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Unit 8: When Chemicals Meet Water—The Properties of Solutions

Section 7: Henry's Law

As we learned earlier in this unit, there are limits to the amount of any given solid that can be dissolved in a specific liquid before the solution becomes saturated. Similarly, the same thing is true if the solute is a gas; the liquid can only absorb a certain amount of the gas before it becomes saturated. In daily life, many liquids have gases dissolved in them; the most common example of this is carbonated sodas. Colas and root beer are composed of flavoring and sugars added to a solution of carbon dioxide gas dissolved in water. (Figure 8-11)

Henry's Law and Soda

Figure 8-11. Henry's Law and Soda

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Henry's Law and Soda

Figure 8-11. Henry's Law and Soda

When a soda container is closed and pressurized under several atmospheres of carbon dioxide pressure, according to Henry's Law, a large amount of carbon dioxide is dissolved in the solution. If the cap is removed, the pressure is removed and bubbles of carbon dioxide begin to form as the concentration is trying to go down to reach the new lower value that Henry's Law predicts.

In 1803, English chemist William Henry (1774–1836) published a paper on his experiments dissolving different quantities of gases in water at varying temperatures and pressures. Henry's Law states that the solubility of a gas in a liquid is proportional to the partial pressure of the gas in contact with the liquid. (See the equation below.) The greater the partial pressure of a gas, the more of that gas will dissolve in the liquid. Understanding Henry's Law allows us to manipulate the amount of gas that dissolves in liquid. In the health field, this means that we can expose the body to higher pressures of oxygen to increase the amount of oxygen in the blood. Or vice versa, we can remove toxic gases like carbon monoxide from the air to which a patient is exposed, to remove it from the solution of blood.

c = kP

concentration of the gas in the solution = Henry's Law constant x partial pressure of the gas above the solution

So, this is really just a simple relationship between the amount of gas that dissolves in the liquid and the partial pressure of the gas above the liquid solution. The constant in that equation is also affected by three other factors. The first is temperature. Henry's Law constants are a function of temperatures. If we leave a glass of tap water out for a long time, bubbles form as the water warms up to room temperature because the air dissolved in the water is actually more soluble when the water is colder. It also depends on what gas is being dissolved and what solvent or liquid is being used. Henry's Law explains why diving with compressed air is dangerous; nitrogen makes up 78 percent of air, and it has a very high Henry's Law constant compared to other gases. Therefore, if we dive down with compressed air for a while, nitrogen begins to dissolve significantly into our blood under the increased pressure of the water. However, returning to the surface quickly removes that pressure; "the bends" occurs when this happens so fast that small bubbles of nitrogen gas form in the small blood vessels. In order to prevent death if this occurs, a diver needs to get to a hospital quickly to get repressurized in a chamber, so that the bubbles will redissolve and the pressure can be released slowly.

Henry's Law tells us that if we put pure water in a jar, the nitrogen and oxygen in the air will dissolve into the water in proportion to their partial pressures. This is important for lakes, streams, and other water reservoirs. The fish and other creatures that live in these bodies of water need certain levels of oxygen to survive. As the water moves around, and comes in constant contact with the air above the surface, a level of oxygen is maintained that allows fish to survive. However, in the ocean, where the depths of the water can be miles deep, only the water near the surface will be in contact with the air, while the water farther down will have little oxygen dissolved in it and can support very little aquatic life. In a fish tank, we have to use a bubbler to force enough oxygen to dissolve in the water so certain types of fish will have enough oxygen to survive. But in the natural world, fish don't need bubblers. The natural motions of certain bodies of water and Henry's Law take care of that.

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