9.3 The Dissolution Process

9.3 The Dissolution Process

Learning Objective

  1. Describe the dissolution process at the molecular level.

 

What occurs at the molecular level to cause a solute to dissolve in a solvent? The answer depends in part on the solute, but there are some similarities common to all solutes.

Recall the rule that like dissolves like. As we saw in Section 9.1 “Solutions”, this means that substances must have similar intermolecular forces to form solutions. When a soluble solute is introduced into a solvent, the particles of solute can interact with the particles of solvent. In the case of a solid or liquid solute, the interactions between the solute particles and the solvent particles are so strong that the individual solute particles separate from each other and, surrounded by solvent molecules, enter the solution. Gaseous solutes already have their constituent particles separated, but the concept of being surrounded by solvent particles still applies. This process is called solvation and is illustrated in Figure 9.4 “Solvation”. When the solvent is water, the word hydration, rather than solvation, is used.  In the case of molecular solutes like sucrose, the solute particles are individual molecules.

9.4 Solvation  of Sucrose, a molecular compound.  Notice that the crystal dissolves into individual complete molecules.  This image was labelled for noncommercial use with modification but did not include attribution information.

 

However, if the solute is ionic, the individual ions separate from each other and become surrounded by solvent particles. That is, the cations and anions of an ionic solute separate when the solute dissolves. This process is referred to as dissociation. Compare the dissociation of a simple ionic solute as shown in Figure 9.5 “Ionic Dissociation” to the process illustrated in Figure 9.4 “Solvation”.

Figure 9.5 Ionic Dissociation.  As NaCl dissolves, Na+ and Cl– ions are pulled away from the salt crystal and are surrounded by water molecules. Attribution:  By Ahazard.sciencewriter [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons

The dissociation of soluble ionic compounds gives solutions of these compounds an interesting property: they conduct electricity. Because of this property, soluble ionic compounds are referred to as electrolytes. Many ionic compounds dissociate completely and are therefore called strong electrolytes. Sodium chloride is an example of a strong electrolyte. Some polar covalent compounds partially ionize when they dissolve, meaning that a few molecule split into ions.  Solutions of such solutes conduct electricity only weakly, so the solutes are called weak electrolytes. Acetic acid (CH3COOH), the compound in vinegar, is a weak electrolyte. Solutes that dissolve into individual neutral molecules without dissociation do not impart additional electrical conductivity to their solutions and are called nonelectrolytes. Table sugar (C12H22O11) is an example of a nonelectrolyte.

Note

The term electrolyte is used in medicine to mean any of the important ions that are dissolved in aqueous solution in the body. Important physiological electrolytes include Na+, K+, Ca2+, Mg2+, and Cl−.

Example 10

The following substances all dissolve to some extent in water. Classify each as an electrolyte or a nonelectrolyte.

  1. potassium chloride (KCl)
  2. fructose (C6H12O6)
  3. isopropyl alcohol [CH3CH(OH)CH3]
  4. magnesium hydroxide [Mg(OH)2]

Solution

Each substance can be classified as an ionic solute or a nonionic solute. Ionic solutes are electrolytes, and nonionic solutes are nonelectrolytes.

  1. Potassium chloride is an ionic compound; therefore, when it dissolves, its ions separate, making it an electrolyte.
  2. Fructose is a sugar similar to glucose. (In fact, it has the same molecular formula as glucose.) Because it is a molecular compound, we expect it to be a nonelectrolyte.
  3. Isopropyl alcohol is an organic molecule containing the alcohol functional group. The bonding in the compound is all covalent, so when isopropyl alcohol dissolves, it separates into individual molecules but not ions. Thus, it is a nonelectrolyte.
  4. Magnesium hydroxide is an ionic compound, so when it dissolves it dissociates. Thus, magnesium hydroxide is an electrolyte.

Note

More information than that provided in this chapter is needed to determine if some electrolytes are strong or weak. We will consider this in Chapter 10 “Acids and Bases”.

Skill-Building Exercise

Classify each of the following as a strong electrolyte, a weak electrolyte, or a non-electrolyte.

  1. acetone (CH3COCH3)

  2. iron(III) nitrate [Fe(NO3)3]

  3. elemental bromine (Br2)

  4. sodium hydroxide (NaOH)

Concept Review Exercise

  1. Explain how the solvation process describes the dissolution of a solute in a solvent.

Answer

  1. Each particle of the solute is surrounded by particles of the solvent, carrying the solute from its original phase.

Key Takeaways

Key Takeaway

  • When a solute dissolves, its individual particles are surrounded by solvent molecules and are separated from each other.

Exercises

  1. Describe what happens when an ionic solute like Na2SO4 dissolves in a polar solvent.

  2. Describe what happens when a molecular solute like sucrose (C12H22O11) dissolves in a polar solvent.

  3. Classify each substance as an electrolyte or a nonelectrolyte. Each substance dissolves in H2O to some extent.

    1. NH4NO3
    2. CO2
    3. NH2CONH2
    4. HCl

  4. Classify each substance as an electrolyte or a nonelectrolyte. Each substance dissolves in H2O to some extent.

    1. CH3CH2CH2OH
    2. Ca(CH3CO2)2
    3. I2
    4. KOH

  5. Will solutions of each solute conduct electricity when dissolved?

    1. AgNO3
    2. CHCl3
    3. BaCl2
    4. Li2O

  6. Will solutions of each solute conduct electricity when dissolved?

    1. CH3COCH3
    2. N(CH3)3
    3. CH3CO2C2H5
    4. FeCl2

Answers

1. Each ion of the ionic solute is surrounded by particles of solvent, carrying the ion from its associated crystal.

3. a. electrolyte

b. nonelectrolyte

c. nonelectrolyte

d. electrolyte

5. a. yes

b. no

c. yes

d. yes