Dissolved gas is a situation which occurs when a gas becomes dissolved in another solute. People often think specifically of a gas-liquid mixture when they think of dissolved gases, but gases can also dissolve in other gases, as in the case of air. Gases may also be dissolved in solids, as in seen in the trace impurities which can be caused in metals when gases are introduced during the casting process.
In order for gas to dissolve in a liquid, it usually needs to be subjected to pressure and held under pressure to remain dissolved. If the pressure is reduced, the gas will start to precipitate out of the liquid. With solids, dissolved gases occur when there is pressure which traps the gas inside, and sometimes the gases precipitate out and form bubbles inside the solid; bread, for example, contains numerous bubbles formed by carbon dioxide released by yeast inside the bread.
In medicine, dissolved gas measurements are sometimes taken to monitor patient health. Healthy human blood has trace amounts of several gases, including oxygen and nitrogen. If these gases are out of balance, it can indicate that the patient has a health problem. Dissolved gas is also an issue when humans are transitioning from high pressure to low pressure environments, as seen when people rise too quickly during a dive and experience “the bends” as the dissolved nitrogen in their blood is released too quickly.
Anesthesiologists also work with dissolved gases. When they deliver anesthesia to patients under general anesthesia, they must blend gases appropriately to keep the patient sedated and to provide the patient with oxygen. This is done by adjusting an anesthesia machine in response to the patient's performance under anesthesia.
Dissolved gas measurements are also used in water quality assessment. Water naturally contains some gases, and interruptions in the normal balance of gases may be indicators that there is a water quality problem. Sites like dams may have monitors in the water continuously to measure the gases. Gas monitoring is also used in aquarium management.
People analyzing unknown samples may use a dissolved gas measurement to gather data about the sample and its origins. Such data can be useful for everything from identifying samples of liquids taken at a crime scene to conducting studies on natural phenomena. Several types of laboratory equipment can be used to determine the level of gases in a solution and to identify which gases are involved.