6.3: Intermolecular Forces in Solutions

Learning Objectives

• Learn some terminology involving solutions.
• Explain the significance of the statement "like dissolves like".
• Explain why certain substances dissolve in other substances.

 

Solutions come in all phases, and the solvent and the solute do not have to be in the same phase to form a solution (such as salt and water). For example, air is a gaseous solution of about 80% nitrogen and about 20% oxygen, with some other gases present in much smaller amounts. An alloy is a solid solution consisting of a metal (like iron) with some other metals or nonmetals dissolved in it. Steel, an alloy of iron and carbon and small amounts of other metals, is an example of a solid solution. Table \(\PageIndex{1}\) lists some common types of solutions, with examples of each.

 

Table \(\PageIndex{1}\) Types of Solutions

Solvent Phase	Solute Phase	Example
gas	gas	air
liquid	gas	carbonated beverages
liquid	liquid	ethanol (C2H5OH) in H2O (alcoholic beverages)
liquid	solid	salt water
solid	gas	H2 gas absorbed by Pd metal
solid	liquid	Hg(ℓ) in dental fillings
solid	solid	steel alloys

 

 

Example \(\PageIndex{1}\): Sugar and Water

A solution is made by dissolving 1.00 g of sucrose (\(\ce{C12H22O11}\)) in 100.0 g of liquid water. Identify the solvent and solute in the resulting solution.

Solution

Either by mass or by moles, the obvious minor component is sucrose, so it is the solute. Water—the majority component—is the solvent. The fact that the resulting solution is the same phase as water also suggests that water is the solvent.

Exercise \(\PageIndex{1}\)

A solution is made by dissolving 3.33 g of \(\ce{HCl(g)}\) in 40.0 g of liquid methyl alcohol (\(\ce{CH3OH}\)). Identify the solvent and solute in the resulting solution.

Answer

solute: HCl(g); solvent: CH₃OH

Like Dissolves Like

A simple way to predict which compounds will dissolve in other compounds is the phrase "like dissolves like". What this means is that polar compounds dissolve polar compounds, nonpolar compounds dissolve nonpolar compounds, but polar and nonpolar do not dissolve in each other.

Even some nonpolar substances dissolve in water but only to a limited degree. Have you ever wondered why fish are able to breathe? Oxygen gas, an nonpolar molecules, does dissolve in water and it is this oxygen that the fish take in through their gills. Or, one more example of a nonpolar compound that dissolves in water is the reason we can enjoy carbonated sodas. Pepsi-cola and all the other sodas have carbon dioxide gas, \(\ce{CO_2}\), a nonpolar compound, dissolved in a sugar-water solution. In this case, to keep as much gas in solution as possible, the sodas are kept under pressure.

This general trend of "like dissolves like" is summarized in the following table:

Table \(\PageIndex{2}\) Summary of Solubilities

Solute	Solvent	Is Solution Formed?
Polar Covalent	Polar	yes
Non-polar Covalent	Non-polar	yes
Polar Covalent	Non-polar	no
Non-polar Covalent	Polar	no
Ionic	Polar	yes
Ionic	Non-polar	no

Note that every time charged particles (ionic compounds or polar substances) are mixed, a solution is formed. When particles with no charges (nonpolar compounds) are mixed, they will form a solution. However, if substances with charges are mixed with other substances without charges a solution does not form.

When an ionic compound is considered "insoluble", it doesn't necessarily mean the compound is completely untouched by water. All ionic compounds dissolve to some extent. An insoluble compound just doesn't dissolve in any noticeable or appreciable amount.

What is it that makes a solute soluble in some solvents but not others?

The answer is intermolecular interactions. The intermolecular interactions include London dispersion forces, dipole-dipole interactions, and hydrogen bonding (as described in the previous section). From experimental studies, it has been determined that if molecules of a solute experience the same intermolecular forces that the solvent does, the solute will likely dissolve in that solvent. So, NaCl—a very polar substance because it is composed of ions—dissolves in water, which is very polar, but not in oil, which is generally nonpolar. Nonpolar wax dissolves in nonpolar hexane, but not in polar water. Liquids that dissolve in one another in all proportions are said to be miscible. Liquids that do not dissolve in one another are called immiscible.

 

Figure \(\PageIndex{2}\) Water (clear liquid) and oil (yellow) do not form liquid solutions. Image used with permission (CC BY-SA 1.0 Generic; Victor Blacus)

 

 

 

 

 

 

Example \(\PageIndex{2}\): Polar and Nonpolar Solvents

Would I₂ be more soluble in CCl₄ or H₂O? Explain your answer.

Solution

I₂ is nonpolar. Of the two solvents, CCl₄ is nonpolar and H₂O is polar, so I₂ would be expected to be more soluble in CCl₄.

Exercise \(\PageIndex{2}\)

Would C₃H₇OH be more soluble in CCl₄ or H₂O? Explain your answer.

Answer

H₂O because both experience hydrogen bonding

Example \(\PageIndex{3}\)

Water is considered a polar solvent. Which substances should dissolve in water?

1. methanol (CH₃OH)
2. sodium sulfate (Na₂SO₄)
3. octane (C₈H₁₈), a non-polar organic compound

Solution

Because water is polar, substances that are polar or ionic will dissolve in it.

1. Because of the OH group in methanol, we expect its molecules to be polar. Thus, we expect it to be soluble in water. As both water and methanol are liquids, the word miscible can be used in place of soluble.
2. Sodium sulfate is an ionic compound, so we expect it to be soluble in water.
3. Like other hydrocarbons, octane is nonpolar, so we expect that it would not be soluble in water.

Exercise \(\PageIndex{3}\)

Toluene (C₆H₅CH₃) is widely used in industry as a nonpolar solvent. Which substances should dissolve in toluene?

1. water (H₂O)
2. sodium sulfate (Na₂SO₄)
3. octane (C₈H₁₈)

Answer

octane (C₈H₁₈) will dissolve. It is also non-polar.

 

Summary

• Solutions are composed of a solvent (major component) and a solute (minor component).
• “Like dissolves like” is a useful rule for deciding if a solute will be soluble in a solvent.
• Liquids that dissolve in one another in all proportions are said to be miscible.
• Liquids that do not dissolve in one another are called immiscible.
• Ion-dipole interactions are formed when ionic compounds dissolve in water.

Contributors and Attributions

• Marisa Alviar-Agnew (Sacramento City College)

• Henry Agnew (UC Davis)

• Libretext: Chemistry for Allied Health (Soult)
• Libretext: The Basics of GOB Chemistry (Ball et al.)