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# 17.2: Solution Formation

Created by: CK-12

## Lesson Objectives

The student will:

• define the terms miscible and immiscible.
• explain why solutions form.
• predict if a solution will form between two substances from their molecular formulas.
• discuss the idea of water as the “universal solvent.”
• explain the solvation of molecules in comparison to ionic solvation.
• discuss the differences between electrolytes and non-electrolytes and give examples of each.
• determine from the formula whether a compound is an electrolyte or a non-electrolyte.
• identify electrolyte and non-electrolyte solutions by their properties.

## Vocabulary

• electrolyte
• hydration
• immiscible
• miscible
• non-electrolyte

## Introduction

We have learned that solutions can be formed in a variety of combinations using solids, liquids, and gases. We also know that solutions have constant compositions, which can be varied to a point. Not all combinations, however, form solutions. Why is it that oil and water will not form a solution, but vinegar and water will? Why can table salt be dissolved in water but not in vegetable oil? The reasons why solutions form will be explored in this lesson, along with a discussion of why water is used most frequently to dissolve substances of various types.

## Compound Structure and Solution Formation

Recall that a polar molecule is one that has a partially positive end and a partially negative end, while non-polar molecules have charges that are evenly distributed throughout the molecule. In fact, during the study of the Valence Shell Electron Pair Repulsion Theory (VSEPR) (from the chapter on “Covalent Bonds and Formulas”), you learned that the chemical structures themselves have built in molecular polarity.

In solution chemistry, we can predict when solutions will form using the saying, “like dissolves like.” This expression means that solubility is based on the two parts of a solution having similar intermolecular forces. For example, suppose you are dissolving methanol in water. Both methanol and water are polar molecules that can form a solution because they both have permanent dipoles (partially positive and partially negative parts of the molecules) that allow the molecules of both substances to be attracted to the other. We say these substances are miscible, which means these two can dissolve in each other and make a solution. If they are immiscible, they are unable to be mixed together.

The same is true for the case of a non-polar substance such as carbon tetrachloride being dissolved in another non-polar substance like pentane. London-dispersion forces are the intermolecular forces that hold the carbon tetrachloride together as a liquid and allow pentane to be a liquid at room temperature. Since both of these substances have the same intermolecular forces, when they are mixed together, a solution will be formed.

In a polar solvent, the molecules of solvent are attracted to each other by the partial charges on the ends of the molecules. When a polar solute is added, the positive polar ends of the solute molecules attract the negative polar ends of the solvent molecules and vice versa. This attraction allows the two different types of molecules to form a solution. If a non-polar solute is added to a polar solvent, the non-polar solute molecules are unable to disrupt the solvent molecules. The polarity of the solvent molecules make the solvent molecules more attracted to each other rather than to the non-polar solute. As a result, the solute and solvent are immisicible. For example, if we were to add table salt, $\mathrm{NaCl}$, to either carbon tetrachloride, we would find that the salt would not dissolve. Since carbon tetrachloride has no permanent dipoles in its molecules, there is no attraction among the charged ions in NaCl and the uncharged molecules of the solvent.

Water is often referred to as the “universal solvent.” The term “universal” is used to describe the fact that water can dissolve many types and kinds of substances. For instance, table salt, $\mathrm{NaCl}$, is an ionic compound, but it easily makes a solution with water. This is true for many ionic compounds. Water also readily dissolves table sugar, a polar covalent compound, and other polar compounds, such as vinegar and corn syrup. Even some non-polar substances dissolve in water to a limited degree. Oxygen gas, a non-polar molecule, can dissolve in water and be taken up by fish through their gills. Carbon dioxide, another non-polar gas, is also soluble in water. Many drinks, such as sodas, beers, seltzer water, and champagne contain carbon dioxide. To keep as much gas in solution as possible, the sodas are kept under pressure.

A student defines solute and solvent and this animation shows the hydrolysis involved in dissolving (6a, 6b): http://www.youtube.com/watch?v=hydUVGUbyvU (1:38).

## Ionic Compounds in Solution

When a solid is formed between a metal and a nonmetal, the ions in the solid are held together by the attraction of the positive metal cation to the negative nonmetal anion. The attraction of oppositely charged particles is called electrostatic attraction (refer to the chapter “Ionic Bonds and Formulas” for more information). For example:

$\mathrm{Na} + \text{ionization energy} \rightarrow \mathrm{Na}^+ + \mathrm{e}^-$
$\mathrm{I} + \mathrm{e}^- \rightarrow \mathrm{I}^- + \text{energy of electron affinity}$
$\mathrm{Na}^+ + \mathrm{I}^- \rightarrow \mathrm{NaI} + \text{energy}$
NaI is therefore held together by electrostatic attraction

Electrostatic attraction is quite strong, so compounds with this type of bonding have high melting and boiling points. Recall that ionic compounds do not form molecules. Instead, the empirical formula represents the lowest whole number ratio of the ions involved in the compound.

Example:

Which of the compounds below would contain an electrostatic attraction as bonds between ions?

1. $\mathrm{MgCl}_2$
2. $\mathrm{Al}_2\mathrm{O}_3$
3. $\mathrm{CH}_4$

Solution:

$\mathrm{MgCl}_2$ and $\mathrm{Al}_2\mathrm{O}_3$ would contain electrostatic attraction because they are both ionic, but $\mathrm{CH}_4$ is not. It does not form bonds by the transfer of electrons but instead by sharing electrons, so it does not have ions for electrostatic attraction.

Since ionic compounds can dissolve in polar solutions, specifically water, we can extend this concept to say that ions themselves are attracted to the water molecules because the ions are attracted to the polar water molecule. When you dissolve table salt in a cup of water, the table salt dissociates into sodium ions and chloride ions, as seen in the equation below:

$\mathrm{NaCl}_{(s)} \rightarrow \mathrm{Na}^+_{(aq)} + \mathrm{Cl}^-_{(aq)}$

The sodium ions get attracted to the partially negative ends of the water molecule, and the chloride ions get attracted to the partially positive end of the water molecule. The process of water molecules attaching to ions is called hydration, as seen in the figures below.

The same is true for any ionic compound dissolving in water. The ionic compound will separate into positive and negative ions, and the positive ion will be attracted to the partially negative end of the water molecules (oxygen) while the negative ion will be attracted to the partially positive end of the water molecules (hydrogen).

This animation shows dissociation of salt in water creating sodium chlorine solution (2c): http://www.youtube.com/watch?v=EBfGcTAJF4o (0:53).

## Covalent Compounds in Solution

Covalent compounds have a different type of attraction occurring between the solute and solvent molecules. Unlike ionic compounds, which result from the transfer of electrons, covalent compounds result from the sharing of electrons between atoms. As a result, there are no distinct charges associated with the atoms in covalent compounds.

In $\mathrm{CO}_2$, each oxygen atom shares two of its electrons with carbon and the carbon shares two of its electrons with each oxygen atom. Look at the figure below:

This sharing of valence electrons represents covalent bonding. However, the electrons are not shared equally. Recall that elements with a greater electronegativity have a stronger attraction for shared electrons. Therefore, they can pull the electrons closer to themselves and away from the element that has a smaller electronegativity. For carbon, the electronegativity value is 2.5, and for oxygen it is 3.5. The result in this molecule is that the electrons are pulled closer to oxygen than carbon. The resultant structure is represented below.

The bonds in this molecule are polar, but the molecule is non-polar overall because the shifting of the shared electrons toward the oxygen atoms are in equal but opposite directions. As a result, there is no overall dipole moment on the molecule.

As ionic solids dissolve into solution, these solids separate into ions. Molecular compounds, however, are held together with covalent bonds, which are not readily broken. For example, when you dissolve a spoonful of sugar into a glass of water, the intermolecular forces between the sugar molecules are disrupted but the intramolecular bonds are not. The sugar will not separate into carbon ions, hydrogen ions, and oxygen ions. The sugar molecules remain intact, but because of their polar properties, they can interact with the polar water molecules to form a solution. You can write the following equation for the dissolution of sugar in water:

$\mathrm{C}_{12}\mathrm{H}_{22}\mathrm{O}_{11(s)} \rightarrow \mathrm{C}_{12}\mathrm{H}_{22}\mathrm{O}_{11(aq)}$

Example:

Which compounds will dissolve in solution to separate into ions?

1. $\mathrm{LiF}$
2. $\mathrm{P}_2\mathrm{F}_5$
3. $\mathrm{C}_2\mathrm{H}_5\mathrm{OH}$

Solution:

$\mathrm{LiF}$ will separate into ions when dissolved in solution:

$\mathrm{LiF}_{(aq)} \rightarrow \mathrm{Li}^+_{(aq)} + \mathrm{F}^-_{(aq)}$

$\mathrm{P}_2\mathrm{F}_5$ and $\mathrm{C}_2\mathrm{H}_5\mathrm{OH}$ are both covalent and will stay as molecules in a solution.

## Electrolytes and Non-Electrolytes

In the late 1800s, Arrhenius classified ionic compounds that dissolved in water as electrolytes. They are considered to be electrolytes because they conduct electricity in water solution. According to Arrhenius (and current theory), the ions in solution provide the charged particles needed to conduct electricity. Look at the equation below for the dissociation of $\mathrm{NaCl}$.

$\mathrm{NaCl}_{(s)} \rightarrow \mathrm{Na}^+_{(aq)} + \mathrm{Cl}^-_{(aq)}$

Once the solid $\mathrm{NaCl}$ is added to the water, it dissolves, which means that the ions move away from their crystalline structure and are now dispersed throughout the water molecules. If two electrodes were to be immersed into a solution of $\mathrm{NaCl}_{(aq)}$, the $\mathrm{Na}^+_{(aq)}$ ions would move toward one electrode and the $\mathrm{Cl}^-_{(aq)}$ ions would move toward the second electrode. This movement of ions allows the electric current to flow through the solution. Therefore, $\mathrm{NaCl}_{(aq)}$ will behave as an electrolyte and conduct electricity because of the presence of $\mathrm{Na}^+_{(aq)}$ and $\mathrm{Cl}^-_{(aq)}$ ions. The more ions that are present in the solution, the stronger the electrolyte solution is.

Example:

Which of the following will form electrolyte solutions and conduct electricity?

1. $\mathrm{CaF}_{2(aq)}$
2. $\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_{6 (aq)}$
3. $\mathrm{KOH}_{(aq)}$

Solution:

$\mathrm{CaF}_{2(aq)}$ and $\mathrm{KOH}_{(aq)}$ are solutions that contain positive cations and negative anions that would separate when dissolved in water. Since ions are separated in solution, they are electrolytes and will conduct electricity.

Calcium fluoride: $\mathrm{CaF}_{2(s)} \rightarrow \mathrm{Ca}^{2+}_{(aq)} + 2 \ \mathrm{F}^-_{(aq)}$
Potassium hydroxide: $\mathrm{KOH}_{(s)} \rightarrow \mathrm{K}^+_{(aq)} + \mathrm{OH}^-_{(aq)}$

2. is not an ionic compound but a covalent compound. This means that when it dissolves in water it stays together as a molecule and is a non-electrolyte.

$\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_{6(s)} \rightarrow \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_{6(aq)}$

Conduction is the result of ions moving through a solution. With covalent compounds, there are no ions moving around in solution; therefore, they are classified as non-electrolytes. Non-electrolytes are solutions that do not conduct electricity. If you were to connect a conductivity meter to these solutions, there would be no reading the wires were placed in a solution containing a non-electrolyte.

In summary, the types of combinations that will form solutions are listed in Table below.

Combinations that Form Solutions
Combination Solution Formed?
Polar substance in a polar substance. Yes
Non-polar substance in a non-polar substance. Yes
Polar substance in a non-polar substance. No
Non-polar substance in a polar substance. No
Ionic substance in a polar substance. Yes
Ionic substance in a non-polar substance. No

## Lesson Summary

• Whether or not solutions are formed depends on the similarity of polarity or the “like dissolves like” rule.
• Polar molecules dissolve in polar solvents, non-polar molecules dissolve in non-polar solvents.
• Ionic compounds dissolve in polar solvents, especially water. This occurs when the positive cation from the ionic solid is attracted to the negative end of the water molecule (oxygen) and the negative anion of the ionic solid is attracted to the positive end of the water molecule (hydrogen).
• Water is considered as the universal solvent since it can dissolve both ionic and polar solutes, as well as some non-polar solutes (in very limited amounts).
• Electrolyte solutions are ones in which free-moving charged particles can conduct an electrical current.
• Non-electrolytes are solutions that do not conduct electricity.

Visit the website for a presentation on solutions.

## Review Questions

1. What is the “like dissolves like” generalization and provide an example to illustrate your answer.
2. Why will $\mathrm{LiCl}$ not dissolve in $\mathrm{CCl}_4$?
3. Will acetic acid dissolve in water? Why?
4. What is the difference between intermolecular and intramolecular bonds?
5. In which compound will benzene $(\mathrm{C}_6\mathrm{H}_6)$ dissolve?
1. carbon tetrachloride
2. water
3. vinegar
4. none of the above
6. In which compound will sodium chloride dissolve?
1. carbon tetrachloride
2. methanol
3. vinegar
4. none of the above
7. In which compound will ammonium phosphate dissolve?
1. carbon tetrachloride
2. water
3. methanol
4. none of the above
8. Thomas is making a salad dressing for supper using balsamic vinegar and oil. He shakes and shakes the mixture but cannot seem to get the two to dissolve. Explain to Thomas why they will not dissolve.
9. Write the reactions for dissolving the following.
1. $\mathrm{NaOH}_{(s)}$
2. $\mathrm{LiOH}_{(s)}$
3. $\mathrm{C}_5\mathrm{H}_{10}\mathrm{O}_{4(s)}$
4. $\mathrm{NH}_4\mathrm{Cl}_{(s)}$
5. $\mathrm{MgCl}_{2(s)}$
10. Which of the following represent physical changes? Explain.
1. explosion of $\mathrm{TNT}$
2. dissolving $\mathrm{KCl}$
3. sharpening a pencil
4. souring milk
11. Which compound contains electrostatic forces?
1. natural gas
2. table salt
3. air
4. sugar
12. Which of the following is a physical change?
1. rotting wood
3. rusting iron
4. molding cheese
13. Which of the following is not a physical change?
1. melting iron
2. pumping gas
3. reaction of chlorine with sodium
4. reaction of magnesium chloride with water
14. Which compound is considered to be an electrolyte when dissolved in water?
1. $\mathrm{HNO}_3$
2. $\mathrm{C}_{12}\mathrm{H}_{22}\mathrm{O}_{11}$
3. $\mathrm{N}_2\mathrm{O}$
4. $\mathrm{CH}_4$
15. Which compound is not considered to be an electrolyte?
1. $\mathrm{AgCl}$
2. $\mathrm{PbSO}_4$
3. $\mathrm{C}_2\mathrm{H}_6$
4. $\mathrm{HClO}_3$
16. Janet is given three solutions. She is to determine if the solutions are electrolytes or not but is not told what the solutions are. She makes the following observations. What can you conclude from her observations and what help can you offer Janet to determine if the solutions are indeed electrolytes?
Solution 1: Clear
Solution 2: Blue but transparent
Solution 3: Clear
1. Describe the intermolecular bonding that would occur between glucose, $\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6$, and water.
2. Define non-electrolyte and give at least one example.
3. How can you tell by looking at a formula that it is most likely a covalent compound? What does this tell you about the bonding?
4. Describe how you could tell the difference between an electrolyte and a non-electrolyte solution.
5. Looking at the periodic table, which pair of elements will form a compound that is covalent?
1. $\mathrm{Ca}$ and $\mathrm{Br}$
2. $\mathrm{Fe}$ and $\mathrm{O}$
3. $\mathrm{Si}$ and $\mathrm{F}$
4. $\mathrm{Co}$ and $\mathrm{Cl}$
6. Which of the following compounds will conduct the least amount of electricity if dissolved in water?
1. $\mathrm{KNO}_3$
2. $\mathrm{BaCl}_2$
3. $\mathrm{CsF}$
4. $\mathrm{CO}_2$
7. Steve is given five solutions in the lab to identify. He performs a conductivity test, a solubility (in water) test, crudely measures the hardness of each substance, and determines the melting point using a melting point apparatus. Some of the melting points, the teacher tells him are too high or low to measure using the laboratory melting point apparatus so she gives him the melting point. For the liquids, he determined the boiling points. He gathers all of his data and puts it into a table. His teacher gives him the names of the five solutions to match his five unknowns to. Can you help Steve match the properties of the unknowns (from Table below) to the solution names (found under the table)?
Table for Problem 23
Unknown Substance Conductivity Solubility (in water) Hardness Melting Point $(^\circ\mathrm{C})$ Boiling Point $(^\circ\mathrm{C})$
1 no (aq) soluble semi- brittle $164$
2 yes (aq) soluble $\mathrm{NA}$(liquid) $100$
3 yes (aq) soluble brittle $\approx 800$
4 no (s) insoluble soft $82$
5 yes (s) soluble $\mathrm{NA}$(liquid) $118$

List of Unknown Names:

• Sodium chloride
• Naphthalene
• Sucrose
• Hydrochloric acid (dilute)
• Acetic acid
1. Predict the type of bonding that will form between the elements sulfur and bromine. Will this molecule conduct electricity in water solution?

Feb 23, 2012

Oct 02, 2014

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