Lesson 1: A Model of Solutions

Part b: Solubility and Structure

Part a: What is a Solution?
Part b: Solubility and Structure
Part c: The Dissolving Process
Part d: Solubility, Temperature, and Pressure
Part e: Dissociation of Ionic Compounds

 

Solubility

Solubility refers to the maximum amount of solute that can be dissolved in a particular solvent. The solubility in water varies significantly from substance to substance. An ionic compound like sodium nitrate (NaNO₃) is very soluble (meaning dissolve-able) in water with more than 90 g of NaNO₃ dissolving in 100 mL of water at room temperature. Other substances like calcium carbonate (CaCO₃) can barely dissolve in water and are considered insoluble (meaning does not dissolve). At room temperature and pressure, a maximum of 0.008 moles of methane gas (CH₄) will dissolve in 100 mL of water. Under the same conditions, 0.38 moles of methane, nearly 50 times as much, will dissolve in 100 mL of hexane (C₆H₁₄). In Lesson 1b, we will learn what makes a solute soluble in a particular solvent.

 
 
 

Like Dissolves Like

One of the catch phrases regarding solubility is that like dissolves like. A solute will dissolve in a solvent if they have experience similar types of intermolecular forces (IMFs). Using the rule, we would predict that a polar solute will dissolve in a polar solvent like water. A nonpolar solute will dissolve in a nonpolar solvent like hexane (C₆H₁₄). We would also predict that nonpolar solutes would not dissolve in polar solvents like water. Similarly, polar solutes seldom dissolve in nonpolar solvents. The concept of polarity becomes critical to predicting solubility. The polarity or nonpolarity of a molecule (and the presence of O-H, N-H, and F-H bonds) determines what type of IMFs the molecule experiences. And because polarity pertains to how molecules are structured, the topic of solubility pertains to molecular structure.
 
Propane, commonly used in outdoor gas grills, is a hydrocarbon. Hydrocarbons contain nonpolar C-C and C-H bonds. The intermolecular forces that hold hydrocarbon molecules together are London dispersion forces. Hexane (C₆H₁₄), also a hydrocarbon, is a great solvent for other compounds that are nonpolar. Yet hexane  has a limited ability to dissolve polar substances. In contrast to hexane, water is a polar molecule. Water molecules are held together in the liquid state by hydrogen bonding, a particularly strong type of dipole-dipole interaction. Water is an effective solvent for polar solutes that are also held together by dipole-dipole interactions or experience hydrogen bonding forces.
 
The table below displays a variety of compounds, their molecular polarity, the intermolecular force (IMF) that is most dominant between its molecules, and their solubility in water and hexane. It is quite evident from Rows 1-5 of that water is a great solvent for polar solutes. And it is quite evident from Rows 6-11 that hexane is a great solvent for nonpolar solutes. These rows give meaning to like dissolves like.

 

 
Like dissolves like is a guideline for predicting the solubility of a particular substance in a solvent like water or hexane. However, it should never be regarded as a rule. The behavior of chemical substances is often more easily explained than it is predicted. This is certainly the case for solubility. Pondering the data from Rows 12 to 17 will help us to build a better model of solubility that goes further than like dissolves like.
 
As seen in Row 12, carbon dioxide (CO₂) is a nonpolar molecule with high solubility in hexane. Based on our simple rule, we might expect it to be insoluble in water. Yet CO₂ is soluble in water. Those of us who love carbonated beverages can be thankful that it is. There are a couple of reasons for the solubility of CO₂ in water. First, as CO₂ dissolves in water, it reacts chemically with water to form carbonic acid. This removes the CO₂ from the aqueous solution, allowing additional CO₂ to take its place. Second, even though CO₂ is nonpolar due to its linear structure, there is a slight interaction of its electronegative O atoms with water molecules. An oxygen atom of CO₂ experiences a dipole-dipole-like interaction with the hydrogen atom of water. This interaction is prevalent enough to contribute to the unexpectedly higher solubility of nonpolar CO₂ in polar water.
 
Rows 13 to 17 contrasts the interaction of five different alcohols (hydrocarbon chains with an O-H group replacing an H atom). The O-H group of each alcohol is the polar part of the molecule. The hydrocarbon chain, consisting of nonpolar C-C and C-H bonds, is the nonpolar part of the molecule. The hydrocarbon chains of two side-by-side molecules will interact through London dispersion forces. The O-H groups on two side-by-side molecules will interact through hydrogen bonding.
 

 

 
As the size of the hydrocarbon chain gets longer, the London dispersion forces become the more dominant intermolecular force. Methanol (Row 13), with a single carbon atom, interacts with water molecules through hydrogen bonding and is very soluble in water. On the other hand, hexanol (Row 17), with a six-carbon atom chain, interacts with hexane through London dispersion forces and is very soluble in hexane. For these alcohols, like dissolves like becomes a guiding principle when looking more closely at the parts of the molecule.

 
 
 

The Soap Trick

You’ve been told countless times to wash your hands with soap. With soap is the critical part of the command. It is not enough to wash your hands with water because water can only be expected to attract, dissolve, and wash away polar solutes. And most of what we wish to cleanse ourselves of - grease, oils, germs, etc. - is nonpolar.  So how do you entice water to wash away nonpolar molecules? You use soap.
 
Soap is a complex molecule that consists of a highly polar, hydrophilic end attached to a long, nonpolar, hydrophobic chain. Hydrophilic means water loving. The hydrophilic end consists of polar C-O bonds, explaining their love for water. This part of the molecule is referred to as the head of the molecule. The rest of the molecule consists of a long chain of carbon atoms with attached hydrogen atoms. This is the nonpolar tail of the molecule. Consisting of nonpolar C-C and C-H bonds, the tail will have nothing to do with water. But this also means that the tail is a down-and-dirty lover of grease and oils.
 
The most common soap is sodium stearate - NaC₁₇H₃₅COO. When the soap dissolves in water, the stearate ion (C₁₇H₃₅COO^-) enters the aqueous solution.

 
The hydrophilic end of the stearate ion, containing the polar C-O bonds, is attracted to water. The hydrophobic end is attracted to the nonpolar substance (the dirt) on your hands. In solution, the stearate ions from a micelle that surrounds the grease and oil. A micelle can be thought of as a three-dimensional, spherical structure that forms a cage around the dirt. The nonpolar tails are pointed towards the dirt while the polar head faces outward towards the water. The tail attracts, adheres to, and traps the dirt while the head is attracted to the water. The micelle and the trapped dirt take a ride with the water straight the drain. And that’s Chemistry for Better Living!
 

 
 

Solubility of Ionic Compounds

Ionic compounds do not consist of molecules. Rather they consist of cations and anions held together by strong ionic bonds in a crystal lattice. Many ionic compounds readily dissolve in water. Others are considered insoluble in water. But none of them, whether they dissolve in water or not, dissolve in nonpolar solvents.
 
The solubility of ionic compounds in water is explained by the ion-dipole interaction between the ions of the crystal lattice and polar water molecule. The positive pole of the water molecules attracts, surrounds, and stabilizes the anion of the compound. The negative pole of the water molecules stabilizes the cation of the compound. Nonpolar solvents like hexane lack a dipole and are unable to stabilize the cation and anion in solution. We will discuss the aqueous solutions of ionic compounds in greater detail in the next part of Lesson 1 and in even more detail in Lesson 1e.

 
 

You will be tempted to quote “like dissolves like” as the answer to any question of the form why does solute X dissolve in solvent Y?  This indicates that you were listening (or reading); but it doesn’t necessarily indicate that you understand solubility. Understanding solubility requires that you understand and give attention to molecular structure and intermolecular forces. It also requires that you understand the mechanism that occurs when a solute is dissolved in a solvent. If structure and IMFs explain the why of solubility, then the mechanism is the how of solubility. In the next part of Lesson 1, we will discuss how things dissolve. But before you leave this page to learn more, take some time to solidify the ideas by following one or more of the suggestions in the Before You Leave section.
 
 
 

Before You Leave

• Download our Study Card on Solubility and Structure. Save it to a safe location and use it as a review tool. (Coming Soon.)
• The Check Your Understanding section below include questions with answers and explanations. It provides a great chance to self-assess your understanding.

 
 
 
 

Check Your Understanding

Use the following questions to assess your understanding. Tap the Check Answer buttons when ready.
 
1. Throughout Lesson 1b, the prototype polar solvent is ___________ and the prototype nonpolar solvent is ___________.
a. hexane, water
b. water, hexane

 
2. Like dissolves like means that _____________.
a. Two identical substances will dissolve in each other.
b. Two substances with the exact same composition of atoms will dissolve in each other.
c. Two substances that are held together by similar types of forces will dissolve in each other.

 

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3. TRUE or FALSE:
Like dissolves like is a hard and fast rule for predicting what solutes would dissolve in certain solvents. There are never exceptions or surpises to this rule.

 

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4. Think about this one:
Substance A is polar. Substance B is nonpolar. Substance C dissolves in Substance A. Substance D does not dissolve in Substance A. All four substances are molecular compounds.
Give your best predictions for the following situations. Explain each answer.

1. Will Substance B dissolve in Substance A? 
   
   Check Answer
   Answer: No
   
   Substance A is polar and substance B is nonpolar. We wouldn't expect them to dissolve in each other.
   
    
   
    
2. Will Substance C dissolve in Substance B? 
   
   Check Answer
   Answer: No
   
   Substance C is likely polar since it dissolves in polar Substance A. Substance B is nonpolar. We wouldn't expect them to dissolve in each other since one is polar and the other is nonpolar.
   
    
   
    
3. Will Substance D dissolve in Substance B? 
   
   Check Answer
   Answer: Yes
   
   Substance D is likely nonpolar since it does not dissolve in polar Substance A. Substance B is nonpolar. We would expect them to dissolve in each other since they are likely both nonpolar.
   
    
   
    
4. Will Substance D dissolve in Substance C? 
   
   Check Answer
   Answer: No
   
   Substance C is likely polar since it dissolves in polar Substance A. Substance D is likely nonpolar since it does not dissolve in polar Substance A. We wouldn't expect them to dissolve in each other since one is polar and the other is nonpolar.
   
    
   
    

 
 
5. Did you know that some vitamins that our bodies need dissolve in water and others do not? Those that dissolve in water are excreted through our urinary tract each day if our body doesn’t use them. Those that don’t dissolve in water will be stored in the fat tissue of our bodies if our body doesn’t need them. The water-soluble vitamins are the B vitamins and vitamin C. The fat-soluble (water-insoluble) vitamins are vitamins A, D, E, and K. Use this information to answer the following questions:

1. The B vitamins and vitamin C are ___________. (polar, nonpolar) 
   
   Check Answer
   Answer: Polar
   
   They dissolve in water. Polar vitamins would dissolve in water.
   
    
   
    
2. Vitamins A, D, E, and K are ___________. (polar, nonpolar) 
   
   Check Answer
   Answer: Nonpolar
   
   They are insoluble in water. Nonpolar vitamins would not dissolve in polar water.
   
    
   
    
3. The vitamins that you must be sure to take on a daily basis are vitamins ________ (B and C, A-D-E-and-K) 
   
   Check Answer
   Answer: B and C
   
   Because they dissolve in water, they are excreted from the body via urination. They are never stored in our fatty tissue. There are no reserves of vitamins B and C in the body. So we must take them everyday.
   
    
   
    
4. Taking too much of vitamins _______________ (B and C, A-D-E-and-K) could be dangerous because they can reach toxic levels in our bodies. 
   
   Check Answer
   Answer: A-D-E-and-K
   
   The body has no way of excreting excess vitamins A, D, E, and K since they do not dissolve in water. Instead, they are stored in the body's fatty tissue. And f megadosing for long periods of time on these vitamins, they can build up to toxic levels.
   
    
   
    
5. Taking mega-doses of vitamins _______________ (B and C, A-D-E-and-K) would be senseless and wasteful if our body isn’t actually using them. 
   
   Check Answer
   Answer: B and C
   
   These are water soluble, so what you don't use is going to end up in the toilet. You'd be flushing vitamins (and money) down the drain.
   
    
   
    

 
 
6. Elemental mercury is a liquid. As indicated in the table, it dissolves in hexane but not in water. Propose a reason why this is.

 

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7. The following compounds either dissolve in water and not hexane or in hexane and not water. For each case, indicate which solvent it would dissolve in and explain why.

1. Carbon disulfide, CS₂ 
   
   Check Answer
   Answer: Hexane
   
   The bonds in CS₂ are nonpolar. The C and S have identical electronegativity values. Its structure is linear; so even if it did have polar bonds, their dipoles could cancel and it would be a nonpolar molecules. We expect high solubility in a nonpolar solvent like hexane.
   
    
   
    
2. Hydrogen fluoride, HF 
   
   Check Answer
   Answer: Water
   
   HF has a very polar bond. It also experiences hydrogen bonding forces. This makes it very much like water. We would expect high solubility in a polar solvent like water.
   
    
   
    
3. Sodium fluoride, NaF 
   
   Check Answer
   Answer: Water
   
   Sodium fluoride is ionic. So we know it doesn't dissolve in hexane. If it dissolves in anything, then it would dissolve in water.
   
    
   
    
4. Carbon tetrachloride, CCl₄ 
   
   Check Answer
   Answer: Hexane
   
   The C-Cl bond is polar. But the molecule is tetrahedral. So the dipoles would cancel each other and the overall molecule would be nonpolar. We would expect high solubility in a nonpolar solvent like hexane. 
    
   
    
5. Boron trichloride, BCl₃ 
   
   Check Answer
   Answer: Hexane
   
   The B-Cl bond is polar. But the molecule has a trigonal planar structure. So the dipoles would cancel each other and the overall molecule would be nonpolar. We would expect high solubility in a nonpolar solvent like hexane.