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A Closer Look

Decompression Sickness

ecompression sickness (DCS) is a con- dissolved in the blood. Dissolved nitrogen One way to prevent DCS is for divers to
D dition caused by the formation of is a normal occurrence and does not pre- make “decompression stops” as they return to
nitrogen bubbles in the blood and tissues sent a problem. However, when a person is the surface. These stops allow the dissolved
of a scuba diver who surfaces too quickly, breathing air under higher pressure, more nitrogen to diffuse into the lungs rather than
causing a rapid drop in pressure. This condi- nitrogen is breathed in and more becomes making bubbles in tissues and the blood-
tion is usually marked by joint pain but can dissolved in the blood and other tissues. stream. Also, it is not a good idea for a diver
include chest pain, skin irritation, and mus- If the person returns to normal pressure to fly on a commercial airliner for a day or so
cle cramps. These symptoms may be barely slowly, the extra dissolved nitrogen is after diving. Commercial airliners are pres-
noticed in mild cases, but severe cases can expelled by the lungs and there is no prob- surized to an altitude of about 2,500 m (about
be fatal. The joint pain is often called bends. lem. If the return to normal pressure is too 8,200 ft). This is safe for the normal passenger
DCS can occur in any situation in rapid, however, nitrogen bubbles form in but can cause more decompression problems
which a person is subjected to a higher air the blood and other tissues, and this causes for the recent diver.
pressure for some period of time and then DCS. The nitrogen bubbles can cause pres- DCS can be treated by placing the
experiences a rapid decompression. Here sure on nerves, block circulation, and affected person in a specially sealed chamber.
is what happens: Air is about 79 percent cause joint pain. These symptoms usually Pressure is slowly increased in the chamber to
nitrogen and 21 percent oxygen. Nitro- appear when the diver returns to the sur- cause nitrogen bubbles to go back into solu-
gen is inert to the human body, and what face or within 8 hours of returning to nor- tion, then slowly decreased to allow the dis-
we inhale is exhaled, but some becomes mal air pressure. solved nitrogen to be expelled by the lungs.

be compared with identifying amounts in the grocery store by saturated solution. A saturated solution is a state of equilibrium
the dozen. You know that a dozen is 12 of something. Now you that exists between dissolving solute and solute coming out of solu-
23
know that 1 mole is 6.02 × 10 of whatever you are mea suring. tion. You actually cannot see the dissolving and coming out of
Chemists use a measure of concentration that is convenient solution that occur in a saturated solution because the exchanges
for considering chemical reactions of solutions. The measure is are taking place with particles the size of molecules or ions.
based on moles of solute since a mole is a known number of Not all compounds dissolve as sugar does, and more or less
particles (atoms, molecules, or ions). The concentration term of a given compound may be required to produce a saturated
of molarity (M) is defined as the number of moles of solute solution at a particular temperature. In general, the difficulty of
dissolved in 1 L of solution. Thus, dissolving a given compound is referred to as solubility. More spe-
cifically, the solubility of a solute is defined as the concentration
__
moles of solute
molarity (M) = that is reached in a saturated solution at a particular temperature.

liters of solution
Solubility varies with the temperature, as the sodium and potas-
equation 11.3 sium salt examples show in Figure 11.9. These solubility curves
describe the amount of solute required to reach the saturation
An aqueous solution of NaCl that has a molarity of 1.0 contains
equilibrium at a particular temperature. In general, the solubili-
1.0 mole NaCl per liter of solution. To make such a solution, you
ties of most ionic solids increase with temperature, but there are
place 58.5 g (1.0 mole) NaCl in a beaker, then add water to make
exceptions. In addition, some salts release heat when dissolved in
1 L of solution.
water, and other salts absorb heat when dissolved. The “instant
cold pack” used for first aid is a bag of water containing a second
SOLUBILITY bag of ammonium nitrate (NH 4 NO 3 ). When the bag of ammo-
There is a limit to how much solid can be dissolved in a liquid. nium nitrate is broken, the compound dissolves and absorbs heat.
You may have noticed that a cup of hot tea will dissolve several You can usually dissolve more of a solid, such as salt or sugar,
teaspoons of sugar, but the limit of solubility is reached quickly as the temperature of the water is increased. Contrary to what
in a glass of iced tea. The limit of how much sugar will dissolve you might expect, gases usually become less soluble in water as
seems to depend on the temperature of the tea. More sugar added the temperature increases. As a glass of water warms, small bub-
to the cold tea after the limit is reached will not dissolve, and solid bles collect on the sides of the glass as dissolved air comes out of
sugar granules begin to accumulate at the bottom of the glass. At solution. The first bubbles that appear when warming a pot
this limit, the sugar and tea solution is said to be saturated. Dis- of water to boiling are also bubbles of dissolved air coming out
solving does not actually stop when a solution becomes saturated, of solution. This is why water that has been boiled usually tastes
and undissolved sugar continues to enter the solution. However, “flat.” The dissolved air has been removed by the heating. The
dissolved sugar is now returning to the undissolved state at the “normal” taste of water can be restored by pouring the boiled
same rate as it is dissolving. The overall equilibrium condition water back and forth between two glasses. The water dissolves
of sugar dissolving as sugar is coming out of solution is called a more air during this process, restoring the usual taste.

11-9 CHAPTER 11 Water and Solutions 283