Gases in Liquids

Overview
  • Gases can exist in significant concentrations within a liquid medium in a variety of chemical forms as discussed below. This feature of gases can be most directly observed when a can or bottle of soda is opened, resulting in release of the carbon dioxide dissolved in the soft drink liquid. The capacity of gases to exist within liquids is a critical feature allowing for transport of gases throughout the circulatory system, without which life would be impossible.
Forms of Gases in Liquids
  • Dissolved Gas
    • This is the most basic form that gases can be present in liquids. Here, gas molecules are simply dissolved in the liquid like any other solute such as salt. Importantly, molecules of gas dissolved in liquid dynamically exchange with molecules of the gas present in the traditional gas phase surrounding the liquid. This means that molecules in the gas phase adjacent to the liquid can spontaneously enter the liquid and become dissolved whereas molecules of gas in the liquid can spontaneously exit the liquid and enter the surrounding gas phase
    • With time, this dynamic exchange of gas molecules between the liquid and the traditional gas phase will establish a dynamic equilibrium. When equilibrium is established, chemists refer to the gas within the liquid as possessing a "Partial Pressure" which is equivalent in value of the partial pressure of the gas in the traditional gas phase bordering the liquid. For example, because the partial pressure of oxygen in the atmosphere at sea level is roughly 150 mm Hg, it is traditional to consider the partial pressure of oxygen dissolved in an open glass of water at sea level to also be 150 mm Hg.
    • Importantly, and this cannot be emphasized enough, dissolved gas is the only form of gas in liquids that contributes to its partial pressure within the liquid. This is because molecules of dissolved gas are the only form of gas within liquid that are in dynamic equilibrium with the molecules of gas in the surrounding traditional gas phase. The other forms of gas described below are not in dynamic equilibrium and thus cannot contribute to the partial pressure of that gas within the liquid.
  • Bound Gas
    • Gas molecules can be tightly bound by molecules in the blood such as proteins. As mentioned, bound gas molecules in liquids are not at equilibrium with the gas molecules in the traditional gas phase surrounding the liquid, and in consequence do not contribute to the partial pressure of the gas within the liquids. Because of this feature, a liquid may possess a large concentration of gas within it in bound form and yet display only a minimal partial pressure of the gas in the dissolved form. This strategy is used to transport large amounts of oxygen within the blood through tight binding of oxygen molecules by hemoglobin.
  • Chemically-modified gas
    • Certain gas molecules can be converted into other non-gas chemical forms and thus become trapped in the liquid. If the reaction is highly reversible, the gas form can be regenerated at a later time and at a distant location. Reversible chemical modification of gases is used as a strategy to transport carbon dioxide within the blood. CO2 gas is converted in tissues to bicarbonate which then flows, trapped within plasma, to the lungs where it is reconverted to carbon dioxide and exhaled. Once again, because bicarbonate within blood is not in equilibrium with carbon dioxide in the traditional gas phase, large amounts of CO2 can be transported in the form of bicarbonate given only low liquid partial pressures of carbon dioxide.


Possible Forms of Gases in Liquids
Gases can exist in liquids as a simple dissolved molecule, bound to a protein, or in a chemically-modified state. Importantly, only the simple dissolved form is in dynamic equilibrium with the traditional gas phase surrounding the liquid. As a result, the simple dissolved form is the only form that contributes to the partial pressure of the gas in the liquid.