Solution Experiment
Collin College
Christian E. Madu, PhD and Michael Jones, PhD
Objectives
•
Predict the polarity of a molecule using the Lewis Dot Formula and molecular
shape.
Determine the polarity of solvents using the ‘Like Dissolves Like’ rule.
Determine the type of Mixtures
Create a solubility curve for KNO3
•
•
•
Safety: SAFETY GOGGGLES MUST BE WORN WHILE WORKING IN THE LABORATORY
Introduction
A mixture is composed of two or more pure substances that are physically mixed, but
not chemically combined. For example, a sugar solution is a mixture of sugar and water.
Also, the air we breathe is a mixture of oxygen, nitrogen, and small amounts of carbon
dioxide, argon, water vapor and other gases.
Although the proportions by mass of the components may vary from sample to sample,
the mixture retains most of the properties of its components.
There are basically two classes of mixtures:
-
Heterogeneous mixture (has no uniform composition)
Homogeneous mixture (has uniform composition)
A solution is a homogeneous mixture in which a varying proportion of particles of a pure
substance (solute) are uniformly dispersed in another substance (solvent). The
particles of the solvent are usually present in a larger quantity while the particles of the
solute are present in a smaller quantity. An aqueous solution is a solution with water as
the solvent and the solute could be a soluble solid, liquid or gas.
In making a solution, the polarity of the solute and solvent must be the same. Usually,
polar solutes are soluble in polar solvents while non-polar solutes are soluble in nonpolar solvents.

This is called the ‘like dissolves like’ rule.
We can use the knowledge of the electron–dot formula to predict the 3D-shape of
the molecule. Then the bond polarity and the shape can in turn be used to predict the
polarity of the molecule. In the valence shell electron-pair repulsion (VSEPR) theory,
the electron groups (lone pairs, single, or multiple bonds) around the central atom are
arranged as far apart as possible to minimize the repulsion between their negative
charges. The total number of electron groups around the central atom is determined by
drawing the electron-dot formula of the molecule, while the 3D shape of the molecule is
determined by the number of atoms and lone-pair of electrons within the electron
group. The polarity of a bond is determined by the electronegativity difference
between the two bonding atoms. Electronegativity is the ability of the bonding atom
to attract the bonding electrons to itself.


A bond between two atoms with similar or identical electronegativities
is said to be a nonpolar bond.
A bond between two atoms with different electronegativities and
therefore an unequal sharing of the bonding electrons is said to be a
polar covalent bond. The polar bond shows a charge separation
called a dipole.
A molecule with two or more covalent bonds could be a nonpolar molecule if:


There are no polar bonds in the molecule.
There are polar bonds with symmetrical arrangement in the molecule
(a symmetrical arrangement cancels the dipoles of the bonds, making
the molecule as a whole nonpolar).
A molecule with two or more polar covalent bonds will be a polar molecule if there are
polar bonds with unsymmetrical arrangements within the molecule.
A.
Predicting the Polarity of Molecule
We are going to use the electron-dot formula, VSEPR theory, 3D shape, and bond
polarity to determine the molecular polarity of water (H2O). Answer the following
questions in the table provided below.
1.
2.
3.
4.
Draw the electron-dot formula of H2O.
How many electron groups are around the central atom?
What is the molecular shape of H2O?
What types of bonds are in a water molecule and what is the polarity of the
bonds (polar or nonpolar bonds)?
5. Is H2O a polar or a nonpolar molecule?
Table 1
Molecule
Electrondot
formula
3D shape
Indicate
the bonds
present
Polarity of
bonds
Molecular
polarity
H2O
B.
Determining the polarity of solvents using the ‘Like Dissolves Like’
Rule
We will use the result obtained from part ‘A’ above to determine the polarity of other
solvents. When a liquid dissolve in another liquid they are said to be miscible liquids.
If they do not dissolve in each other and instead form different layers when mixed they
are said to be immiscible liquids.
Materials:
Test tubes (7), test tube rack, stirring rods, H2O, hexane, methanol,
pentane, cyclohexane.
B.1
Label the test tubes 1-7 and set them up in a test tube rack.
B.2
To each test tube 1-4 add 3 mL of water and to each test tube 5-7 add 3 mL
of hexane.
B.3
Add the following materials to the specified test tubes:
Table 2
Test tube #
1
2
3
4
5
6
7
Add 3 mL
H2O
H2O
H2O
H2O
Hexane
Hexane
Hexane
Then add 2 mL
Hexane
Methanol
Pentane
Cyclohexane
Methanol
Pentane
Cyclohexane
Miscibility
B.4
Use the result in the table above to determine the polarity of the solvents
in the table below
Table 3
Solvent
Hexane
Methanol
Pentane
Cyclohexane
**
Molecular Polarity
Please note that all waste should be discarded into the appropriate waste
containers provided in the lab, not in the sink.
C. Polarity of solvents and solutes using the ‘Like Dissolves Like’ Rule
Here we are going to determine the solubility of some solids in polar and nonpolar
solvents and then use the ‘like dissolves like’ rule to determine the polarity of the solids
and the type of mixture formed (homogeneous or heterogeneous mixture).
Materials:
Test tubes (6), test tube rack, spatulas, stirring rods, H2O, hexane, NaCl,
sucrose, biphenyl.
C.1
Label the test tubes 1-6 and set them up in a test tube rack.
C.2
To each test tube 1-3 add 2 mL of water and to each test tube 4-6 add 2 mL
of hexane.
C.3
Add a few crystals of the following materials to the specified test tubes:
Table 4
Test tube
#
1
2
3
4
5
6
Add 2 mL
H2O
H2O
H2O
Hexane
Hexane
Hexane
Then add few
crystals of
NaCl
Sucrose
Biphenyl
NaCl
Sucrose
Biphenyl
Solubilty
Type of
mixture
C.4
Use the result in the table above to determine the polarity of the solutes in
the table below
Table 5
Solvent
NaCl
Sucrose
Biphenyl
Molecular Polarity
** Please also note that all waste should be discarded into the appropriate waste
containers provided in the lab, not in the sink.
D.
Solubility curve of KNO3
Solubility of a compound is defined as the maximum amount of solute that can be
dissolved in 100 g of water at a given temperature. Solubility is usually stated in g/100 g
H2O. The solubility of solids generally increases as the temperature increases. When the
solution that holds a maximum amount of solute at a particular temperature (saturated
solution) is cooled to a lower temperature, the excess solute will precipitate out at the
lower temperature. A solubility curve is the plot of solubility (g/100 g H2O) versus
temperature (degree Celsius).
Materials:
Large test tube, weighing bowl, 400 mL beaker, buret clamp, 10 mL
graduated cylinder, KNO3(s), and hot plate.
To reduce the amount of KNO3 used, each group of students will be assigned a given
amount of KNO3 to use, and the results obtained by each table will be shared with the
entire class to be used for plotting the solubility curve.
D.1
Each table will be assigned to carefully weigh out one of the following amounts of
KNO3 (4 g, 5 g, 6 g, 7 g, 8 g, or 9 g KNO3). Place the weighed out amount of KNO3
in the large test tube.
D.2
Add 10 mL of H2O to the test tube at room temperature and clamp the test tube
to a ring stand and place the test tube in a beaker of water.
D.3
Use the hot plate to heat the water while stirring the mixture in the test tube until
all the KNO3 solid dissolves completely. Continue heating to 60 °C.
D.4
Remove the test tube from the hot water and allow the test tube and content to
cool while stirring gently with the thermometer. Look closely for the first
appearance of crystals and record the temperature of the solution at the
appearance of the crystals.
D.5
Repeat the heating and cooling steps until you get two or three temperature
readings that agree.
D.6
Record your result and those of the other lab groups in the table below and use
the table to plot a solubility curve of solubility (g solute/100 g H2O) versus
temperature (°C).
Table 6
Mass KNO3 (g)
Temperature (°C)
Crystals Appear
Solubility
(g KNO3/100 mL H2O)
References
1. Karen C. Timberlake; Essential Laboratory Manual for general, organic, and
biological chemistry. Second Edition, Pearson Prentice Hall, 2011.
2. Molecular structure