Name: Date: Class:
Intermolecular Forces Lab Activity – Lab Worksheet Answer Key
Intermolecular Forces Lab Worksheet - Answer Key
Directions: Today you will complete two experiments that compare the properties
of water to isopropanol (a type of rubbing alcohol). Before conducting each
experiment, you will first read about the experiment and then make a prediction
about what you think will happen. Afterward you do the experiment, you will write an
explanation of what was happening at the atomic level that let us observe these
properties.
Introduction: In many ways, water is a miracle liquid. It is essential for all living things on earth and it
is often referred to as a universal solvent because many substances dissolve in it. Water displays
1. Draw the structural formula (with lone pairs) for the two molecules we will be comparing today,
water (H
2
O) and isopropanol (C
3
H
8
O). Draw a dipole arrow to show the direction the electrons are
being pulled and label the areas of partial positive and negative charges.
2. Which molecule is more polar? ___________ water______________________________________
Experiment #1: Penny
How many drops of water fit on the head of a penny? How many drops of isopropanol fit on the head of
a penny?
Prediction:
a. I think _________ drops of water will fit on the head of the penny because ___________________
_______________________________________________________________________________
_______________________________________________________________________________
H
2
O
or
C
3
H
8
O
Note: the bond between C and H is NOT POLAR.
Do not draw dipole arrows or partial charges for
those bonds.
Name: Date: Class:
Intermolecular Forces Lab Activity – Lab Worksheet Answer Key
b. I think _________ drops of isopropanol will fit on the head of the penny because _______________
_______________________________________________________________________________
_______________________________________________________________________________
Procedure:
1. Place one penny on a paper towel.
2. Drop water onto the surface of the penny, slowly, one drop at a time using the
dropper bottle with
water
.
3. Count how many drops it takes until the water spills off the penny.
4. Record the number of drops.
5. Repeat steps A through D for isopropanol using the dropper bottle with isopropyl alcohol.
6. Compare the number of drops of isopropanol to the number of drops of water.
Observations:
Conclusions:
Were your predictions correct? Explain. __Answers will vary_______________________________________________
___________________________________________________________________________________________________________
___________________________________________________________________________________________________________
___________________________________________________________________________________________________________
Reading:
Since the hydrogen and oxygen atoms in the water molecule carry opposite
(partial) charges, nearby water molecules are attracted to each other. The
attraction between the δ+ hydrogen and the δ- oxygen in adjacent molecules is
called hydrogen bonding.
Hydrogen bonding is an intermolecular force. Intermolecular forces are forces
between molecules in a substance. Hydrogen bonding is one of the strongest
intermolecular forces and causes water to have many unusual properties.
Drops that fit on the penny
Water
Answers will vary
Isopropanol
Answers will vary
Name: Date: Class:
Intermolecular Forces Lab Activity – Lab Worksheet Answer Key
Neighboring water molecules are
attracted to one another. Molecules at the
surface of liquid water have fewer
neighbors and, as a result, have a greater
attraction to the few water molecules that
are nearby. This enhanced (greater)
attraction is called surface tension. It
makes the surface of the liquid water
slightly more difficult to break through
than the interior of the water.
Surface tension (seen on the left) is the force between water molecules at the surface of a sample of
liquid water. These molecules form stronger hydrogen bonds with each other than the molecules below.
A water strider is an insect that takes advantage of this property of water – the insect can walk along
the surface without breaking through.
Explanation: Why did more water drops fit on the surface of the penny then isopropanol drops? Use
evidence from the reading about hydrogen bonding and intermolecular forces in your response.
____ Answers will vary but should make note of the fact that isopropanol does not have surface
tension in the same way that water does because it does not feature hydrogen bonding between
molecules due to the lack of polarity. __________________________________________________
Experiment #2: Evaporation Rates
Which evaporates faster, water or isopropanol?
Prediction: Do you think water or isopropanol will evaporate faster? Why? ___ Answers will vary__
__________________________________________________________________________________
__________________________________________________________________________________
Procedure:
1. Dip one end of a Q-tip in water. Shake of excess water.
2. Dip one end of a different Q-tip in isopropanol. Shake off excess isopropanol.
3. At the same time, draw (streak) the tips of each Q-tip across a paper towel
in two parallel lines.
4. Time how long it takes for each streak to evaporate.
5. Watch and wait to determine which liquid evaporates first. (This may take a few minutes.) Record
your observations.
Observations:
Name: Date: Class:
Intermolecular Forces Lab Activity – Lab Worksheet Answer Key
Conclusions:
Were your predictions correct? Explain. __Answers will vary_______________________________________________
___________________________________________________________________________________________________________
___________________________________________________________________________________________________________
___________________________________________________________________________________________________________
Reading:
Since the hydrogen and oxygen atoms in the molecule carry opposite
partial charges, nearby water molecules are attracted to each other.
The attraction between the δ+ hydrogen and the δ- oxygen in adjacent
molecules is a special type of intermolecular force called hydrogen
bonding that causes water molecules to “stick” together in liquid form.
This force has to be overcome in order for liquid water to become a
gas. It takes a lot of energy to overcome the force of hydrogen
bonding.
When enough energy has been added to water it boils and turns
from liquid to gas. This happens at a temperature of 100˚C.
Isopropanol turns from liquid to gas at 82.5˚ C. Since the
vaporization of isopropanol occurs at a lower temperature than
water, this means it takes less energy to turn isopropanol into a gas,
and therefore it will evaporate faster than water. Isopropanol has
weaker intermolecular forces holding its molecules together so it
takes less energy (a lower temperature) to separate the molecules
to enter the gas phase.
Explanation: Why does isopropanol evaporate faster than water? Use the terms hydrogen bonding
and intermolecular forces in your response.
__ Answers will vary but should make note of the fact that isopropanol does not have hydrogen
bonding between molecules due to the lack of polarity which results in the molecules “sticking
together” more. Since the molecules are more stuck together, it takes more energy to separate
their bonds resulting in a higher boiling point.__________________________________________
Time to evaporation
Water
Answers will vary
Isopropanol
Answers will vary
