9.1 Solutions

Learning Objectives

  1. Learn some terminology involving solutions.
  2. Recognize which terminology is qualitative and which terminology is quantitative.
  3. Explain why certain substances dissolve in other substances.

The major component of a solution is called the solvent. The minor component of a solution is called the solute. By major and minor we mean whichever component has the greater presence by mass or by moles. Sometimes this becomes confusing, especially with substances with very different molar masses. However, here we will confine the discussion to solutions for which the major component and the minor component are obvious.

Solutions exist for every possible phase of the solute and the solvent. Salt water, for example, is a solution of solid NaCl in liquid water; soda water is a solution of gaseous CO2 in liquid water, while air is a solution of a gaseous solute (O2) in a gaseous solvent (N2). In all cases, however, the overall phase of the solution is the same phase as the solvent.

Example 9.1

A solution is made by dissolving 1.00 g of sucrose (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.

Test Yourself

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

Answer
The solute is HCl(g); the solvent is CH3OH.

One important concept of solutions is in defining how much solute is dissolved in a given amount of solvent. This concept is called concentration. Various words are used to describe the relative amounts of solute. Dilute describes a solution that has very little solute, while concentrated describes a solution that has a lot of solute. One problem is that these terms are qualitative; they describe more or less but not exactly how much.

In most cases, only a certain maximum amount of solute can be dissolved in a given amount of solvent. This maximum amount is called the solubility of the solute. It is usually expressed in terms of the amount of solute that can dissolve in 100 g of the solvent at a given temperature. Table 9.2 “Solubilities of Some Ionic Compounds” lists the solubilities of some simple ionic compounds. These solubilities vary widely: NaCl can dissolve up to 31.6 g per 100 g of H2O, while AgCl can dissolve only 0.00019 g per 100 g of H2O.

Table 9.2 Solubilities of Some Ionic Compounds
Solute Solubility (g per 100 g of H2O at 25°C)
AgCl 0.00019
CaCO3 0.0006
KBr 70.7
NaCl 36.1
NaNO3 94.6

When the maximum amount of solute has been dissolved in a given amount of solvent, we say that the solution is saturated with solute. When less than the maximum amount of solute is dissolved in a given amount of solute, the solution is unsaturated. These terms are also qualitative terms because each solute has its own solubility. A solution of 0.00019 g of AgCl per 100 g of H2O may be saturated, but with so little solute dissolved, it is also rather dilute. A solution of 36.1 g of NaCl in 100 g of H2O is also saturated but rather concentrated. Ideally, we need more precise ways of specifying the amount of solute in a solution.

In some circumstances, it is possible to dissolve more than the maximum amount of a solute in a solution. Usually, this happens by heating the solvent, dissolving more solute than would normally dissolve at regular temperatures, and letting the solution cool down slowly and carefully. Such solutions are called supersaturated solutions and are not stable; given an opportunity (such as dropping a crystal of solute in the solution), the excess solute will precipitate from the solution.

It should be obvious that some solutes dissolve in certain solvents but not others. NaCl, for example, dissolves in water but not in vegetable oil. Beeswax dissolves in liquid hexane but not water. 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. 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. This concept leads to the general rule that “like dissolves like” for predicting whether a solute is soluble in a given solvent. However, this is a general rule, not an absolute statement, so it must be applied with care.

The diagram shows eight purple spheres labeled K superscript plus and eight green spheres labeled C l superscript minus mixed and touching near the center of the diagram. Outside of this cluster of spheres are seventeen clusters of three spheres, which include one red and two white spheres. A red sphere in one of these clusters is labeled O. A white sphere is labeled H. Two of the green C l superscript minus spheres are surrounded by three of the red and white clusters, with the red spheres closer to the green spheres than the white spheres. One of the K superscript plus purple spheres is surrounded by four of the red and white clusters. The white spheres of these clusters are closest to the purple spheres.
Figure 9.1 As potassium chloride (KCl) dissolves in water, the ions are hydrated. The polar water molecules are attracted by the charges on the K+ and Cl− ions. Water molecules in front of and behind the ions are not shown.
Image from OpenStax: licensed under a Creative Commons Attribution 4.0 International License

Example 9.2

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.

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

Answer
H2O, because both experience hydrogen bonding.

Key Takeaways

  • Solutions are composed of a solvent (major component) and a solute (minor component).
  • Concentration is the expression of the amount of solute in a given amount of solvent and can be described by several qualitative terms.
  • Solubility is a specific amount of solute that can dissolve in a given amount of solvent.
  • “Like dissolves like” is a useful rule for deciding if a solute will be soluble in a solvent.

Exercises

Questions

  1. Define solute and solvent.
  2. Define saturated, unsaturated, and supersaturated.
  3. A solution is prepared by combining 2.09 g of CO2 and 35.5 g of H2O. Identify the solute and solvent.
  4. A solution is prepared by combining 10.3 g of Hg(ℓ) and 45.0 g of Ag(s). Identify the solute and solvent.
  5. Use Table 9.2 “Solubilities of Some Ionic Compounds” to decide if a solution containing 45.0 g of NaCl per 100 g of H2O is unsaturated, saturated, or supersaturated.
  6. Use Table 11.5 to decide if a solution containing 0.000092 g of AgCl per 100 g of H2O is unsaturated, saturated, or supersaturated.
  7. Would the solution in Exercise 5 be described as dilute or concentrated? Explain your answer.
  8. Would the solution in Exercise 6 be described as dilute or concentrated? Explain your answer.
  9. Identify a solute from Table 9.2 whose saturated solution can be described as dilute.
  10. Identify a solute from Table 9.2 whose saturated solution can be described as concentrated.
  11. Which solvent is Br2 more likely soluble in — CH3OH or C6H6?
  12. Which solvent is NaOH more likely soluble in — CH3OH or C6H6?
  13. Compounds with the formula CnH2n + 1OH are soluble in H2O when n is small but not when n is large. Suggest an explanation for this phenomenon.
  14. Glucose has the following structure:

    Glucose

    What parts of the molecule indicate that this substance is soluble in water?

Answers

  1. The solvent is the majority component of a solution, whereas the solute is the minority component of a solution.
  1. solute: CO2; solvent: H2O
  1. supersaturated
  1. concentrated, because there is a lot of solute
  1. AgCl or CaCO3
  1. C6H6
  1. The nonpolar end dominates intermolecular forces when n is large.

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