Like Dissolves Like Hydrophilic and Hydrophobic Molecules
Soaps, Detergents, and Dry-Cleaning Agents Units of Concentration

Like Dissolves Like

By convention, we assume that one or more solutesdissolve in a solvent to form a mixtureknown as the solution. The photographsthat accompany this section illustrate what happens when we add apair of solutes to a pair of solvents.


The solutes have two things in common. They are both solids,and they both have a deep violet or purple color. The solventsare both colorless liquids, which do not mix.

The difference between the solutes is easy to understand.Iodine consists of individual I2 molecules heldtogether by relatively weak intermolecular bonds. Potassiumpermanganate consists of K+ and MnO4-ions held together by the strong force of attraction between ionsof opposite charge. It is therefore much easier to separate the I2molecules in iodine than it is to separate the ions in KMnO4.

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There is also a significant difference between the solvents:CCl4 and H2O. The difference between theelectronegativities of the carbon and chlorine atoms in CCl4is so small (EN = 0.56) that there is relatively littleionic character in the CCl bonds.

Even if there was some separation of charge in these bonds,the CCl4 molecule wouldn"t be polar, because it has asymmetrical shape in which the four chlorine atoms point towardthe corners of a tetrahedron, as shown in the figure below. CCl4is therefore best described as a nonpolar solvent.


The difference between the electronegativities of the hydrogenand oxygen atoms in water is much larger (EN = 1.24),and the HO bonds in this molecule are therefore polar. If the H2Omolecule was linear, the polarity of the two OH bondswould cancel, and the molecule would have no net dipole moment.Water molecules are not linear, however, they have a bent, orangular shape. As a result, water molecules have distinctpositive and negative poles, and water is a polar molecule, asshown in the figure below. Water is therefore classified as a polarsolvent.

Because water molecules are bent, or angular, they have distinct negative and positive poles. H2O is therefore an example of a polar solvent

Because the solvents do not mix, when water and carbontetrachloride are added to a separatory funnel, two separateliquid phases are clearly visible. We can use the relativedensities of CCl4 (1.594 g/cm3) and H2O(1.0 g/cm3) to decide which phase is water and whichis carbon tetrachloride. The denser CCl4 settles tothe bottom of the funnel.

When a few crystals of iodine are added to the separatoryfunnel and the contents of the funnel are shaken, the I2dissolves in the CCl4 layer to form a violet-coloredsolution. The water layer stays essentially colorless, whichsuggests that little if any I2 dissolves in water.

When this experiment is repeated with potassium permanganate,the water layer picks up the characteristic purple color of theMnO4- ion, and the CCl4 layerremains colorless. This suggests that KMnO4 dissolvesin water but not in carbon tetrachloride. The results of thisexperiment are summarized in the table below.

Solubilities of I2and KMnO4 in CCl4and Water

This table raises two important questions. Why does KMnO4dissolve in water, but not carbon tetrachloride? Why does I2dissolve in carbon tetrachloride, but not water?

It takes a lot of energy to separate the K+ and MnO4-ions in potassium permanganate. But these ions can form weakbonds with neighboring water molecules, as shown in the figurebelow.

KMnO4 dissolves in water because the energy released when bonds form between the K+ ion and the negative end of the neighboring water molecules and between the MnO4- ion and the positive end of the solvent molecules compensates for the energy it takes to separate the K+ and MnO4- ions.

The energy released when these bonds form compensates for theenergy that has to be invested to rip apart the KMnO4crystal. No such bonds can form between the K+ or MnO4-ions and the nonpolar CCl4 molecules. As a result,KMnO4 can"t dissolve in CCl4.

The I2 molecules in iodine and the CCl4molecules in carbon tetrachloride are both held together by weakintermolecular bonds. Similar intermolecular bonds can formbetween I2 and CCl4 molecules in a solutionof I2 in CCL4. I2 thereforereadily dissolves in CCl4. The molecules in water areheld together by hydrogen bonds that are stronger than mostintermolecular bonds. No interaction between I2 and H2Omolecules is strong enough to compensate for the hydrogen bondsthat have to be broken to dissolve iodine in water, so relativelylittle I2 dissolves in H2O.

We can summarize the results of thisexperiment by noting that nonpolar solutes (such as I2)dissolve in nonpolar solvents (such as CCl4),whereas polar solutes (such as KMnO4)dissolve in polar solvents (such as H2O). Asa general rule, we can conclude that like dissolves like.

Practice Problem 1:

Elemental phosphorus is often stored under water because it doesn"t dissolve in water. Elemental phosphorus is very soluble in carbon disulfide, however. Explain why P4 is soluble in CS2 but not in water.

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Practice Problem 2:

The iodide ion reacts with iodine in aqueous solution to form the I3-, or triiodide, ion.

I-(aq) + I2(aq)


What would happen if CCl4 was added to an aqueous solution that contained a mixture of KI, I2, and KI3?

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Hydrophilic andHydrophobic Molecules

The family of compounds known as the hydrocarbonscontain only carbon and hydrogen. Because the difference betweenthe electronegativities of carbon and hydrogen is small (EN= 0.40), hydrocarbons are nonpolar. As a result, they do notdissolve in polar solvents such as water. Hydrocarbons aretherefore described as immiscible (literally,"not mixable") in water.

When one of the hydrogen atoms in a hydrocarbon is replacedwith an -OH group, the compound is known as an alcohol,as shown in the figure below. As might be expected, alcohols haveproperties between the extremes of hydrocarbons and water. Whenthe hydrocarbon chain is short, the alcohol is soluble in water.Methanol (CH3OH) and ethanol (CH3CH2OH)are infinitely soluble in water, for example. There is no limiton the amount of these alcohols that can dissolve in a givenquantity of water. The alcohol in beer, wine, and hard liquors isethanol, and mixtures of ethanol and water can have anyconcentration between the extremes of pure alcohol (200 proof)and pure water (0 proof).

The structure of the alcohol known as ethanol.

As the hydrocarbon chain becomes longer, the alcohol becomesless soluble in water, as shown in the table below.

Solubilities of Alcohols in Water

Formula Name Solubility in Water (g/100 g)
CH3OH methanol infinitely soluble
CH3CH2OH ethanol infinitely soluble
CH3(CH2)2OH propanol infinitely soluble
CH3(CH2)3OH butanol 9
CH3(CH2)4OH pentanol 2.7
CH3(CH2)5OH hexanol 0.6
CH3(CH2)6OH heptanol 0.18
CH3(CH2)7OH octanol 0.054
CH3(CH2)9OH decanol insoluble in water

One end of the alcohol molecules has so much nonpolarcharacter it is called hydrophobic (literally,"water-hating"), as shown in the figure below. Theother end contains an -OH group that can form hydrogen bonds toneighboring water molecules and is therefore said to be hydrophilic(literally, "water-loving"). As the hydrocarbon chainbecomes longer, the hydrophobic character of the moleculeincreases, and the solubility of the alcohol in water graduallydecreases until it becomes essentially insoluble in water.

One end of this alcohol molecule is nonpolar, and therefore hydrophobic. The other end is polar, and therefore hydrophilic.

People encountering the terms hydrophilic and hydrophobicfor the first time sometimes have difficulty remembering whichstands for water-hating and which stands for water-loving. If youcan remember that Hamlet"s girlfriend was named Ophelia (notOphobia), you might be able to remember that the prefix philo-is commonly used to describe love for example, in philanthropist,philharmonic, philosopher, and so on.

The data in the table above show one consequence of thegeneral rule that like dissolves like. As molecules become morenonpolar, they become less soluble in water. The table belowshows another example of this rule. NaCl is relatively soluble inwater. As the solvent becomes more nonpolar, the solubility ofthis polar solute decreases.

Solubility of Sodium Chloride in Water andin Alcohols

Formula of Solvent Solvent Name Solubility of NaCl (g/100 g solvent)
H2O water 35.92
CH3OH methanol 1.40
CH3CH2OH ethanol 0.065
CH3(CH2)2OH propanol 0.012
CH3(CH2)3OH butanol 0.005
CH3(CH2)4OH pentanol 0.0018

Soaps, Detergents,and Dry-Cleaning Agents

The soimg.orgistry behind the manufacture of soap hasn"t changedsince it was made from animal fat and the ash from wood firesalmost 5000 years ago. Solid animal fats (such as the tallowobtained during the butchering of sheep or cattle) and liquidplant oils (such as palm oil and coconut oil) are still heated inthe presence of a strong base to form a soft, waxy material thatenhances the ability of water to wash away the grease and oilthat forms on our bodies and our clothes.

Animal fats and plant oils contain compounds known as fattyacids. Fatty acids, such as stearic acid (see figure below),have small, polar, hydrophilic heads attached to long, nonpolar,hydrophobic tails.


Fatty acids are seldom found by themselves in nature. They areusually bound to molecules of glycerol (HOCH2CHOHCH2OH)to form triglycerides, such as the triglyceride known as trimyristin,which can be isolated in high yield from nutmeg, shown in thefigure below.


These triglycerides break down in the presence of a strongbase to form the Na+ or K+ salt of thefatty acid, as shown in the figure below. This reaction is calledsaponification, which literally means "the makingof soap."

The saponification of the trimyristin extracted from nutmeg.

Part of the cleaning action of soap results from the fact thatsoap molecules are surfactants they tendto concentrate on the surface of water. They cling to the surfacebecause they try to orient their polar CO2-heads toward water molecules and their nonpolar CH3CH2CH2...tails away from neighboring water molecules.

Water can"t wash the soil out of clothes by itself because thesoil particles that cling to textile fibers are covered by alayer of nonpolar grease or oil molecules, which repels water.The nonpolar tails of the soap molecules on the surface of waterdissolve in the grease or oil that surrounds a soil particle, asshown in the figure below. The soap molecules therefore disperse,or emulsify, the soil particles, which makes it possibleto wash these particles out of the clothes.

Most soaps are more dense than water. They can be made tofloat, however, by incorporating air into the soap during itsmanufacture. Most soaps are also opaque; they absorb rather thantransmit light. Translucent soaps can be made by adding alcohol,sugar, and glycerol, which slow down the growth of soap crystalswhile the soap solidifies. Liquid soaps are made by replacing thesodium salts of the fatty acids with the more soluble K+or NH4+ salts.

Forty years ago, more than 90% of the cleaning agents sold inthe United States were soaps. Today soap represents less than 20%of the market for cleaning agents. The primary reason for thedecline in the popularity of soap is the reaction between soapand "hard" water. The most abundant positive ions intap water are Na+, Ca2+, and Mg2+ions. Water that is particularly rich in Ca2+, Mg2+,or Fe3+ ions is said to be hard. Hard water interfereswith the action of soap because these ions combine with soapmolecules to form insoluble precipitates that have no cleaningpower. These salts not only decrease the concentration of thesoap molecules in solution, they actually bind soil particles toclothing, leaving a dull, gray film.

One way around this problem is to "soften" the waterby replacing the Ca2+ and Mg2+ ions with Na+ions. Many water softeners are filled with a resin that contains-SO3- ions attached to a polymer, as shownin the figure below. The resin is treated with NaCl until each-SO3- ion picks up an Na+ ion.When hard water flows over this resin, Ca2+ and Mg2+ions bind to the -SO3- ions on the polymerchain and Na+ ions are released into solution.Periodically, the resin becomes saturated with Ca2+and Mg2+ ions. When this happens, it has to beregenerated by being washed with a concentrated solution of NaCl.

When a water softener is "charged," it is washed with a concentrated NaCl solution until all of the -SO3- ions pick up an Na+ ion. The softener then picks up Ca2+ and Mg2+ ions from hard water, replacing these with Na+ ions.

There is another way around the problem of hard water. Insteadof removing Ca2+ and Mg2+ ions from water,we can find a cleaning agent that doesn"t form insoluble saltswith these ions. Synthetic detergents are examples of suchcleaning agents. Detergent molecules consist of long, hydrophobichydrocarbon tails attached to polar, hydrophilic -SO3-or -OSO3- heads, as shown in the figurebelow.

By themselves, detergents don"t have the cleaning power ofsoap. "Builders" are therefore added to syntheticdetergents to increase their strength. These builders are oftensalts of highly charged ions, such as the triphosphate (P3O105-)ion.

Cloth fibers swell when they are washed in water. This leadsto changes in the dimensions of the cloth that can cause wrinkles-- which are local distortions in the structure of the fiber or evenmore serious damage, such as shrinking. These problems can beavoided by "dry cleaning," which uses a nonpolarsolvent that does not adhere to, or wet, the cloth fibers. Thenonpolar solvents used in dry cleaning dissolve the nonpolargrease or oil layer that coats soil particles, freeing the soilparticles to be removed by detergents added to the solvent, or bythe tumbling action inside the machine. Dry cleaning has theadded advantage that it can remove oily soil at lowertemperatures than soap or detergent dissolved in water, so it issafer for delicate fabrics.

When dry cleaning was first introduced in the United Statesbetween 1910 and 1920, the solvent was a mixture of hydrocarbonsisolated from petroleum when gasoline was refined. Over theyears, these flammable hydrocarbon solvents have been replaced byhalogenated hydrocarbons, such as trichloroethane (Cl3C-CH3),trichloroethylene (Cl2C=CHCl), and perchloroethylene(Cl2C=CCl2).

Units ofConcentration

The concentration of a solution is defined asthe amount of solute dissolved in a given amount of solvent orsolution.

There are many ways in which the concentration of a solutioncan be described.

The molarity (M) of a solution isdefined as the ratio of the number of moles of solute in thesolution divided by the volume of the solution in liters.

Practice Problem 3:

At 25oC, a saturated solution of chlorine in water can be prepared by dissolving 5.77 grams of Cl2 gas in enough water to give a liter of solution. Calculate the molarity of this solution.

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A 3.5% solution of hydrochloric acid, for example, has 3.5grams of HCl in every 100 grams of solution. The concentration ofa solution in units of moles per liter can be calculated from themass percent and density of the solution.

It is also possible to describe the concentration of asolution in terms of the volume percent. This unit isused to describe solutions of one liquid dissolved in another ormixtures of gases. Wine labels, for example, describe thealcoholic content as 12% by volume, because 12% of the totalvolume is alcohol.

Molarity is the concentration unit most commonly used bysoimg.orgists. It has one disadvantage. It tells us how much solutewe need to make a solution, and it gives us the volume of the solutionproduced, but it doesn"t tell us how much solvent willbe required to prepare the solution. We can make a 0.100 Msolution of CuSO4, for example, by dissolving 0.100mole of CuSO4 5 H2O in enough water togive one liter of solution. But how much water is enough? Becausethe CuSO4 5 H2O crystals occupy somevolume, it takes less than a liter of water, but we have no ideahow much less.

When it is important to know how much solute and solvent arepresent in a solution, soimg.orgists use two other concentrationunits: molality and mole fraction.

The molality (m) of a solution isdefined as the number of moles of solute in the solution dividedby the mass in kilograms of the solvent used to make thesolution.

A 0.100 m solution of CuSO4,for example, can be prepared by dissolving 0.100 mole of CuSO4in 1 kilogram of water. Because the density of water is about 1g/cm3, or 1 g/mL, the volume of water used to preparethis solution will be approximately one liter. The total volumeof the solution, however, will be larger than 1 liter because theCuSO4 5 H2O crystals will undoubtedlyoccupy some volume. As a result, a 0.100 m solution isslightly more dilute than a 0.100 M solution of the samesolute.

Practice Problem 4:

A saturated solution of hydrogen sulfide in water can be prepared by bubbling H2S gas into water until no more dissolves. Calculate the molality of this solution if 0.385 grams of H2S gas dissolve in 100 grams of water at 20oC and 1 atm.

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Molality has an important advantage over molarity. Themolarity of an aqueous solution changes with temperature, becausethe density of water is sensitive to temperature. Becausemolality is defined in terms of the mass of the solvent, not itsvolume, the molality of a solution does not change withtemperature.

The ratio of solute to solvent in a solution can also bedescribed in terms of the mole fraction of the solute or thesolvent in a solution. By definition, the mole fractionof any component of a solution is the fraction of the totalnumber of moles of solute and solvent that come from thatcomponent. The symbol for mole fraction is a Greek capital letterchi: C. The mole fraction ofthe solute is defined as the number of moles of solutedivided by the total number of moles of solute and solvent.

Conversely, the mole fraction of the solvent is thenumber of moles of solvent divided by the total number of molesof solute and solvent.

In a solution that contains a single solute dissolved in asolvent, the sum of the mole fraction of the solute and thesolvent must be equal to 1.

Csolute+ Csolvent= 1

Practice Problem 5:

Calculate the mole fractions of both the solute and the solvent in a saturated solution of hydrogen sulfide in water at 20oC and 1 atm.