Fellow name: Kim Cross
Title of Lesson: What is so special about water?
School: University High School
Grade Level: 10th-11th
Subject(s): Chemistry
Summary
Students will be exposed to the importance of surface tension, and its effect on life.
For example, water striders are able to walk on water because of water’s surface tension.
Plants rely on the surface tension to transport water inside them.
Although a water molecule has an overall neutral charge, the actual structure of a
water molecule makes it a polar molecule. The polarity of the water molecule causes it to
be attracted to other water molecules as well as molecules of other substances. The
attraction between water molecules is called cohesion. The attraction of water molecules
to other substances, like soil or glass, is called adhesion. The cohesive force that occurs
between water molecules is so strong that when comes in contact with another medium,
such as air, the water creates a "sticky skin", which is known as surface tension. These
bonds are so strong that they can support insects,
In what way is your lesson/activity inquiry-based?
-When the class period starts there will be a beaker of boiling water in front of the room
and the students will be asked “what are the bubbles coming off the beaker?”
-Then I will ask questions based on previously known information such as
-What is an atom?
-What is a molecule & provide and example
-How are atoms held together?
-What is a covalent bond & provide an example of a molecule with a covalent bond
- What is an ionic bond & provide an example of a molecule with an ionic bond
-What is the chemical formula for table salt?
-What happens when you put salt in water & is this a chemical or physical change
This is a good time to recap of physical and chemical changes, specifically in
relation to bonding. When physical changes occur the molecule remains the same,
for example with water you have ice at OC, liquid water at room temperature, and
water vapor above boiling temperature, yet they all have the same compositing H2O.
However, when chemical changes occur the molecular formula differs, chemical
bonds are broken and reformed.
Time Required
1 class period 90 minutes
Group Size
4 Students per group
Estimated cost
Cost of the detergent, wax paper and needles ($15-$20)
Introduction / Motivation
Each student will get a picture of water molecule and on the bottom of the page that says
“You depend on me. I help you exist.”
“You depend on me.
I help you exist”
The students will be told, “Last night some one slipped this note under the door of the
office at my research lab. I was very curious why someone mysteriously left this for me. So I
brought it here to get some assistance from all of you. What do you guys think this is or
what it means?”
-Note if someone responds that its water. I will ask how they know.
-Then further ask which atom is oxygen and which is hydrogen?
- What are the positive and negative signs for?
-If positive and negative charges attract each other, what causes something to be
more positive or negative?
-Covalent bonds share electrons, and it has been explained that water is
covalently bonded, so how could there be a charge?
Water is covalent bonded; however there is more than meets the eye with this molecule.
Yes, oxygen and hydrogen share electrons; however the electrons are not shared equally.
There is a slight inequality involved. Here is an example to imagine what’s occurring. A
husband and wife are sleeping in the same bed right now during the winter. The husband is
a tall husky guy and the wife is a short petite woman. They have a nice electric blanket to
keep them warm however it just barely covers both of them. The husband, since he is
bigger, needs more of the blanket, so he takes more of the blanket than the smaller wife.
This is like the relationship between oxygen and hydrogen in the water molecule. The
oxygen “tugs” at the electrons more than the hydrogen.
Question: What is occurring when the oxygen pulls/ attracts the electron a little bit
more? Think in terms of overall charge.
The oxygen becomes slightly negative while the hydrogen becomes slightly
positive
Electronegativity is the tendency for one atom to pull the electrons of another atom.
-Which atom in water is more electronegative?
-What do you think this means in terms of a bunch of water molecules together?
-What will happen in terms of attraction?
The hydrogen atoms in one water molecule are attracted to the oxygen
atom of another molecule. So the result is water molecules are attracted to
other water molecules.
Water molecules form weak bonds with one another. These are called Hydrogen bonds.
We are going to look at an important property of water called surface tension in a lab
today. When we’re done, we’ll come back and talk about what you discovered, and what
that means in terms of how water helps sustain life on earth. The tendency of molecules to
stick together is called cohesion, which is stronger for water than for most other liquids.
Cohesion is very important in plants. Trees depend on cohesion to help transport water
from their roots to their leaves. Cohesion helps water rise against the force of gravity.
Surface tension is a measure of how difficult it is to stretch or break the surface of a
liquid. Hydrogen bonds give water an unusually high surface tension, making it behave as
though it were coated with an invisible film.
Today’s ActivityWalter the Water Strider has a dilemma. Water
Striders are insects that live on the surface of water
in slow moving streams. They are very sensitive to
the motion around them, and can move at a rate of
about a meter per second. Approximately three
weeks ago, someone reported that the river was
changing a few miles upstream. Something was
causing the striders to fall in the water, instead of
being able to walk like they usually do. In the past week, several of Walter’s family
members strode upstream to investigate, but, according to witnesses, they fell through the
water. The new and dangerous river water seems to be getting closer to Walter’s home, and
he’s concerned for his life. Walter never thought about what enables him to walk on water.
He always took it for granted. But now as his life seems in danger, he wonders what helps
him walk on water. Perhaps if he can figure that out, then he can learn how to save himself.
What might be causing the Water Striders to fall in the water?
Materials List:
-dropper
-water
-detergent
- Pepper
-50 pennies
-Sheet of wax paper
-2 needles
-Clear plastic cup
- Alcohol
-Petri Dish
Part 1 (Needle in a Petri Dish): Water, Alcohol, Needle, Petri Dish, and Detergent
Part 2 (Penny Drop): Water, Dropper, Pennies
Part 3 (Over the Top): Water, Dropper, Pennies, Cup
Part 4 (Pepper Time): Water, Petri Dish, Pepper, and Detergent
Part 5 (Wax Paper): Water, Wax Paper, and Dropper
Procedure
Part 1: Needle in a Petri Dish
Fill a Petri dish with water. Put the needle on a fork and slowly lower the fork over
the water, then into the water, then down so the needle floats. Were you able to
make it float?
Then fill a Petri dish with alcohol. Try floating the needle in it by dipping it in with
the fork. Were you able to make it float?
Put the needle in water again. Make it float. Then dip a second needle in detergent.
Try to make it float. What happened?
Part 2: Penny Drop
Guess how many drops of water you can place on the surface of a penny without it spilling
over:___________
With a penny, conduct two separate trials to see how many drops the penny can hold. Use
heads or tails and use two different pennies.
Draw what the water on the penny looks like before the water spills over:
Penny Drawing
Trial 1 ________ Trial 2 _________ Calculate the average:________
Part 3: Over the Top
Fill a small cup with water to the top without spilling over.
How many pennies do you think you could add before the water spills over?________
Slowly add pennies to the lass of water one at a time. What does the side of the glass look
like as you add more pennies? Draw the side view below:
How many pennies did you add before it spilled over?_______
Part 4: Pepper Time
Fill a Petri dish with water. Sprinkle pepper over the top of the water. What do you
observe?
How many pepper pieces are immersed in the water?
Place one drop of detergent in the dish. What did you observe?
Are there more or less pepper pieces immersed in the water?
Part 5: Wax Paper
Take a piece of wax paper and place some drops of water on the wax paper. Try to push the
water drops together. What do you observe?
Draw how the water looks on the wax paper.
Lesson Closure
1. What enables Walter to walk on water?
2. What is surface tension?
3. What caused the water molecules to stick together and create surface tension?
4. Why did the needle float easier on the water than on the alcohol?
5. How did the addition of detergent affect the surface tension of water?
6. Why does detergent have this effect on water?
7. What shape did water take while on top of the glass or on top of the penny?
8. Why did water take on a dome shape, and not a flat one when on the wax paper?
9. Based on the evidence you collected in this activity, what do you think might be causing
all the Water striders to fall into the water?
10. What are hydrogen bonds?
11. Draw four water molecules, and show how they are attracted to one another (show the
charges).
Is this lesson based upon or modified from existing materials? If yes, please specify
source(s) and explain how related:
- Floating Paperclip and other Surface Tension Experiments:
www.exo.net/~jillj/activities/surfacetension.pdf
-Surface Tension - an introduction to surface tension:
physics.about.com/od/physicsexperiments/.../surfacetension.htm
-Walking On Water: Water Striders and Surface Tension:
http://www.woodrow.org/teachers/bi/1998/waterstrider/student_lab.html
References
Smith, Robert W. Hands On Science, Copyright, 1989, Instructional Fair
Publisher
Walpole, Brenda, 175 Experiments to Amuse and Amaze Your Friends Copyright
1988, Random House Publisher
John W. M. Bush (May 2004). "MIT Lecture Notes on Surface Tension, lecture 5" (PDF).
Massachusetts Institute of Technology. http://web.mit.edu/1.63/www/Lecnotes/Surfacetension/Lecture5.pdf. Retrieved April 1 2007.
John W. M. Bush (April 2004). "MIT Lecture Notes on Surface Tension, lecture 1" (PDF).
Massachusetts Institute of Technology. http://web.mit.edu/1.63/www/Lecnotes/Surfacetension/Lecture1.pdf. Retrieved April 1 2007.
John W. M. Bush (May 2004). "MIT Lecture Notes on Surface Tension, lecture 3" (PDF).
Massachusetts Institute of Technology. http://web.mit.edu/1.63/www/Lecnotes/Surfacetension/Lecture3.pdf. Retrieved April 1 2007.
"Surface and Interfacial Tension". Langmuir-Blodgett Instruments.
http://www.ksvinc.com/surface_tension1.htm. Retrieved 2007-09-08.
^ Calvert, James B.. "Surface Tension (physics lecture notes)". University of Denver.
http://mysite.du.edu/~jcalvert/phys/surftens.htm. Retrieved 2007-09-08.
List CA Science Standards addressed
-Atomic and Molecular Structure
a. Students know how to relate the position of an element in the
periodic table to its atomic number and atomic mass.
b. Students know how to use the periodic table to identify metals,
semimetals, nonmetals, and halogens.
c. Students know how to use the periodic table to identify alkali metals, alkaline earth
metals and transition metals, trends in ionization energy, electronegativity, and the
relative sizes of ions and atoms.
-Chemical Bonding
a. Students know atoms combine to form molecules by sharing electrons to form
covalent or metallic bonds or by exchanging electrons to form ionic bonds.
b. Students know chemical bonds between atoms in molecules