet their ears don't implode. My ears begin to ache at the bottom of a swimming pool, just 3 meters down. Over millions of years, most marine mammals have lost their ear canals and sinus cavities. These air-filled cavities would make deep dives impossible, so they have been selected away. A sea lion still has traditional ears, but his ears fill with fluid during a dive, which prevents his eardrums from bursting inward.

Deep diving mammals and birds still have lungs; what can we do about that? Ironically, whales exhale, blowing most of the air out of their lungs just before a deep dive. You and I take a deep breath in, but that makes us buoyant, requiring more energy to dive. Marine mammals employ the opposite strategy. A whale can expel 90% of the air in his lungs; whereas an athletic human might be able to blow out 10%. His lungs are virtually empty when he dives. The remaining air is compressed, but it isn't a lot of air, and his rib cage is remarkably flexible. The lungs squash flat, until he returns to the surface. Read more about the adaptations that make deep dives possible. Then see how humans compare.

Nitrogen and the Bends

Nitrogen hardly dissolves in water at standard pressure, but this can change during a deep dive. Remember that our air is 80% nitrogen, so there is always some nitrogen in your lungs. If nitrogen is forced into your blood under pressure, and then you come to the surface quickly, it can bubble out of solution, like the bubbles in your carbonated drink. A capillary can be as thin as a human hair, whence the tiniest nitrogen bubble will block it. The first symptom is joint pain, which is why the condition is called the bends. More extreme cases can lead to organ failure, and even death.

A whale or elephant seal experiences 200 atmospheres of pressure or more, but he avoids the bends by forcing almost all of the air out of his lungs before each dive. Just one more advantage to breathing out, not in. Yes, a little bit of nitrogen is forced into his blood during a deep dive, but it's a lot of blood, and a small volume of nitrogen, so it doesn't cause any trouble when he surfaces.

A free diving human rarely gets the bends, because she doesn't descend to more than 6 atmospheres, with only the air in her lungs. A scuba diver, however, has a lot to worry about. She takes a tank of air with her, and breathes it in under pressure for an hour or more - plenty of time for nitrogen to work its way into her blood. Thus there is an ascent schedule; she can only rise at a certain speed. Perhaps she has been exploring a coral reef 30 meters below the surface. This is the limit of an air breathing scuba diver, for reasons that are described below. She might rise to 20 meters, then swim around and exercise for 10 minutes to work the nitrogen out of her blood. Next she ascends to a depth of 10 meters, and finally she is ready to surface. Always leave enough air in the tank for a slow and steady ascent. Scuba divers take classes, and are certified, because this is not a trivial undertaking.

Something else goes wrong if our scuba diver descends to 40 meters or more. It is called nitrogen narcosis. At high concentrations, dissolved nitrogen in the blood acts like alcohol, and leads to impaired judgment. Our diver is very happy, but she has no idea what she is doing. She may ascend too quickly, or simply run out of air in her euphoria. At further depths, even more nitrogen is forced into the blood, and she may drift into blissful unconsciousness. Well our intrepid diver wants to go below 30 meters, because humans always need to explore, so what does she do? She breathes something other than air, a mixture of oxygen and helium with virtually no nitrogen. She doesn't have to worry about narcosis or the bends. Problem solved. The mixture is called heliox, and it's expensive! She might use a rebreather, which captures the helium instead of venting it out to sea. Her CO2 is scrubbed, and more oxygen added, whereupon she breathes the heliox in again. Another mixture, trimix, contains helium, oxygen, and nitrogen, and is less expensive. This supports deeper and longer dives than traditional scuba gear, but it still contains some nitrogen, so care must be taken.

This is not the first chapter to explore the properties of nitrogen. Let's review some of the characteristics of nitrogen, as described in chapter 6. Each nitrogen atom has 3 bonds, and a molecule of nitrogen consists of two atoms joined by a triple bond. This is written N2, or N#N in smiles notation. It's a compact, light, symmetric molecule, thus a gas, even at low temperatures. It takes tremendous energy and finesse to break the triple bond and join nitrogen and hydrogen to make ammonia, NH3, but that is the first step in the manufacture of fertilizer, which we need to grow crops, to feed the billions of people on earth. Ammonia is also instrumental in the production of explosives. All this we've seen before, but there's always something new to learn about the elements, even if you've studied them for years. N2 is a harmless gas that you breathe in every day, but under pressure it dissolves in the blood, and can lead to narcosis, or even unconsciousness. Modest concentrations of nitrogen in the blood can cause the bends if you decompress too quickly.

Many gases can produce narcosis, and some are more powerful than N2. Perhaps you have experienced laughing gas, a compound of nitrogen and oxygen, N2O. You don't have to step into a high pressure chamber, laughing gas dissolves in the blood and produces narcosis under ordinary conditions. The effect is similar to alcohol, but much stronger.

We've barely met xenon, element number 54, but this is a good time to bring him back into the picture. This atom has no bonds, and is therefore a nobel gas. It doesn't react with other atoms, and is safe, in modest concentrations, however, it is surprisingly anesthetic. It dissolves readily in the blood, and can produce narcosis even under ordinary conditions. Breathe in a mixture of 80% xenon and 20% oxygen and you will quickly fall asleep. A surgeon can operate, and you won't feel a thing. If xenon weren't so expensive it might be used as a general anesthetic in hospitals.

Helium is a nobel gas, but it doesn't dissolve in the blood, which is why deep divers breathe helium and oxygen to avoid narcosis and the bends. Helium might be the only gas that is safe to breathe in a high pressure environment, i.e. more than 5 atmospheres. Even hydrogen causes a decrement in mental function and judgment.