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Title:
Surface Tension of Water on a Penny.
How many drops of water can you put on a penny before it spills?
Principle(s) Investigated:
1. Properties of chemical substance
2. Cohesive attraction or Cohesive force
3. Surface tension
4. Wan der Waals Forces
Standards:
CA Standards For Science
GRADES NINE THROUGH TWELVE—CHEMISTRY
Atomic and Molecular Structure
Standard 2, Chemical Bonds
Biological, chemical, and physical properties of matter result from the ability of atoms to form bonds from electrostatic forces between electrons and protons and between atoms and molecules. As a basis for understanding this concept.
Section 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.
Materials:
· Two penny
· Two eyedropper
· Two sample solutions, A (water) & B (soap with water)
· Paper towel
· Two pieces of cloth, or clean surface
· Two small plastic container or beaker
Procedure:
1. The hook, ask the questions how many drops of water can a penny hold? Students can show their responses with their fingers or whiteboards.
2. Show amount of water that corresponds approximately to 30 drops of water, ask again, if it is possible to hold such amount of water.
Part A
3. Place a dry penny on a clean surface area.
4. With the eyedropper, place drops of solution A (water) on the penny. Purr one drop at a time counting each of the drops.
5. Stop the count of drops when the water runs over the edge of the penny.
6.Record the number of drops in a table
7. Repeat the process from steps from 3 to 6.
8. Calculate the average of drops that a penny can hold.
Part B.
Testing solution B
9. Start with a clean and dry penny
10. Place the penny of a clean surface area
11. With a different eyedropper, place drops of solution B (water & Soap) on the penny
12. Place one drop at a time and count them.
13. Stop the count of drops when the water runs over the edge of the penny
14. Record the number of drops in a second table
15. Repeat the process form steps 10 to 14.
16. Determine the average of drops that a penny can hold
For Kinesthetic Students
Perform a physical representation with 4 to 5 students. Each Student should be a water molecule and their arms should be Hydrogen atoms.
Procedure
1. Instruct to 4 or 5 students to stand in front or in the middle of the classroom
2. Have all students to form a line standing next to each other
3. Have all students except the last one of the line to cross both of their arms in front of their chest. The last student will cross only one arm.
4. Have all students to hold hands with one another
5. Form a human- chain
6. Demonstrate the strong bonds with students holding both hands. Simulate braking of two strong bonds.
7. Demonstrate the weak bond with the student that holds one hand only; simulate the breakage of one strong and one weak bond.
Student prior knowledge:
1. Properties of chemical substance
2. Electrical Attraction
3. Molecules
4. Concepts of Covalent, Ionic, and Metallic Bonding
Explanation:
For the teacher
Teachers could use graphic organizers to teach or develop a concept. For this demonstration it is useful that the teacher provide definitions of Chemical Bonds.
For instance it can be used the following definition, “Chemical Bond is an attraction between atoms brought about by a sharing of electrons between to atoms or a complete transfer of electrons. There are three types of chemical bonds: Ionic, Covalent, and Polar Covalent. In addition chemists recognize another type of bond called a hydrogen bond.” Moreover, it is suggested the used of chart to defined Cohesion and Surface Tension.1
Samples
or
http://mdilleymillbrook.wikispaces.com/D+-+Chemical+Bonding
For the Experiment
In Chemistry, a chemical substance is a form of matter that has constant chemical composition and properties. They can be solids, liquids or gases.
In the case of a drop of water, it exist an intermolecular hydrogen bonding, The hydrogen atoms carry a relative positive charges in comparison to the oxygen atom. This charge polarization within the molecule allows it to align with adherent molecules through strong intermolecular hydrogen bonding, which results in the property of cohesive. A hydrogen bond is also known as electromagnetic forces where a hydrogen atom interacts with an electronegative atom; such is the case of nitrogen and oxygen. The hydrogen is known to have a polar bonding to another electronegative atom to create the bond. These bonds can occur between molecules (intermolecular) or within different parts of a single molecule (intermolecular). In other words, the cohesive force is the property of similar molecules sticking together, generating this way surface tension. This surface tension can be described as a contractive tendency of the surface of a liquid that allows resisting an external force. Surface tension has the dimension of force per unit length, or energy per unit area. The two are equivalent, but when referring to energy per unit of area, people use it for the tern of surface energy, which is a more general term that applies to solids as well as liquids.
See Diagram
Diagram shows, in cross-section, a needle floating on the surface of water. Its weight, Fw, depresses the surface, and is balanced by the surface tension forces on either side, Fs, which are each parallel to the water's surface at the points where it contacts the needle. Notice that the horizontal components of the two Fs arrows point in opposite directions, so they cancel each other, but the vertical components point in the same direction and therefore add up[1] to balance Fw.2
The surface tension is measured in forces per unit length. In the SI system, these units are known as Newton per meter; likewise, in the cgs system, the unit is known as dyne per cm. One dyne/cm corresponds to 0.001 N/m3
In simple terms, Surface Tension can be defined as the force that pulls adjacent molecules on the surface of a liquid. Therefore, the greater the surface tension is, the more difficult it is to penetrate the surface of a liquid.4
Conclusion:
Hydrogen boding presents a strong intermolecular force that increases surface tension by bonding surface molecules to each other.
References:
1. http://staff.jccc.net/pdecell/chemistry/bonds.html
4. Herr, N., & Cunningham, J. (1999). Hands-On Chemistry Activities with real-Life Applications. San Francisco, CA: Jossey-Bass.
Questions & Answers:
1. Q: Compare and contrast the average of drops in each part of the experiment, which substance can hold more drops?
A: Substance A can hold more drops
2. Q: Explain in terms of cohesion and surface tension why you provided that answer in question 1?
A: Substance A is pure water, and its Cohesive force is the property of similar molecules to stick together; they are mutually attractive to each other because they create an electrical attraction that can maintain a macroscopic structure like in the case of the drops of water with its formula of H2O. The cohesion property allows a surface tension which creates a solid-like state upon which low density material can be placed on top. In other words, the cohesive forces among liquids molecules are responsible for the phenomenon of surface tension. In the water, each molecule is pulled equally in every direction by neighboring molecules, resulting in a net force of zero. The molecules of water at the surface do not have other molecules on all sides of them; therefore, they are pulled inwards, which generates an internal pressure that forces water surfaces to the minimal area.
3. Q: Why substance B could not hold the same amount of water? Explain in terms of cohesion and surface tension?
A: The soap added to the second substance creates a breakdown in the polarity of the water molecules. The soap gets involved and breaks cohesive bonding. In other words, the soap weakened the surface tension and does not allow to the molecules to be attracted towards each other. The result is a weak cohesion due to the Van der Waals forces that operate by induced polarity in a non-polar molecule.
Applications to Everyday Life:
1. Flotation of objects denser than water.
Flotation of objects denser than water occurs when the object is nonwettable and its weight is small enough to be borne by the forces arising from surface tension. For example, insects such as the water strider use surface tension to walk on the surface of a pond. The surface of the water behaves like an elastic film: the insect's feet cause indentations in the water's surface, increasing its surface area. that relies on the surface tension of water to walk on top of it; they live on the surface of ponds, slow streams, marshes, and other quiet waters
2. Separation of oil and water.
For the case of water and liquid wax, is caused by a tension in the surface between dissimilar liquids. This type of surface tension is called "interface tension", but its physics are the same.
3. Surface tension also allows plants to move water (and dissolved nutrients) from their roots to their leaves.
Photographs:
Solution A (Water):
Solution B (Water + Soap):
Videos:
Instructional videos For Cohesion and Surface Tension
http://www.youtube.com/watch?v=CT4pURpXkbY
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