Solution Formation
CK12 Editor
Say Thanks to the Authors
Click http://www.ck12.org/saythanks
(No sign in required)
To access a customizable version of this book, as well as other
interactive content, visit www.ck12.org
CK-12 Foundation is a non-profit organization with a mission to
reduce the cost of textbook materials for the K-12 market both
in the U.S. and worldwide. Using an open-content, web-based
collaborative model termed the FlexBook®, CK-12 intends to
pioneer the generation and distribution of high-quality educational
content that will serve both as core text as well as provide an
adaptive environment for learning, powered through the FlexBook
Platform®.
Copyright © 2012 CK-12 Foundation, www.ck12.org
The names “CK-12” and “CK12” and associated logos and the
terms “FlexBook®” and “FlexBook Platform®” (collectively
“CK-12 Marks”) are trademarks and service marks of CK-12
Foundation and are protected by federal, state, and international
laws.
Any form of reproduction of this book in any format or medium,
in whole or in sections must include the referral attribution link
http://www.ck12.org/saythanks (placed in a visible location) in
addition to the following terms.
Except as otherwise noted, all CK-12 Content (including
CK-12 Curriculum Material) is made available to Users
in accordance with the Creative Commons Attribution/NonCommercial/Share Alike 3.0 Unported (CC BY-NC-SA) License
(http://creativecommons.org/licenses/by-nc-sa/3.0/), as amended
and updated by Creative Commons from time to time (the “CC
License”), which is incorporated herein by this reference.
Complete terms can be found at http://www.ck12.org/terms.
Printed: September 18, 2012
AUTHOR
CK12 Editor
www.ck12.org
C ONCEPT
Concept 1. Solution Formation
1
Solution Formation
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
a solution that contains ions and is capable of conducting an electric current
hydration
the process of water molecules attaching to ions
immiscible
liquids that do not have the ability to dissolve in each other
miscible
liquids that have the ability to dissolve in each other
non-electrolyte
a solution that does not contain ions and is not capable of conducting an electric current
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.
1
www.ck12.org
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, 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, 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).
MEDIA
Click image to the left for more content.
2
www.ck12.org
Concept 1. Solution Formation
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:
Na + ionization energy → Na+ + e−
I + e− → I− + energy of electron affinity
Na+ + I− → NaI + 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?
a. MgCl2
b. Al2 O3
c. CH4
Solution:
MgCl2 and Al2 O3 would contain electrostatic attraction because they are both ionic, but CH4 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:
−
NaCl(s) → Na+
(aq) + 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.
3
www.ck12.org
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.c
om/watch?v=EBfGcTAJF4o (0:53).
MEDIA
Click image to the left for more content.
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 CO2 , 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:
4
www.ck12.org
Concept 1. Solution Formation
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:
C12 H22 O11(s) → C12 H22 O11(aq)
Example:
Which compounds will dissolve in solution to separate into ions?
a. LiF
b. P2 F5
c. C2 H5 OH
Solution:
LiF will separate into ions when dissolved in solution:
−
LiF(aq) → Li+
(aq) + F(aq)
P2 F5 and C2 H5 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 NaCl.
5
www.ck12.org
−
NaCl(s) → Na+
(aq) + Cl(aq)
Once the solid 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 NaCl(aq) , the Na+
(aq) ions would move toward one electrode and the Cl(aq) ions would move toward the
second electrode. This movement of ions allows the electric current to flow through the solution. Therefore, NaCl(aq)
−
will behave as an electrolyte and conduct electricity because of the presence of Na+
(aq) and 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?
a. CaF2(aq)
b. C6 H12 O6(aq)
c. KOH(aq)
Solution:
CaF2(aq) and 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: CaF2(s) → Ca2+
(aq) + 2 F(aq)
−
Potassium hydroxide: KOH(s) → K+
(aq) + 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.
C6 H12 O6(s) → C6 H12 O6(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 1.1.
6
www.ck12.org
Concept 1. Solution Formation
TABLE 1.1: Combinations that Form Solutions
Combination
Polar substance in a polar substance.
Non-polar substance in a non-polar substance.
Polar substance in a non-polar substance.
Non-polar substance in a polar substance.
Ionic substance in a polar substance.
Ionic substance in a non-polar substance.
Solution Formed?
Yes
Yes
No
No
Yes
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.
Further Reading / Supplemental Links
Visit the website for a presentation on solutions.
• http://www.public.asu.edu/ jpbirk/CHM-113_BLB/Chpt13/sld002.htm
Review Questions
1.
2.
3.
4.
5.
What is the “like dissolves like” generalization and provide an example to illustrate your answer.
Why will LiCl not dissolve in CCl4 ?
Will acetic acid dissolve in water? Why?
What is the difference between intermolecular and intramolecular bonds?
In which compound will benzene (C6 H6 ) dissolve?
a.
b.
c.
d.
carbon tetrachloride
water
vinegar
none of the above
6. In which compound will sodium chloride dissolve?
a.
b.
c.
d.
carbon tetrachloride
methanol
vinegar
none of the above
7
www.ck12.org
7. In which compound will ammonium phosphate dissolve?
a.
b.
c.
d.
carbon tetrachloride
water
methanol
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.
a.
b.
c.
d.
e.
NaOH(s)
LiOH(s)
C5 H10 O4(s)
NH4 Cl(s)
MgCl2(s)
10. Which of the following represent physical changes? Explain.
a.
b.
c.
d.
explosion of TNT
dissolving KCl
sharpening a pencil
souring milk
11. Which compound contains electrostatic forces?
a.
b.
c.
d.
natural gas
table salt
air
sugar
12. Which of the following is a physical change?
a.
b.
c.
d.
rotting wood
rising of bread dough
rusting iron
molding cheese
13. Which of the following is not a physical change?
a.
b.
c.
d.
melting iron
pumping gas
reaction of chlorine with sodium
reaction of magnesium chloride with water
14. Which compound is considered to be an electrolyte when dissolved in water?
a.
b.
c.
d.
HNO3
C12 H22 O11
N2 O
CH4
15. Which compound is not considered to be an electrolyte?
a.
b.
c.
d.
AgCl
PbSO4
C2 H6
HClO3
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?
8
www.ck12.org
Concept 1. Solution Formation
Solution 1: Clear
Solution 2: Blue but transparent
Solution 3: Clear
17. Describe the intermolecular bonding that would occur between glucose, C6 H12 O6 , and water.
18. Define non-electrolyte and give at least one example.
19. 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?
20. Describe how you could tell the difference between an electrolyte and a non-electrolyte solution.
21. Looking at the periodic table, which pair of elements will form a compound that is covalent?
a.
b.
c.
d.
Ca and Br
Fe and O
Si and F
Co and Cl
22. Which of the following compounds will conduct the least amount of electricity if dissolved in water?
a.
b.
c.
d.
KNO3
BaCl2
CsF
CO2
23. 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 1.2) to the
solution names (found under the table)?
TABLE 1.2: Table for Problem 23
Unknown Substance
1
2
3
4
5
Conductivity
no (aq)
yes (aq)
yes (aq)
no (s)
yes (s)
Solubility
water)
soluble
soluble
soluble
insoluble
soluble
(in
Hardness
semi- brittle
NA(liquid)
brittle
soft
NA(liquid)
Melting
(◦ C)
164
Point
Boiling
(◦ C)
Point
100
≈ 800
82
118
List of Unknown Names:
•
•
•
•
•
Sodium chloride
Naphthalene
Sucrose
Hydrochloric acid (dilute)
Acetic acid
24. Predict the type of bonding that will form between the elements sulfur and bromine. Will this molecule
conduct electricity in water solution?
9