Based On The Animation, Which Of The Following Observations Is True?

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Lesson 5.2

Surface Tension

Key Concepts

• The attraction of molecules at the surface of a liquid is called surface tension.
• The polarity of water molecules tin can help explicate why h2o has a strong surface tension.

Summary

Students will notice several phenomena related to the polarity of water molecules. They volition discover a demonstration of a paper clip being placed on the surface of water. Students will place drops of water in an already-filled test tube and on the surface of a penny. They will compare the way water behaves with the less polar liquid isopropyl booze and will see how detergent affects water's surface tension. Students will relate these observations to an explanation of surface tension at the molecular level.

Objective

Students volition be able to explicate, on the molecular level, the furnishings of polarity on h2o's surface tension.

Evaluation

Download the educatee activity sheet, and distribute one per student when specified in the action. The action sheet volition serve as the "Evaluate" component of each v-E lesson program.

Safe

• Be sure you and the students habiliment properly fitting goggles. Isopropyl booze is flammable. Keep it away from flames or spark sources. Read and follow all warnings on the label. Use in a well-ventilated room.
• Paper towels wet with alchohol should be allowed to evaporate. Dry paper towel can and then exist placed in the trash.
• Minor amounts of isopropyl alcohol can exist disposed of down the drain or co-ordinate to local regulations.

Materials for the Demonstration

• 1 clear plastic cup
• Water
• one standard size paper prune
• i big paper clip

Materials for Each Group

• Water
• Isopropyl alcohol (70% or higher)
• Dish detergent in cup
• Test tube
• 2 pennies
• ii droppers
• 2 toothpicks
• 2 paper towels

1. Do a sit-in to show water's surface tension.

   Either exercise the following demonstration for students or show them the video Water'south surface tension.

   Materials

   • one clear plastic loving cup
   • Water
   • 1 standard size paper clip
   • one large paper clip

   Teacher Preparation

   Unbend the big newspaper clip until it is straight. Then bend it into a "U" shape. Bend the bottom of each end out a little bit every bit shown. This will be your device for picking up and placing the smaller paper prune onto the surface of the water. It works like tweezers, but in contrary: allow it to spread apart in order to pick upwardly the newspaper clip and squeeze it to release the paper prune.

   

   Procedure

   1. Place water in one loving cup until it is about ¾ full.

   2. Use your device to choice up a paper prune. Practise this past squeezing the ends of the device together a bit and placing them inside the paper clip. And so allow the ends to spread apart so that the tension of the ends pushes confronting the inside of the paper clip and holds it in place.

      
   3. Very carefully lower the paper clip so that it lies flat on the surface of the water. Slowly clasp the device to release the paper clip.

   Expected Results

   The paper clip should rest on the surface of the water. This may take a couple of tries.

   Ask students:

   Why practise you think a paper clip, which is more than dense than h2o, tin stay on the surface of h2o?

   Remind students that newspaper clips are more dense than water and would usually sink. Help students realize that the paper prune in the demo stayed on the surface of the water because of something having to do with the water molecules at the water's surface, called surface tension.

2. Accept students relate their observations in the demo to a water strider standing on the surface of water.

   Project the image Water Strider and Molecule.

   Signal out how the surface of the h2o seems to bend merely not break under the water strider'southward legs. Tell students that what they see is some other example of water'south surface tension.

   Ask students:

   Why do you think water has such a strong surface tension?

   Encourage students to remember about what they already know about the stiff attraction between h2o molecules. Students should recall that water molecules are very attracted to each other and this attraction is the basis for surface tension. This volition be explained in more item afterwards in this lesson.

   Give each student an activity sheet.

   Students will record their observations and answer questions most the activity on the activity canvass. The Explain It with Atoms & Molecules and Have It Further sections of the activity canvas volition either be completed every bit a form, in groups, or individually depending on your instructions. Look at the teacher version of the activity sail to observe the questions and answers.

3. Have students carefully add together single drops of water to a filled test tube.

   Tell students that at that place is another phenomenon that is caused by water'southward surface tension. It is h2o's ability to fill beyond the top of a container.

   Question to Investigate

   How much water tin can you lot add to a full test tube?

   Materials for Each Group

   • Water
   • Dropper
   • Exam tube
   • Penny
   • ii paper towels

   Process

   1. Pour h2o into a examination tube so that the water is very near the top of the test tube.

   2. Hold the exam tube up to center level and use a dropper to advisedly add drops of h2o, one at a fourth dimension, to the test tube.

      
   3. Spotter the water at the top of the test tube while you add the drops. Keep calculation drops until the h2o spills.
   4. Identify a penny on a paper towel.
   5. While watching from the side, add together single drops of h2o to the penny. Continue adding drops until the water spills.

   Expected Results

   While looking from the side, students will come across that the water forms a dome on the top of the test tube and on the penny.

   Ask students:

   What did the h2o await like as you lot added information technology to the top of the test tube and the penny?

   The water makes a "dome" or "loma" of water above the top of the test tube and penny.

4. Explain how attractions between h2o molecules requite water its stiff surface tension.

   Projection the image Why Water Beads.

   Explain to students that water's surface tension is based on the attractions between water molecules at the surface and the water molecules in the rest of the h2o. A water molecule beneath the surface feels attractions from all the molecules around it. Only the molecules at the surface just experience attractions from the molecules side by side to them and beneath them. These surface molecules are pulled together and inward by these attractions. This inward pull has the result of compressing the surface molecules which form a tight arrangement over the water's surface. This tight organization at the surface is called surface tension.

   The inward pull from the attractions of the molecules results in the smallest possible surface for a volume of water, which is a sphere. This is why water forms a round drop or dome at the peak of the filled test tube and on the surface of a penny.

5. Take students compare the surface tension of water and alcohol.

   Ask students:

   How could we compare the surface tension of water and alcohol?

   Students may suggest placing an equal number of drops of each liquid on wax newspaper, overfilling a test tube, or comparing the number of drops that can exist added to the meridian of a penny.

   Note: Even though at that place are many means to compare the surface tension of water and booze, the process written below compares each liquid on the surface of a penny. In guild to make this as off-white a test every bit possible, students should identify each liquid on 2 similar pennies that are either both "heads" or "tails." They should also exist certain to add single drops of each liquid slowly and advisedly.

   Question to Investigate

   Which has a greater surface tension, h2o or alcohol?

   Materials for Each Group

   • ii pennies
   • ii droppers
   • Water
   • Isopropyl alcohol (70% or higher)
   • Paper towel

   Procedure

   1. Place two pennies on a newspaper towel.

   2. Use a dropper to add drops of water to the surface of a penny. Count the drops until the water overflows.

      
   3. Use a dropper to add drops of booze to the surface of the other penny. Count the drops until the alcohol overflows.

   Expected Results

   The h2o beads up on the penny and the booze spreads out flat. Many more drops of h2o tin be added to the penny than drops of alcohol.

6. Hash out student observations and why students were not able to go as many drops of alcohol on a penny.

   Project the image H2o and Alcohol.

   Review that water molecules are polar and that they are very attracted to each other. Indicate out that booze molecules are polar in only one area, making them somewhat attracted to each other. They are non as attracted to other alcohol molecules as h2o is to other h2o molecules.

   H2o and Alcohol on Pennies

   Explain that h2o'southward attraction pulls itself together into a tight system. Explain that alcohol molecules don't have a structure that is as good equally water's for attraction to itself. Booze molecules only accept 1 O–H bond and they have some C–H bonds that are pretty not polar. At that place is not equally strong an attraction between them every bit there is betwixt h2o molecules.

   The shape of the water molecule and its polarity at the top and the bottom requite water molecules lots of opportunities to attract. Almost anywhere two water molecules meet they can exist attracted to each other.

   Just alcohol has a different size and shape and has its polar part on ane end. Booze molecules tin meet at areas where they would not attract as strongly. The h2o is more attracted to itself than to the metal of the penny. The alcohol is a bit less attracted to itself and so information technology spreads more than on the penny.

7. Have students add detergent to the water on a penny.

   Question to Investigate

   How does detergent bear upon h2o's surface tension?

   Materials

   • Dish detergent in cup
   • 2 pennies
   • Dropper
   • 2 tooth picks
   • Newspaper towel

   Procedure

   1. Place 2 clean dry pennies on a flat surface like a table or desk.
   2. Use a dropper to add together water to both pennies. Add together the same number of drops to each penny so that the h2o stacks upwardly in a dome shape near the same height on both.

   3. Gently impact the water on ane penny with a toothpick. Lookout man the surface of the water equally you touch it.

      
   4. Dip the toothpick in liquid detergent and then touch the water on the other penny with the toothpick.

   Note: This activity works all-time if the dome of h2o on the pennies is pretty high.

   Expected Results

   Touching the water with the toothpick causes the surface of the water to be pressed down and bend. Touching the h2o with the toothpick and detergent causes the water to collapse and spill off the penny.

8. Explain how detergent interferes with water's surface tension.

   Project the epitome H2o and Detergent.

   Explain that detergent is made from molecules that have a charged terminate and a longer uncharged terminate. The detergent molecules spread out over the surface of the water with the charged end in the water and the uncharged end sticking out. The water molecules at the surface are attracted to the charged end of the detergent molecules. As the surface water molecules are attracted outward, this acts against their in attraction that was creating the surface tension. This reduces the surface tension, and the water does non concur its circular shape and thus spills.

9. Talk over how the polarity of the material that the water is placed on affects how the water absorbs or beads up.

   Ask students:

   If water absorbs into a paper towel but does non absorb into wax newspaper, what does that say about the polarity of paper and wax paper?

   The molecules that brand upwards paper are probably polar, and the molecules that make up wax are probably nonpolar.

   Project the animation Water on Paper Towel.

   Explicate that paper towel and other paper is made from cellulose. Cellulose is made from repeating molecules of glucose that are bonded together. The glucose molecule has many O–H bonds, which are polar. Polar water molecules are attracted to polar cellulose.

   Project the animation Water on Wax Newspaper.

   Tell students that wax is made from methane series, which is repeating carbon-hydrogen bonds. The C–H bond is not very polar, so water is more attracted to itself than to the wax. This causes the water to dewdrop up on wax paper.

Source: https://www.middleschoolchemistry.com/lessonplans/chapter5/lesson2

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