NAME _______________________________ Chemistry 162 SFCC
Win 2010
_______
Solutions
In this learning activity, you will examine, on the molecular level, why some substances mix well together
and some do not. You will compare molecular characteristics and relate these characteristics to solubility.
You will study mixtures and the dissolving process. When different substances evenly disperse in each other
with a clear and transparent appearance, the mixture is called a solution. A solution is specified by the
concentration of each substance present in it. You will learn several common ways used to express solution
concentration in modern society.
After completing this learning activity packet, you should be able to do the following:
(1) Describe the process of dissolving on a molecular level; explain how molecular characteristics
influence this process.
(2) State the like-dissolves-like rule and give actual examples.
(3) Define each of these terms in relation to a solution: solvent, solute and concentration.
(4) Explain and compare different concentration units: percentage, mg/L, ppm, ppb and molarity.
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I. Why Substances Dissolve
Both salt and sugar dissolve in water, yet neither oil nor gasoline do. Why do some substances mix well
together and others do not? The answer has to do with the interactions among different molecules or ions.
The diagrams in Figure 1 illustrate dissolving on a molecular level: B molecules are pulled apart by the
attractive forces between molecules A and B (indicated by the dotted lines) to disperse evenly among A.
Figure 1.
Dissolving
Process On
The Molecular
Level
molecule A
molecule B
...... A-B attractive forces. (the A-A and B-B
attractive forces are not shown)
A. The Like - Dissolves - Like Rule
Numerous experiments have verified that compounds with compatible characteristics of molecular structure
and polarity mix well with each other. This fact is called the like-dissolves-like rule. The Like-dissolves-like
rule is basic to our understanding of the dissolving process on the molecular level. Similar structural
features and polarity cause molecules of one compound to attract those of another compound. If this
intermolecular attractive force is stronger than the existing attractive forces among molecules of the same
type, the two compounds will dissolve in each other.
For example, as discussed in LAP FC.4, water is a polar compound. The water molecule, H 2O, has two O-H
bonds with a partial charge separation between the H atoms (+ pole) and the oxygen atom (- pole). Sugar
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molecule (sucrose, C12H22O11) has a similar structural feature of multiple O-H bonds with - oxygen and +
hydrogen atoms. This is why sugar and water mix well together. Sugar is one of the hydrophilic or "waterloving" substances (hydro- means water, -philic is derived from the Greek word philos, meaning, loving).
Q1. (a) Circle and identify all the O-H bonds in the structure of
Figure 2. Structure of Sucrose
sucrose in Figure 2.
CH2OH
(b) How many OH bonds are present in this structure? _____
CH
Similar to sugar, table salt is also a hydrophilic substance. Table
salt (sodium chloride, NaCl) is composed of sodium cations
(Na+1) and chloride anions (Cl-1). The + and - charges are
comparable to the + and - polarity in water. This is why salt is
very soluble in water. In addition, ions dissociate in water. Refer
to LAP FC.4, Water and Its Properties, section V, for a more
detailed discussion concerning dissociation of ions in water.
CH2OH
O
O
CH
HO
CH
OH
CH
C
CH
HO
O
CH
OH
CH
CH
CH2OH
OH
Oil and gasoline are examples of hydrophobic or "water-fearing" substances (-phobic is derived from the
Greek word phobos, meaning, a fear). Every time you use oil-and-vinegar dressing on your salad, you have
to first shake it up, because the dressing’s oil and water naturally stay separated. Gasoline also stays separate
from water instead of dissolving in it. On the other hand, the two hydrophobic substances, gasoline and oil,
mix very well with each other. All of these phenomena show that like-dissolves-like rule is at work!
Let us examine the structures of two hydrophobic compounds. Figure 3(a) shows the structure of heptane, a
typical molecule found in gasoline. Heptane has a structural feature of repeating -CH2- groups. This feature
is also found in Figure 3(b), which shows a typical structure of corn oil molecules.
Figure 3 (a) Typical
H
H H
H
H H
H
Molecule Found In
│
│ │ │
│ │
│
Gasoline Oil
H C  C  C  C  C  C  C  H
│ │ │
│
│ │
│
H H
H
H
H H
H
Figure 3 (b) Typical
Molecule Found In
Corn Oil (written in
the condensed way.)
This structure can be written in a condensed way
as: CH3CH2CH2 CH2CH2CH2CH3
O
H H

 
CH2-O-CCH2CH2CH2CH2CH2CH2CH2C=CCH2CH2CH2CH2CH2CH2CH2CH3
CH-O-COCH2 CH2CH2CH2CH2 CH2CH2CH2CH2CH2CH2CH2CH2 CH2CH3
CH2-O-C-CH2CH2CH2CH2CH2CH2CH2C=CCH2CH2CH2CH2 CH2CH2CH2CH3
 
O
H H
Q2. The structures of molecules in gasoline and oil are dominated by many C-C and C-H bonds.
(a) Is the C-C bond polar or nonpolar? __________ (b) Is the C-H bond polar or nonpolar? ___________
(c) Explain why gasoline and oil do not mix well with water on the molecular level.
______________________________________________________________________________________
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B. Applications Of The Like - Dissolves - Like Rule
Perform the following hands-on exercise to examine how different compounds mix with water.
Put On Safety Goggles and Protective Clothing !!
Obtain a designated test tube tray marked for solubility test. There are five test tubes in this tray, each
containing a different compound: test tube #1, methanol (CH3OH), #2 acetone (CH3COCH3) , #3 methylene
chloride (CH2Cl2), #4 hexane (C6H14), and #5 sodium iodide (NaI). The compounds in test tubes #1 through
#4 are volatile, which means they vaporize easily at room temperature. Avoid inhaling any vapor by keeping
the test tubes stoppered. Open a stopper only briefly as directed. Never direct the mouth of a test tube toward
any persons while shaking. If skin contact occurs, wash with soap and plenty of tap water.
With a plastic transfer pipette, deliver 1 mL distilled water into each test tube. Stopper and gently invert
each test tube to mix the contents. Due to possible vapor build-up, vent the test tube after mixing by opening
the stopper briefly.
Q3. (a) Record in Table 1 whether the compound in each test tube mixes well with water.
(b) Write the formula for each compound in Table 1.
(c) Calculate the difference in electronegativity of each of the following types of chemical bond. These
bonds are found in the compounds given in Table 1.
C-H _______
C-O ______ O-H _______
C-C _______
C-Cl _______
Na - I _______
(d) Label the polar (electronegativity difference > 0.5) and ionic (electronegativity difference > 1.7) bonds
in each model structure in Table 1 as shown for the structure of methanol.
Table 1. Solubility of Different Compounds in Water
Hydrogen
Test tube #
Oxygen
carbon
1
methanol
2
acetone
chlorine
3
methylene chloride
Formula
Model
Structure
Label the polar
and ionic bond(s)
in each structure.
C-O
O-H
Does this
compound mix
well with water?
3
iodide
4
hexane
sodium
5
sodium iodide
Q4. Does the solubility of each compound in Table 1 correlate with its molecular characteristics according
to the like-dissolves-like rule? ________ Explain.
_______________________________________________________________________________________
The next solubility tests involve iodine (I2). Complete the following.
Q5. (a) Is iodine, I2, polar or nonpolar? _______________
(b) Based on the like-dissolves-like rule, predict whether I2 will mix better with water or methylene chloride
(CH2Cl2). _______ Explain why.
_______________________________________________________________________________________
Obtain a grain of I2 crystal from its reagent vial with a pair of tweezers. Drop it into test tube #3 (containing
water and CH2Cl2). Stopper and gently invert the test tube to mix its contents.
(c) Report your observations. _______________________________________________________________
(d) Did the observation in (c) verify your prediction in (b)? _______ Explain.
_______________________________________________________________________________________
Q6. (a) Predict whether iodine will mix better with hexane (C6H14) or water. __________ Explain.
_______________________________________________________________________________________
Drop a grain of I2 crystal into test tube #4 (containing water and hexane). Stopper and gently invert the test
tube to mix its contents.
(b) Report your observations. _______________________________________________________________
(c) Did the observation in (b) verify your prediction in (a)? _______ Explain why.
_______________________________________________________________________________________
II. Solutions are Homogeneous Mixtures
If two or more substances are put together and the mixture does not have a uniform appearance, it is called a
heterogeneous mixture. If the mixture has a uniform, clear and transparent appearance, it is called a
solution. A solution is a homogeneous mixture of two or more elements and /or compounds. Usually the
major component in a solution is labeled as the solvent and all other components are each called a solute.
A solution and a pure compound differ in terms of composition. A pure compound has a definite
composition; the elements in the compound combine in definite ratios, specified by its chemical formula. A
solution, on the other hand, has variable composition; the components in a solution do not have any definite
combining ratios. This is why a solution may be diluted or concentrated. To specify a particular solution, we
need to tell what chemicals are in it, as well as the concentration of each compound or element present.
Q7. Examine the rack of test tubes labeled P through W to identify the type in each tube as either a solution
or mixture. (Do not open the stopper on any test tube.) Complete Table 2. Each test tube contains either
a solution or a heterogeneous mixture. If a tube contains a solution, tell which component is the solvent.
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Table 2. Test Tubes of Mixtures
P
mixture / solution
Name
Content
Cobalt nitrate & water
H2O (15 mL), Co(NO3)2 (0.1 g)
Q Copper sulfate & water
H2O (15 mL), CuSO4 (0.1 g)
R
Cobalt chloride & water
H2O (15 mL), CoCl2 (0.1 g)
S
Isopropyl alcohol and
water (Rubbing alcohol)
C3H8O (7 mL), H2O (3 mL)
T
Acetic acid and water
(vinegar)
H2O (9.5 g) , C2H4O2 (0.5 g)
U Hexane and water
hexane (4 mL), water (4 mL)
V Biphenyl and water
water (10 mL) , biphenyl (0.1 g)
W Orange juice
water and orange pulp
solvent used
Note that most of the solutions in Table 2 use water, these are aqueous solutions, which are the most
common type.
III. Solution Concentrations
Solution concentration specifies the amount of solute present in certain standard quantity of solvent. There
are many different ways to express the concentration of a solution. Commonly used concentration units
include percentage (%), parts-per-million (ppm), parts-per-billion (ppb) and molarity (M). These are
summarized in Table 3. Each concentration unit can be seen as a ratio of the amount of solute over a certain
amount of solvent or solution. In this section, we will discuss the practical significance of each concentration
unit, and how to convert or relate one concentration unit to another.
Table 3. Commonly Used Concentration Units
Unit of
concentration
Molarity
Percentage or Parts
per hundred
Parts per million
Parts per billion
Expression
M
%
ppm
ppb
solute in unit of:
mole
part
part
part
amount of solution
1 Liter
100 parts
1,000,000 parts
1,000,000,000 parts
Ratio format
x mole

1L
x part

100 parts
x part

1,000,000 parts
x part

1,000,000,000 parts
common usage
In chemistry
laboratories
relatively high
concentrations
relatively low
concentrations
extremely low
concentrations
Example
Stomach juice
contains
0.15 M HCl.
Rubbing alcohol is
a 70% solution of
alcohol and water.
Danger level for
carbon monoxide
(CO) in air is 9 ppm
in 8 hour-period.
Maximum
contamination level of
lead (Pb) in drinking
water is 50 ppb.
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A. Molarity (M)
Molarity (M) is the concentration unit most frequently used in chemistry. Molarity specifies the number of
moles of a solute present in one liter (L) of a solution. Each mole equals 6.02  1023 individual units. For
examples: every mole of acetone contains 6.02  1023 molecules of acetone, C3H6O. Every mole of sodium
chloride, NaCl, contains 6.02  1023 pairs of the ions sodium (Na+1) and chloride (Cl-1). An aqueous solution
of 1 M sucrose, for example, contains 1 mole of C12H22O11 per liter of this solution.
Q8. Calculate the mole(s) of solute present in solutions (a) through (d).
(a) A 0.05 M KCl solution contains ______________ mole(s) of KCl in one liter of the solution.
(b) A 1.0 M HCl solution contains ___________ mole(s) of HCl in two liters of the solution.
(c) A 0.15 M NaOH solution contains _____________ mole(s) of NaOH in 0.5 liter of the solution.
(d) A 0.83 M HC2H3O2 solution contains ____________ mole(s) of HC2H3O2 in 2.6 liters of the solution.
(e) The common arithmetic procedure you used to find the answers to (a) through (d) is:
_______________________________________________________________________________________
B. Percentage (%), Parts Per Million (ppm) and Parts Per Billion (ppb)
Solution concentrations are also expressed in three other related units: percentage (%), parts per million
(ppm) and parts per billion (ppb). These are related because they all designate concentration by the parts of
a solute in a specified parts of the solution.

Percentage (%) concentration tells the amount of solute present in 100 parts of the solution.
% = (amount of solute  amount of solution )  102

Parts per million (ppm) measures the parts of solute in one million parts of the solution.
ppm = (amount of solute  amount of solution )  106

Parts per billion (ppb) measures the parts of the particular solute in one billion parts of the solution.
ppb= (amount of solute  amount of solution )  109
In calculating concentrations using any of the above three units, the “amount” of solute and that of solution
should be given in the same units. For example, if the amount of solute is given in grams (g), the amount of
solution is also given in grams. If the amount of solute is given in milliliters (mL), that of the solution is also
given in mL. However, since 1 g = 1 mL for most aqueous solutions, in some cases, solute is given in grams
and solution is given in mL, or vice versa.
Q9. (a) A solution contains 2 L of a solute in 1 L of the solution. What is its concentration in ppm? Explain.
______________________________________________________________________________________________
(b) A solution contains 0.5 mL of a solute in 1 L of the solution. What is its concentration in ppm? Show
work.
_______________________________________________________________________________________
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(c) A 1% acetone solution contains _______ L acetone in 10 mL of the solution. Show work.
_______________________________________________________________________________________
Q10. (a) A solution contains 5 g of a solute in 1 Kg of the solution. What is its concentration in ppb?
Show work
_______________________________________________________________________________________
(b) A solution contains 3.5 mg per Kg. What is its concentration? Explain or show work.
_______________________________________________________________________________________
FURTHER STUDIES
Further Application Of The Like-Dissolves-Like Rule
You have learned that some molecules are hydrophobic; they do not mix with water. Other molecules are
hydrophilic; they mix with water well. A third type of molecules have double nature: a part of such a
molecule is hydrophobic, and the other part of it is hydrophilic. An example of this type of molecules is
soap. Soap is a chemical you use frequently to clean greasy or oily hands. Without using soap, the more you
try to rinse grease off of your hands with water, the more grease sticks to your hands. This is because grease,
like oil, is hydrophobic. When soap is present, grease is easily rinsed off with running water. Soap acts as a
middle agent to bring grease and water together. Soap does this by having a molecular structure that contains
both a polar segment and a nonpolar segment. The structure of a typical soap molecule is shown in Figure 4.
Figure 4. A
Typical Soap
Molecule
O

-1
+1
CH3CH2CH2CH2 CH2CH2 CH2CH2 CH2CH2 CH2CH2CH2CH2 CH2CH2 CH2  CO Na
Head
Tail
Q11. (a) Which section in the molecule is polar, the head or the tail? _______
(b) When coming in contact with grease, which part of soap will mix with grease, head or tail? _________
(c) Which part of soap will stay in water? _______
As soap molecules pull grease together with it to water, they carry grease away with them in rinse water.
Examples of Aqueous Solutions
Q12. Almost all of the liquid solutions we encounter in everyday life are aqueous solutions, with water as
the solvent. Look around your house or consider what you drink each day, to list a few examples of
these aqueous solutions in Table 4. Check product labels, if necessary, to find the names of solutes in
each solution. These you create on your own.
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Table 4.
Common Aqueous Solutions
Solution type
solvent
possible solute(s)
water
water
water
water
Solution Concentrations
Q13. Complete the following:
(a) A normal saline solution of 0.017 M NaCl contains ____________ mole(s) of NaCl in 2.5 liters of the
solution. Show work.
____________________________________________________________________________________
(b) There are 5 gram of acetic acid in every 100 grams of vinegar. The percentage of this solution is ______
(c) There are 7 g of minerals in every 1000 grams of milk. The percentage of minerals in milk is _________
Show work. __________________________________________________________________________
(d) There are 75 g of potassium iodide (KI) in every gram of iodized table salt (NaCl).
The concentration of KI in this iodized salt is __________________ ppm.
(e) The maximum contamination level of arsenic allowed in drinking water is 0.05 milligrams per liter
(mg/L). In water and most aqueous solutions, 1 L = 1 Kg.
This concentration of 0.05 mg/L is equivalent to ________________ ppm, and ________________ ppb.
Show work.
____________________________________________________________________________________
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