9.4.1: How Solutions Form

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Learning Objectives

Explain the significance of the statement "like dissolves like."
Explain why certain substances dissolve in other substances.

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, a nonpolar molecule, does dissolve in water—it is this oxygen that the fish take in through their gills. The reason we can enjoy carbonated sodas is also due to a nonpolar compound that dissolves in water. 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 (Polarity of Compound)	Solvent (Polarity of Compound)	Dominant Intermolecular Force	Is Solution Formed?
Polar	Polar	Dipole-Dipole Force and/or Hydrogen Bond	yes
Non-polar	Non-polar	Dispersion Force	yes
Polar	Non-polar		no
Non-polar	Polar		no
Ionic	Polar	Ion-Dipole	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 Chapter 10). 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.

Figure \(\PageIndex{2}\): Water (clear liquid) and oil (yellow) do not form liquid solutions. (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?

methanol (CH₃OH)
sodium sulfate (Na₂SO₄)
octane (C₈H₁₈)

Solution

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

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.
Sodium sulfate is an ionic compound, so we expect it to be soluble in water.
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?

water (H₂O)
sodium sulfate (Na₂SO₄)
octane (C₈H₁₈)

Answer: Octane (C₈H₁₈) will dissolve. It is also non-polar.

Summary

“Like dissolves like” is a useful rule for deciding if a solute will be soluble in a solvent.

Contributions & Attributions

This page was constructed from content via the following contributor(s) and edited (topically or extensively) by the LibreTexts development team to meet platform style, presentation, and quality:

Marisa Alviar-Agnew (Sacramento City College)
Henry Agnew (UC Davis)