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PHY_10_14_V2_TG_Surface tension.Wetting,capillary phenomena.docx
Theoretical material for the lesson, definitions for concepts
Surface tension is the energy, or work, required to increase the surface area of a liquid due to intermolecular forces. Since these intermolecular forces vary depending on the nature of the liquid (e.g. water vs. gasoline) or solutes in the liquid (e.g. surfactants like detergent), each solution exhibits differing surface tension properties. Whether you know it or not, you already have seen surface tension at work. Whenever you fill a glass of water too far, you may notice afterward that the level of the water in the glass is actually higher than the height of the glass. You may have also noticed that the water that you spilled has formed into pools that rise up off the counter. Both of these phenomena are due to surface tension.
In a sample of water, there are two types of molecules. Those that are on the outside, exterior, and those that are on the inside, interior. The interior molecules are attracted to all the molecules around them, while the exterior molecules are attracted to only the other surface molecules and to those below the surface. This makes it so that the energy state of the molecules on the interior is much lower than that of the molecules on the exterior. Because of this, the molecules try to maintain a minimum surface area, thus allowing more molecules to have a lower energy state. This is what creates what is referred to as surface tension.
The water molecules attract one another due to the water's polar property. The hydrogen ends, which are positive in comparison to the negative ends of the oxygen cause water to "stick" together. This is why there is surface tension and takes a certain amount of energy to break these intermolecular bonds. Same goes for other liquids, even hydrophobic liquids such as oil. There are forces between the liquid such as Van der Waals forces that are responsible for the intermolecular forces found within the liquid. It will then take a certain amount of energy to break these forces, and the surface tension. Water is one liquid known to have a very high surface tension value and is difficult to overcome.
There are several other important concepts that are related to surface tension. The first of these is the idea of cohesive and adhesive Forces. Cohesive forces are those that hold the body of a liquid together with minimum surface area and adhesive forces are those that try to make a body of a liquid spread out. So if the cohesive forces are stronger then the adhesive forces, the body of water will maintain its shape, but if the opposite is true than the liquid will be spread out, maximizing its surface area. Any substance that you can add to a liquid that allows a liquid to increase its surface area is called a wetting agent.
https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Physical_Properties_of_Matter/States_of_Matter/Properties_of_Liquids/Surface_Tension
Additional guidelines for organizing a lesson
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Teacher greets the class, notes the date and registers students. Students indicate their presence verbally. The teacher asks about the previous lesson content. Students provide answers |
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Teacher sets a Problem situation Teacher introduces the Learning Objective and Lesson Objectives. |
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The teacher sorts students into pairs or groups of three depending on class size. Students sit according to the groupings created. Each group is given a medium/media to research about and create a presentation. In small classes, some students may prepare individually. They should explore the topic that they had in the last lesson
Group1: Surface tension Syringe, coin, water Group 2: The capillary bulb, sand with different grains, water, roller-bandage Group 3: Surface tension Glass, paper clips, water |
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The students prepare PowerPoint presentations and explain their work to other groups. They can write formulas on the board, draw graphs, and answer questions from other students. The teacher can support students and give feedback to each group. |
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On a note card or in small groups, have students brainstorm to come up with plausible answers to the following questions. Ask for the most in-depth answers they can provide. In conclusion the teacher asks questions |
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Three-Minute Writing: Hand each student a note card. Instruct them to write down — in three minutes — everything they learned in today's lesson. It doesn't matter if the information is out of order. Small diagrams or drawings are fine. Encourage students to try to fill the note card front and back in the time given. Require that they do this from memory — they may not refer to their notes. Teacher reviews the lesson objectives with students Teacher issues homework for students Additional multilevel (on differentiation) tasks The following 10 questions can be assigned to weak students: 1, 2,3,4, 5,8, 9, 10,11, 15, 16. The following 10 questions can be assigned to the average students: 2, 6, 7, 8, 9, 11, 13, 14, 17, 23. The following 15 questions can be assigned to the strong students: 2, 4, 6, 7, 8, 9, 10, 11, 12, 13, 14, 17, 18, 19, 20.
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List of useful links and literature
• Surface tension https://youtu.be/3Cf68jizW0Q
• Capillary effect https://youtu.be/451-Xg1Drfk
• Surface Tension and Capillary Action http://www.wwnorton.com/COLLEGE/chemistry/gilbert/tutorials/interface.asp?chapter=chapter_09&folder=capillary_action
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