393

Water and SolutionsIntroduction to Chapter 23

Many of the foods you eat and the products you use (like shampoo) are solutions orother types of mixtures. In this chapter you will learn about solutions and solubility.Since many solutions are critical to the human body, you will also learn howsolutions affect health and athletic performance.

Investigations for Chapter 23

In this Investigation you will construct an apparatus to view the Tyndall effect. TheTyndall effect is a test for determining the characteristics of a mixture. Your testswill tell you whether the mixture is a solution, colloid, or suspension.

In this Investigation you will design three methods for dissolving rock salt in waterand calculate the dissolving rates for each method. Your tests will determine theeffect of changing conditions on solubility, such as heat and stirring.

When you want to dissolve sugar in water, it helps to heat things up. In thisInvestigation, you will observe how temperature influences how fast a substancedissolves. Using your observations, you will develop an explanation for howtemperature affects solubility. In addition, using carbonated water and a balloon,you will have the opportunity to explore how pressure affects the solubility of a gasin a liquid.

23.1 What is a Solution? Can you identify mixtures as solutions,suspensions, or colloids?

23.2 Dissolving Rate How can you influence dissolving rates?

23.3 Solubility What factors affect solubility?

Chapter 23Solutions

8

Chapter 23: Solutions

394

Learning Goals

In this chapter, you will:

Categorize mixtures as solutions, suspensions, or colloids.

Define solubility.

Describe saturated, unsaturated, and supersaturated solutions.

Define and calculate dissolving rate.

List factors which influence dissolving rate.

Evaluate the effectiveness of different methods of influencing dissolving rates.

Explain how temperature and pressure influence solubility.

Understand solubility values.

Interpret temperature-solubility graphs.

Vocabulary

alloys equilibrium solubility value supersaturatedatmospheres hydrated solutes systemcolloid saturated solution Tyndall effectdissolved solubility solvent unsaturateddissolving rate

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23.1 What is a Solution?

Figure 23.1: The variety of containers for bottled water. Spring water and seltzer are often purchased for drinking. Distilled water is used in steam irons and for performing experiments in the laboratory. Why would distilled water be a good choice for these uses?

23.1 What is a Solution?If you walk down the beverage aisle of your local grocery store, you might be surprised by the manydifferent ways water is bottled for sale. You might see mineral water, spring water, potable water,distilled water, and carbonated (or seltzer) water. To complicate matters more, there may be severalbrands of each kind of water to choose from! Is there truly any difference between them?

What is in bottled water?

Bottled watercontains morethan pure H20

The types of bottled water mentioned above have unique characteristics. Mineralwater, potable water (that is, water suitable for drinking), and carbonated watercontain elements other than pure H20.

Mineral watercontains naturally

present minerals

Mineral water, according to government regulations, must contain at least 250milligrams per liter of dissolved minerals such as calcium, potassium, andmagnesium. The minerals must be present naturally and cannot be added at thebottling plant to make mineral water. Also, the water must be collected from anunderground source.

Potable watercontains additives

Potable water is bottled from a city or town water source. In addition to minerals,potable water may contain sodium fluoride, chlorine, or other additives.Consumers sometimes purchase this water if they know their home plumbingcontains lead pipes or lead solder and they want to avoid ingesting any traces oflead that might be present in their tap water.

Distilled wateris nearly

mineral-free

Distilled water has had most of the minerals removed. First, the water is boiled.The minerals are left behind when the water molecules enter the gas phase. Thewater vapor is then collected and cooled to room temperature so that it exists onceagain as a liquid. Sometimes the water is further purified by processes calleddeionization and reverse osmosis. Salt water is converted to fresh water usingthese processes. However, even treated water contains traces of other elements.

Carbonated water Carbonated water contains carbon dioxide gas evenly distributed throughout theliquid to make it bubbly.

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Types of bottled water are examples of solutions

A solution ishomogeneous at

the molecularlevel

In chemistry terms, we call mineral water, potable water, tap water, carbonatedwater, and even distilled water solutions. A solution is a mixture of two or moresubstances that is homogeneous at the molecular level. The word homogeneousmeans the particles in the water are evenly distributed. For example, in mineralwater, there are no clumps of hundreds of mineral ions. The particles in a solutionexist as individual atoms, ions, or molecules. Each has a diameter between 0.01and 1.0 nanometer.

An alloy isa solution of two

or more metals

Although we often think of solutions as mixtures of solids in liquids, solutionsexist in every phase, be it solid, liquid, or gas. Carbonated water is a solution of agas in a liquid. Fourteen-karat gold is a solution of two solids, silver and gold.“Fourteen-karat” means that 14 out of every 24 atoms in the solution are goldatoms. Likewise, ten-karat means that 10 out of every 24 atoms in the solution aregold. Solutions of two or more metals are called alloys.

A solution isa mixture of solute

dissolved ina solvent

Scientists generally refer to the component of the mixture that is present in thegreatest amount as the solvent. The remaining components are called the solutes.When the solute particles are evenly distributed throughout the solvent, we saythat the solute has dissolved.

Table 23.1: Different solutions

Solution Solvent Solute(s) State of solution

air nitrogen (gas) O2, CO2, He, H2, H20, etc. (gases)

gas

carbonated water water (liquid) CO2 (gas) liquid

saline solution water (liquid) salt (solid) liquid

rubbing alcohol alcohol (liquid) water (liquid) liquid

sterling silver silver (solid) copper (solid) solid

Figure 23.2: Solutions are made when solutes dissolve in solvents. Here, salt is the solute, and water is the solvent.

What is a nanometer?

A nanometer is one-billionthof a meter. It is represented bywriting “nm.” In addition toparticles, wavelengths of lightare measured in nanometers.For example, the range ofwavelengths of visible light is400 to 700 nm.

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23.1 What is a Solution?

Figure 23.3: Mayonnaise is a colloid. Water and silt make a suspension.

Figure 23.4: The Tyndall effect helps you tell the difference between colloids and solutions. Here, the beam of the flashlight is visible as it shines through the colloid in the beaker. The beam would not be visible if the beaker contained a solution.

Colloids and suspensions

Colloid particlesare larger than

those in truesolutions, but

smaller than thosein suspensions

Mixtures such as mayonnaise, egg whites, and gelatin are colloids. They look likesolutions, but the particles in these mixtures, at one to 1,000 nanometers, arelarger than those found in solutions. True solutions contain single atoms andmolecules (less than 1 nanometer in size). By comparison, colloid particles areformed of clusters of atoms or molecules. Nevertheless, colloid particles are toosmall (1-1,000 nanometers) to settle to the bottom of their container. Instead, theystay evenly distributed throughout the mixture because they are constantly tossedabout by the movement of the liquid particles.

Suspensionssettle upon

standing

You may have noticed that when you step into a pond or lake to go swimming, yousuddenly make the water cloudy. Your feet cause the mud on the bottom of thepond or lake to mix with the water. However, if you stand very still, eventually thewater becomes clear again. This is because the individual mud particles sink. Insuspensions like muddy water, the particles are greater than 1,000 nanometers indiameter. Atoms and molecules are much smaller than 100 nanometers.Suspensions are mixtures that settle upon standing (figure 23.3). Suspensions canbe separated by filtering.

The Tyndall effect It isn’t possible to separate colloids by filtering. However, there is a way tovisually distinguish colloids from true solutions. It is called the Tyndall effect. Ifyou shine a flashlight through a jar of a translucent colloid, the particles scatter thelight, making the beam visible. Fog is an example of a colloid. This is why anautomobile’s headlight beams can be seen on a foggy evening.

Table 23.2: Properties of solutions, suspensions, and colloids

Approximate size of solute particles

Solute particles settle

Can be separated by filtering

Particles scatter light

solutions 0.01 - 1.0 nm no no no

colloids 1.0 - 1,000 nm no only with special equipment yes, if transparent

suspensions >1000 nm with time yes yes, if transparent

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Figure 23.5: Water from lakes, rivers, and streams must be treated to make it safe to drink. A microscopic view of two Giardia in water is shown in the upper left corner of the figure. Giardia is a parasite that lives in the small intestine of wild animals. You can become infected with Giardia if you drink untreated water that contains infective cysts, a life stage of Giardia that permits this parasite to spread from animal to animal or from animals to humans.

23.2 Dissolving RateOn long backpacking trips, hikers must make sure that they have a safe, reliable source of drinkingwater. Drinking from an icy cold mountain stream may seem appealing to a hot, tired backpacker, but itcould bring the trip to an unpleasant end. Most streams, rivers, and lakes in the United States containGiardia and other microorganisms that can cause serious intestinal disturbances.

Consequently, wise backpackers always carry water-treatment supplies. One of the safest and leastexpensive ways to purify the water is to add an iodine tablet. Iodine tablets are effective against manymicroorganisms, bacteria, and viruses. These tablets are especially useful because they add so littleweight to the pack and can be used with refreshingly cold water.

What could a thirsty backpacker do to get an iodine tablet to dissolve faster? In this section, you willlearn about factors that affect the dissolving rate of various substances.

Molecular motion and dissolving rate

Stirring a mixturespeeds dissolving

One of the simplest ways to increase the dissolving rate of the iodine tablet is tostir the water or shake the water bottle. To understand why this method works, weneed to take a look at what is happening on a molecular level.

A solute such as iodine can dissolve when solvent molecules collide with clumpsof solute particles. In this case, water molecules collide with iodine ions in thetablet. Stirring helps this process by increasing the rate of the collisions. The resultof collisions between molecules is that solute particles are “pulled” into solutionand surrounded by solvent molecules.

Stirring exposesfresh solvent tonewly exposed

solute

Stirring a solution does more that just increase collisions between solute andsolvent molecules. Stirring moves the molecules around. Solvent molecules thathave collided with the solute have to get out of the way so that new ones cancollide with the exposed surface of the solute.

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23.2 Dissolving Rate

Surface area and dissolving rate

Crushing a solutetablet increasesthe surface areaexposed to the

solvent

If the hiker were to crush the iodine tablet, the dissolving rate would increasedramatically. When the tablet is whole, many billions of iodine atoms remaincompletely surrounded by other iodine atoms, but the atoms inside the tablet areprotected from the water molecules. Crushing the tablet increases the surface areathat is available for solvent molecules to interact with solute molecules.

Here is a 1-centimeter cube. This means that each edge of thecube is 1 centimeter wide. The area of each face of the cube is1 cm by 1 cm or 1 cm2. Cubes have six faces. Therefore, thetotal surface area of the cube is: 6 × 1 cm2 = 6 cm2.

Every time thepiece is dividedthe surface area

increases

What do you think happens to surface area if we cut thecube in half? Now, in addition to the original 6 cm2, youhave added two additional faces of 1 cm2 each, for atotal surface area of 8cm2.

Cut the halves of the cube in half, and you havefour new faces for a total surface area of 12 cm2.

Stack up the four pieces and make avertical cut down the center. You haveadded two new faces, for a totalsurface area of 14 cm2.

Cut each of your new stacks in half, to add four more faces. Now you have18 cm2, or three times your original surface area. Imagine how much greater thesurface area would be if you crushed the cube into a powder!

Disinfecting with iodine

Iodine, like all halogens(fluorine, chlorine, andbromine), has seven valenceelectrons. It reacts withsubstances to gain an electronand satisfy the “octet rule”—the need for eight electrons inthe outermost energy level ofan atom.

Halogens are so reactive thatthey can be used asdisinfectants. However, of thehalogens, iodine (the only oneexisting as a solid at roomtemperature) and chlorine (incombination with otherelements) are the most easilyused in this way. In theproper concentrations,chlorine and iodine tabletsdissolved in water can killmicroorganisms like Giardiawithout being toxic to theperson drinking the water.

Fluorine and bromine are notuseful for sterilizing water.Fluorine is a highly reactivegas, and bromine is a liquidthat is very damaging to skin.

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Figure 23.6: Grinding substances to make medicines is an ancient practice that is still used today. Powdered substances dissolve quickly.

Figure 23.7: Timed-release capsules contain microencapsulated medicine that appear as tiny beads within the capsule. The different dissolving rates of the beads allow you to take less medicine to feel better.

Timed-release capsules: making less medicine more effective

Timed-releasemedicine

Have you ever taken cold or allergy medicine in the form of a clear capsule withmulti-colored round beads inside? If so, you are familiar with timed-releasemedicine. Understanding dissolving rates made the invention of this type ofmedicine possible.

What is micro-encapsulation?

There are several ways to manufacture timed-release medication. One of the mostcommon is called microencapsulation. Using this method, pharmaceuticalmanufacturers divide a dose of medicine into tiny particles. Some of theseparticles are placed into the capsule unchanged. These particles of medicinebecome active in the patient’s body soon after the capsule is swallowed. Theremaining particles are coated with a polymer usually derived from gelatin,cellulose, or silicone. The coating dissolves slowly in the stomach, releasing themedicine inside over a period of time. By changing the thickness or varying thecoating material, manufacturers alter the dissolving rate of the coating. Thisallows them to control the amount of time it takes for the medicine to be releasedin the body. The different colored beads inside the capsule show the differentcoatings used to encapsulate the medicine particles.

Timed-releasecapsules mean that

you can take lessmedicine to feel

better

Timed-release capsules have several advantages. First, they make smaller doses ofmedicine more effective. Previously, when a patient swallowed a dose ofmedicine, there would be a peak level in the body followed by a steady declineuntil the next pill was swallowed. To maintain the minimum effective amount ofthe medicine in the body at all times, the peak level had to be significantly higherthan the ideal dose. This meant the patient had to consume larger quantities of themedicine, thereby increasing the risk of side effects. Timed-release capsules helpto ensure a steady supply of the minimum effective dose.

Timed-releasecapsules are

safer and morecost-effective

Timed-release capsules are safer and more convenient for the patient. Forexample, a patient who previously had to take a pill four times a day might switchto a once-a-day, timed-release capsule. Taking only one capsule reduces thechances of forgetting how much medicine you have taken.

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23.3 Solubility

Figure 23.8: With vigorous exercise, you can lose up to a half-gallon of water per hour by sweating and exhaling.

23.3 SolubilityThe human body is mostly water. For every hour of vigorous exercise, you may lose as much as a half-gallon of your body’s water supply through sweating and exhaling! You also lose small amounts ofsalts, lactic acid, and urea when you sweat. Lactic acid and urea are the breakdown products of sugarand proteins, respectively. The more you exercise, the more water and salts you lose, and the more youbreak down sugar and protein. You can replenish lost fluid by drinking water. To quickly replace saltsand sugar as well, many athletes consume sports drinks. Sweat and sports drinks are both examples ofsolutions—both are mostly water with dissolved substances. In this section, you will learn about thefactors that affect how solutes dissolve in solutions.

Using systems to talk about solutions

Systems arecollections of

matter andprocesses that can

be studied

Your body is a system. A system is a collection of matter and processes that takeplace in a certain space and can be observed and studied. Your body with itsnumerous metabolic activities is an excellent example of a system that is “open.”This means that your body constantly interacts with its environment by taking inand releasing substances. For example, you eat food, exhale carbon dioxide, andgive off heat.

Systems can beopen or closed

Scientists often find open systems difficult to work with because it is hard tocontrol the variables in open systems. Scientists have an easier time studyingclosed or nearly closed systems. A reaction that takes place in a stoppered test tubeis a good example of a closed system.

A solute and asolvent make up a

system

The system of a solution includes the solute and the solvent. The kind of containerthat holds the solute and solvent is not important. For the rest of this section, thefactors that affect the system of a solute and a solvent will be discussed.

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What happens when a solute dissolves in a solvent?

NaCl isan example of

a solute

Let’s use sodium chloride (NaCl) as an example of a solute being dissolved in thesolvent, water. If you look closely at a single crystal of NaCl, you will notice thatit is a cube. Millions of sodium (Na) and chlorine (Cl) atoms, each too small to seewith your naked eye, are a part of a single crystal of NaCl.

The bond betweenNa and Cl is ionic

Within a crystal of NaCl, ionic bonds are formed between Na and Cl. Theoxidation state of Na is (1+), and the oxidation state of Cl is (1-). Na and Cl are agood match because their oxidation states add up to zero: (1+) + (1-) = 0. Thismeans that a single NaCl molecule is neutral and stable. However, the Cl end ofthe molecule is more negatively charged than the Na end because the Cl hasattracts an electron away from Na to form the ionic bond.

Water moleculesbond witheach other

Although a water molecule does not have ionic bonds, the molecule does have apartially charged positive end and a partially charged negative end. Watermolecules weakly connect to each other by matching their partial positive end tothe partial negative end of a neighboring molecule. These links between watermolecules are called hydrogen bonds.

Water moleculeshydrate Na and Cl

When a NaCl crystal is mixed with water, a reaction occurs. The partially chargedends of the water molecules are attracted to Na and Cl in the crystal. The processresults in the formation of Na+ and Cl- ions that are completely surrounded bywater molecules. When this happens, we say the ions are hydrated and write “aq”next to the ions. “Aq” stands for aqueous which refers to a water solution.Hydrated Na(aq)

+ and Cl(aq)- ions are able to freely move in a solution.

Hydrationcontinues until the

solution issaturated

As one layer of molecules on a NaCl crystal is brought into solution by watermolecules, another layer is exposed. If the conditions are right, this processcontinues until the entire crystal is dissolved. It seems, in fact, to havedisappeared! All that has happened, however, is that Na+ and Cl- ions from NaClhave been separated and completely surrounded by water molecules.

Figure 23.9: When a crystal of NaCl molecules is mixed with water, hydrated Na+ and Cl- ions are formed.

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Chapter 23

23.3 Solubility

Figure 23.10: Solubility values for common substances.

Common name

Solubility at 25 °C

(grams per100 mL H2O)

table salt

(NaCl)

37.7

sugar

(C12H22O11)

200

baking soda

(NaHCO3)

approx. 10

chalk

(CaCO3)

insoluble

talc

(Mg silicates)

insoluble

Solubility

What issolubility?

The term solubility means the amount of solute that can be dissolved in a specificvolume of solvent under certain conditions. A solute’s solubility depends on thechemical nature of the solvent. Another important factor that influences solubilityis the temperature of the system (the solute and solvent).

Volume affectssolubility

For a solute to dissolve completely, you need a certain volume of solvent. Thevolume of solvent provides enough solvent molecules to surround all the solutemolecules. For example, to dissolve an amount of sodium chloride (NaCl), youneed enough water molecules to pull apart and surround all the Na+ and Cl- ions.

The solubility of asolid usually

increases withtemperature

The solubility of a solid substance usually increases as temperature increases. Theeffect of temperature on solubility has to do with molecular motion and the energyof the solute-solvent system. At higher temperatures, molecules move faster sothat there are more molecular collisions between solute and solvent molecules.The rate of collisions between these molecules is usually directly related to therate at which the solute dissolves.

Solubility values The solubility value for table salt (NaCl) is 1 gram per 2.8 milliliters of water at25°C. The solubility value for NaCl tells you how much can dissolve in a certainvolume (or, sometimes, mass) of water as long as the water is at 25°C. Using thisinformation, how much salt would dissolve in 280 milliliters of water at thattemperature? If you said 100 grams, you are correct!

Some substancesdo not dissolve in

water

The table in figure 23.10 shows the solubility values for common substances.Notice that chalk and talc do not have solubility values. Because these substancesdo not dissolve in water, they are said to be insoluble.

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Temperature-solubility graphs

Temperature-solubility graphsshow how much

substancedissolves at a

given temperature

The solubility values for solutes are easily determined if you have a temperature-solubility graph. The y-axis on these graphs represents how many grams of solute(in this case, salts) will dissolve in 100 milliliters of water. The x-axis representstemperature in degrees Celsius.

You will notice in the graph that the salts (NaCl, KNO3, NaNO3) dissolvedifferently as temperature increases. For something to dissolve in water, the watermolecules need to break the bonds between the solute molecules. Water dissolvessubstances differently because the chemical bonds between atoms are not all thesame.

Interpreting thegraph

The graph above is a temperature-solubility graph for sodium chloride (NaCl),potassium nitrate (KNO3), and sodium nitrate (NaNO3). The solubility of NaCldoes not change much as temperature increases. The effect of temperature on thesolubility of KNO3 and NaNO3 is more noticeable. More KNO3 and NaNO3 willdissolve in 100 milliliters of water at higher temperatures than NaCl.

Example

How many grams of potassium

nitrate (KNO3) will dissolve in 200 mL

of water at 60oC?

Solution:

(1) You are asked for the mass in

grams of solute.

(2) You are given temperature and

volume.

(3) The relationship between solubility

and temperature for KNO3 can be

seen on a graph to the left.

(4) From the graph, you see that

110 grams of KNO3 dissolve in

100 mL of water at 60oC.

(5) Plug in numbers.

200 mL / 100 mL = 2

2 x 110 g = 220 g

(6) Answer:

220 grams of KNO3 will dissolve in

200 mL of water at 60oC.

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23.3 Solubility

Figure 23.11: The CO2 in a can of soda like ginger ale has been dissolved in water with the use of pressure.

How did soda get its name?

In 1767, Joseph Priestly, anEnglish chemist best knownfor discovering oxygen,figured out how to artificiallycarbonate beverages.Initially, carbon dioxide wasobtained by acidifyingbaking soda (sodiumbicarbonate). This is why weoften use the name “soda”for carbonated beverages.

The solubility of gases

For the same reasons that temperature tends to increase the solubility of solids in liquids, temperaturetends to decrease the solubility of gases in liquids. You may have noticed that a can of soda at roomtemperature is more likely to fizz and spill over when opened than a cold can of soda. As temperatureincreases, the gas molecules and water molecules begin to move around more. The increased motionmeans that more dissolved gas molecules encounter the surface of the soda and escape.

Pressure influences the solubility of gases

The solubility ofgas depends on

temperature andpressure

The solubility of gases depends on temperature and pressure. When you drinkfizzy, carbonated beverages, you are consuming carbon dioxide (CO2) that hasbeen dissolved in water with the use of pressure (figure 23.11). Soda is fizzybecause carbon dioxide has been dissolved in the liquid by using pressure. Whenyou pop the tab on a can of soda, you release pressure. You can hear carbondioxide rapidly escaping. Shaking a can of soda before opening it also forces somecarbon dioxide to come out of solution by getting more carbon dioxide moleculesto the surface of the liquid.

Fish and otheraquatic organisms

in lakes, rivers,and oceans need

dissolved oxygento live

Dissolved oxygen is an important component of lake, river, and ocean water.Oxygen is mixed into the water through wave action and produced by underwaterplants as a by-product of photosynthesis. When the water temperature rises, theamount of dissolved oxygen decreases. Less dissolved oxygen means less oxygenfor fish, and they depend on this dissolved oxygen for respiration. When theweather is very warm, fish stay near the bottom of ponds and rivers where there iscooler, more oxygenated water.

How temperatureaffects the amount

of dissolvedoxygen in water

Electrical generating facilities are often built near bodies of water so that theyhave an inexpensive source of water for their cooling system. However, when thiswater is discharged back into the river or bay while it is still warm, it cansignificantly reduce the amount of dissolved oxygen available in the waterway. Atthe same time, the warming of the water increases the metabolic rate of the fish sothat their need for dissolved oxygen increases. The combination of these twofactors can spell trouble for the fish and cause large disturbances to the localecosystem.

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How much will dissolve?

The solubility of asubstance stops at

equilibrium

When talking about solubility, equilibrium is the balance of solute moleculescoming and going from a solution for a given set of conditions. In other words, theprocess of dissolving a solute is a “two-way street.” For every set of conditions, asolute will dissolve in and come out of solution at a certain rate. When the rate ofdissolving equals the rate of coming out of solution, we say equilibrium has beenreached (figure 23.12).

Saturated meansthe maximum

amount hasdissolved

One way to describe a solution that contains 100 grams of NaCl in 280 millilitersof water at 25°C is to say that it is saturated, meaning no more salt will dissolveunder these conditions. If you raise the temperature of this system, however, youwill be able to dissolve a little more salt. What happens when the solution coolsback down again? Some of the dissolved salt will recrystallize (figure 23.12).

Unsaturatedmeans more solute

can be dissolved

This sequence of processes also describes how rock candy is made. Rock candyconsists of large sugar crystals usually attached to a rough surface such as a pieceof cotton string. The candy is made by heating water to boiling and then stirring ingranulated sugar. As long as the sugar dissolves, the solution is said to beunsaturated. When no more sugar will dissolve, the solution is said to besaturated. Next, the saturated, sugar-water solution is poured into a jar with asuspended cotton string. As the solution cools, it becomes supersaturated.

Supersaturatedmeans more isdissolved than

normally possible

Supersaturated solutions are unstable. In this case, if the jar of supersaturatedsugar-water is jiggled, the suspended string moves even slightly, or anothergranule of sugar is dropped in, crystals of solute begin to form. The crystals stickto the rough surface of the string. After five to seven days, all the excess sugarreturns to its solid form, and the string is covered with large crystals. The solutionleft behind now contains only the amount of sugar that can remain dissolved atroom temperature. It is once again a saturated solution.

Figure 23.12: A solute dissolves until equilibrium is reached. This diagram shows gas molecules (the open circles) dissolving and coming out of solution at the surface of the solution. At the bottom of the glass, molecules of a solid (the closed circles) are dissolving and recrystallizing.

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23.3 Solubility

�Science in the Real World: Scuba diving

You now know that gases dissolve in liquids and that pressure is an important factor in determininghow much gas will dissolve in a liquid. In scuba diving, a deep descent underwater poses someproblems that have to do with the solubility of gases in blood.

Atmospheric pressure is measured in units called atmospheres. The abbreviation of this unit is “atm.”At the Earth’s surface, atmospheric pressure is 1 atm. The pressure increases by 1 every 10 meters(about 33 feet) as a diver descends through sea water. In other words, at a depth of 10 meters, thepressure acting on the diver has doubled to 2 atmospheres, or twice what we are used to on the Earth’ssurface. At 30 meters (99 feet), the pressure has quadrupled to 4 atm. Because one atm is equal to14.5 pounds per square inch (psi), at 40 meters, you would be under 72.5 psi. That’s equal to abouttwice as much air pressure as a car tire!

Because a diver is under increased pressure during a dive,the concentrations of gases in the blood and tissues of thediver are higher. The diver can easily process oxygen andcarbon dioxide. However, nitrogen is an unreactive gas. Itstays in the tissues when a diver is in deep water. Highconcentrations of nitrogen in the body cause a conditioncalled nitrogen narcosis. This causes divers to be eitherextremely carefree, extremely suspicious, or fearful. Ineither case, the diver loses his or her ability to function

safely underwater. Diving partners (called “dive buddies”) keep up constant communication to checkthat they are not confused because of nitrogen narcosis. The best way to treat nitrogen narcosis is toslowly rise to the water surface with a partner or dive buddy. A slow ascent with normal breathingallows gases to come back out of the blood and tissues easily. Scuba divers should never hold theirbreath underwater. Expanding gases as a diver rises to the surface can rupture lung tissue!

Decompression sickness occurs when body tissues get supersaturated with nitrogen. Bubbles ofnitrogen form in the bloodstream and tissues. These bubbles can block important arteries and cause astroke or heart attack. When they are trapped in one’s joints or back or abdomen, the bubbles arepainful. To release pressure in the back or stomach due to bubble formation, individuals withdecompression sickness bend over, which is why decompression sickness is often called “the bends.”

Figure 23.13: How pressure changes with water depth.

Depth(meters)

Pressure (atm)

0 110 220 330 440 5

How did scuba get started?

SCUBA stands for self-contained underwaterbreathing apparatus. Anumber of inventors havecontributed to developing thetechnology for scuba diving.The invention of theaqualung by Jaques-YvesCousteau and Emile Gagnanin 1943 made scuba divingavailable to anyone whowanted to do underwaterexploring. This device madebreathing air underwatereasy, safe and reliable.

Chapter 23 Review

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Chapter 23 Review

Vocabulary review

Match the following terms with the correct definition. There is one extra definition in the list that will not match any of the terms.

Set One Set Two1. solution a. A substance particles that dissolves in a solvent 1. dissolve a. Contains less than the maximum amount of

solute that can dissolve for a given set of conditions

2. solvent b. A solution of two or more metals 2. solubility b. To spread a solute evenly throughout a solvent

3. solute c. A mixture of two or more substances that is homogeneous at the molecular level

3. unsaturated c. Unstable solution containing more solute than will usually dissolve for a given set of conditions

4. suspension d. A mixture that cannot be separated by filtering but does scatter light rays

4. saturated d. To undergo a phase change

5. colloid e. The component of a solution that is present in the greatest amount

5. supersaturated e. Contains the maximum amount of solute that will dissolve for a given set of conditions

f. A mixture that will separate if left to stand for a period of time

f. The amount of solute that will dissolve in a given amount of solvent for a given set of conditions

Set Three1. soluble a. A substance in a solution that is in the smallest

amount

2. solubility value b. The ability of a substance to be dissolved by a solvent

3. equilibrium c. The state of the formation of a solution in which as many solute molecules are dissolving as are coming back out of the solution

d. A number indicating the mass of solute that can dissolve in a mass or volume of solvent at a certain temperature

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Chapter 23 Review

Concept review

1. Give an example of a solution in which the solute is not a solidand the solvent is not a liquid.

2. Name two ways to distinguish between suspensions andcolloids.

3. What would happen to the solubility of potassium chloride inwater as the water temperature increased from 25°C to 100 °C?

4. What happens to the solubility of oxygen in a pond as the pondtemperature decreases from 25°C to 10°C?

5. When you open a can of orange soda at room temperature, whyis it more likely to fizz and spill over than a can that has beenrefrigerated?

6. What happens to a supersaturated solution when more solute isadded?

7. Name three ways to increase the dissolving rate of sugar inwater.

8. What information goes on the x-axis and on the y-axis of atemperature-solubility graph?

9. A piece of rock candy in a sugar solution is at equilibrium withthe solution. What does this mean on a molecular level? In otherwords, what are the rock candy molecules doing, and what arethe sugar molecules in solution doing? You may want to draw apicture to help answer this question.

Problems

1. If you had a cube measuring 10 centimeter per side, what is itstotal surface area?

2. If you cut the cube into eight identical small cubes, what is itstotal surface area?

3. One solubility value of NaCl is 1 gram/2.8 milliliters of water at25°C. Use this value to figure out the following: (a) The volumeof water needed to dissolve 100 grams of NaCl, (b) The mass ofNaCl that would dissolve in 100 ml of water.

4. Use the temperature-solubility graph on page 404 to answer thefollowing questions:

a. What is the solubility value of NaNO3 at 25°C?

b. Which salt has the highest solubility value at 75°C?

c. Temperature affects the solubility of which salt the least?

d. Temperature affects the solubility of which salt the most?

Chapter 23 Review

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�Applying your knowledge

1. Create an interesting handout for fourth graders that explainshow to make rock candy. The handout should include safetyinstructions, including supervision, and definitions of thesewords: dissolve, unsaturated, saturated, and supersaturated.

2. Part of a drug manufacturer’s job is designing medicines thatare easy to take and work effectively.

a. Design a medicine that dissolves quickly in water. Whatfeatures allow it to dissolve quickly?

b. Design a timed-release medicine. What features allow themedicine to be released in small doses once it is taken?

3. In scuba diving, a counting test is performed by dive buddies tocheck to see if both buddies are thinking clearly. The test isperformed correctly when the buddies are able to counttogether with their fingers because in scuba diving, it is easierto communicate with hands than with voices. For example, ifone buddy holds up two fingers, the other buddy should hold

up three fingers. If one buddy holds up four fingers, the otherbuddy should hold up five fingers, and so on. Use thisinformation to answer this question:You are scuba diving with a buddy in deep water and decide tocheck if she is all right by doing the counting test with her. Youhold up two fingers. She should hold up three fingers, but sheholds up five fingers! You suspect your buddy has nitrogennarcosis. You decide to take your buddy slowly to the surface.At the surface, you check again to see if she is all right. Youhold up two fingers, and she holds up three fingers. You arerelieved she is thinking clearly! When someone asks whathappened, you tell the story in detail. Explain what you thinkhappened to your friend and why. Also, explain why youbrought her to the surface slowly instead of quickly.

4. Why should scuba divers never hold their breath whenunderwater?

Rock Candy Recipe

You will need: 1 kg sugar, 450 mL water, saucepan, 2 liter glass jar, candy thermometer, cotton string,plastic wrap, pencilPrepare the jar: Tie one end of the clean cotton string to the middle of the pencil. Cut the string to a lengthequal to the height of the jar. Tie a knot in the end of the string to serve as a small weight. Place the pencilon top of the jar so that the string is suspended inside. The string should not touch the sides or bottom of the jar. Heat the water to boiling. Slowly stir in the sugar until the solution is saturated. When no additional sugar will dissolve, pour the solutioninto the jar. Make sure the string does not stick to the side of the jar. Cover with plastic wrap.Set the jar aside where it can rest undisturbed for 5 to 7 days. At the end of this time, remove the string from the solution. Examine thelarge crystals with a magnifying glass.