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Principle(s) Investigated: Surface Tension, Cohesion, Adhesion, van der Walls forces, dipole-dipole forces, hydrogen bonding, hydrophobic materials, surfactants, emulsifiers, spheres
Chemistry Grades 9-12
2.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.
2.d Students know the atoms and molecules in liquids move in a random pattern relative to one another because the intermolecular forces are too weak to hold the atoms or molecules in a solid form
2.g Students know how electro-negativity and ionization energy relate to bond formation.
2.h Students know how to identify solids and liquids held together by van der Waals forces or hydrogen bonding and relate these forces to volatility and boiling/ melting point temperatures.
Physics Grades 9-12
1.c Students know how to apply the law F=ma to solve one-dimensional motion problems that involve constant forces (Newton's second law).
Materials:
Soap Boat:
Water
Cookie tray (Your Household or Department Store)
Dishwashing soap (Dawn, etc) (Grocery Store)
Craft Foam or Balsa Wood (Craft Store like Michaels)
Pepper Plate:
Bowl (preferably shallow)
Water
Pepper (Grocery Store)
Liquid Dishwashing Soap (Dawn, etc) (Grocery Store)
Milk Art:
Dish (slightly curved) or bowl (Your Household or Department Store)
Milk (Grocery Store)
Food Coloring (Grocery Store)
Q-tips (Grocery Store)
Liquid Dishwashing Soap (Dawn, etc) (Grocery Store
Procedure: Each of these demonstrations could be done by the students individually or in small groups for fun, hands-on activities.
(1) Soap Boat
a. Cut a boat shape out of craft foam, include a slot or slits at the back
b. Fill shallow cookie tray with cool tap water.
c. Apply a small amount of liquid soap to the back area of the boat, over the slotted area (you may want to use a toothpick or q-tip to apply a small amount of the soap)
d. Place the boat in the water. It should move quickly. If not put a little more soap at the back of the boat or just behind the boat.
(2) Pepper Plate
a. Fill a shallow bowl or curved plate with cool tap water.
b. Sprinkle household pepper in the water until it covers the whole area evenly
c. Add one drop of liquid soap to the center of the bowl. The pepper should disperse instantly to the edges of the bowl.
(3) Milk Art
a. Fill a shallow bowl or curved plate with milk
b. Place a few drops of each color of food coloring in the center area of the milk
c. Dip 1 or 2 Q-tips in liquid soap.
d. Place the soap covered Q-tips in the center of the bowl and hold them there for a few second. The food coloring should instantly disperse.
e. Keep placing the soap covered Q-tips into other areas of the bowl to create interesting designs.
Student prior knowledge: Basic chemistry with atomic structure and positive and negative charges, covalent bonding, basic physics, forces
Explanation:
Demonstrations:
(1) Soap Boat: The water’s surface tension is broken when the soap hits the water at the back of the boat through the slits. Since there is no longer the same force of surface tension at the back of the boat as there is at the front of the boat, the boat is pulled forward.
(2) Pepper Plate: When the pepper is floating evenly on top of the water, the surface tension of the water is the same across bowl. When the soap is dropped into the middle of the bowl, it breaks the surface tension at that center point, so the pepper is drawn to the sides of the bowl by the unbroken surface tension. The soap breaks acts so quickly that the pepper is dispersed to the edges of the plate in an instant.
(2) Milk Art: The food coloring liquids float in the center of the milk bowl because all of the milk’s surface tension is even across the bowl. When the soap is added into the middle, the surface tension is broken and unbroken surface tension draws the majority of the food coloring to the outside of the bowl, creating a colorful design.
Discussion:
The intermolecular forces of water are based on the covalent bonds in the water molecules. The two hydrogen atoms are unevenly distributed on the oxygen, creating positive and negative ends of the water molecule. The positive end of one molecule is attracted to the negative end of the adjacent molecule. This attraction is spread evenly throughout the liquid, except at the surface where there are no molecules above the water pulling in that direction. Therefore, there is a downward force pulling the water to itself on the surface of the water. It requires force to break that bond. Insects who are light and have hydrophilic feet can rest on top of the water because they do not exert enough force to break the surface tension.
Soap and detergents, called surfactants (short for surface action) are able to break through the surface tension because they have two polar ends, one which is hydrophilic (water attracting) and one which is hydrophobic (water repelling). The hydrophilic pole bonds with the water, breaking the water’s intermolecular bonds, and thus the surface tension.
The Force of Surface Tension:
Surface tension has the dimensions of force per unit length required to change the surface shape of the liquid. It is expressed as a proportionality constant (γ) between the change in the surface area (dσ) and the work required to do the change:
dw= γdσ
This produces energy/area expressed as mN/m.
This chart shows the different surface tensions of some liquids at 20 degrees Celsius, since it is dependent upon temperature. The units here are dynes/cm:
Measuring Surface Tension:
Surface tension can be measured using the drop method:
It can also be measured using the maximum bubble pressure method where air is blown into a liquid through a tube, and the surface tension can be calculated from the maximum bubble pressure and radius:
Questions & Answers:
(1) How do you think surface tension is related to the boiling point of liquids?
Liquids with a greater surface tension have stronger intermolecular bonds and, thus, a higher boiling point because more energy is required to break through the stronger bonds than in a liquid with lower surface tension.
(2) How does the high surface tension of water affect different biomes?
If the surface tension of water was lower, it would evaporate more easily, leaving less available water in the environment.
(3) Explain how the surface tension of water could be a danger to some insects.
Some insects are so light that they are unable to overcome the force of surface tension and leave the water.
Applications to Everyday Life: Explain (don't just list) three instances where this principle can be used to explain other phenomenon.
(1) Capillary Action in plants: Plants transport water and nutrients from their roots up to their leaves and throughout the plant in part by using both the cohesion and adhesion property of water. The water adheres to the sides of the plant (adhesion), and pulls up more water with it due to cohesion.
(2) Cleaning with soap: We use soap to clean clothes, dishes and ourselves because it breaks the surface tension of water which allows the water to penetrate the dirt and object we wish to clean.
(3) Gulf Oil Spill: Cleanup companies are using surfactants to disperse the oil in the water more quickly because oil and water do not mix or disperse due to their different bonds and intersurface properties. However, these dispersants are also causing the oil to penetrate fish bodies more readily, causing the fish to be poisoned or die.
Photographs:
Videos:
Here is a quick version of my video without the power point discussion:
Here is a link to the full version of the video with narrated demonstrations and power point discussion Intermolecular Forces
Herr, N. Hands On Chemistry Activities with Real Life Applications, (1999) Jossey-Bass, Intermolecular Forces pp. 210-216, and Making Colloids pp. 475-477.
Jesus Lizard, You Tube
Liquid Metal Man, You Tube
Wikipedia: Surface Tension, Adhesion, Cohesion, Surfactants, Emulsifiers
http://alpha.chem.umb.edu/chemistry/ch314/documents/Experiment3Lecture.pdf
http://sciencesquad.questacon.edu.au/activities/soap_powered_boat.html
http://www.youtube.com/watch?v=lOWg2cTfA9w
http://findarticles.com/p/articles/mi_m3289/is_9_175/ai_n16753595/
http://www.chem1.com/acad/sci/aboutwater.html
Dispersants Speed Up Bioaccumlation of BP Gulf Oil Spill Toxins | Alexander Higgins Blog
http://www.chemistryexplained.com/Co-Di/Colloids.html
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