Overview

Lesson Procedure

60-70 minutes of class time across 5 phases.

1. Warm-Up5-10 minutes
2. Direct Instruction15-20 minutes
3. Guided Practice15 minutes
4. Independent Practice15 minutes
5. Closure10 minutes

1. Warm-Up5-10 minutes

   Hold up a glass of water and a dropper, and let one drop fall so the class can see it. Then give the prompt. Pairs have two minutes.

   Warm-Up Prompt

   "You want to learn how a drop of water leaves a dropper. You could read a scientist's description, watch a slow-motion video, do the experiment yourself or use a computer simulation. Name one thing you think only the video could show you, and one thing you think only the reading could tell you."

   Take answers and sort them on the board under video and reading. Students usually say a video shows motion that is too fast to see, and a reading explains why. Define the words the lesson uses: an experiment is a test you carry out and measure yourself; a simulation is a computer model that acts out a process, often one too small, too fast or too slow to watch; a multimedia source combines words with pictures, sound or motion, such as a web page with photos and an animation. Tell students that RST.6-8.9 asks them to compare (say what two sources both show) and contrast (say what only one of them shows) the information they get from these sources and from a text on the same topic. Today's topic is the "skin" on the surface of water, which scientists call surface tension. In this lesson every video, simulation and web page is described in words, so the whole class works from the same information.

2. Direct Instruction15-20 minutes

   Part 1: Three questions and a why. Show Diagram 1. For any text and any other source on the same topic, ask: (1) What do both tell me? (2) What does only the text tell me? (3) What does only the other source tell me? Then ask why they differ. A text can explain, compare and report measurements from many trials. An experiment gives you firsthand results, but only for your materials. A video shows what the camera saw, even events too fast for the eye, but not why they happen. A simulation shows parts too small to see, but it is simplified. Where two sources seem to disagree, check what each one could really detect before deciding which is wrong.

   Part 2: The modeled pair. Read the context note aloud: C. V. Boys was a British physicist. In the winter of 1889-1890 he gave three lectures on soap bubbles to an audience of young people in London, doing experiments in front of them as he talked, and he published them as a book in 1890. In the first lecture he shows that the surface of water acts like a stretched skin. Words to know: issuing (flowing out), definite (fixed, the same each time), india-rubber (natural rubber), tripod (a three-legged stand), fancy (something imagined), pailful (a bucketful). Read Source 1 aloud, then Source 5, a written description of a slow-motion video. Think aloud with the three questions.

   1I shall now try another experiment as simple as the last. I have a pipe from which water is very slowly issuing, but it does not fall away continuously; a drop forms which slowly grows until it has attained a certain definite size, and then it suddenly falls away. I want you to notice that every time this happens the drop is always exactly the same size and shape. Now this cannot be mere chance; there must be some reason for the definite size, and shape. Why does the water remain at all? It is heavy and is ready to fall, but it does not fall; it remains clinging until it is a certain size, and then it suddenly breaks away, as if whatever held it was not strong enough to carry a greater weight. Mr. Worthington has carefully drawn on a magnified scale the exact shape of a drop of water of different sizes, and these you now see upon the diagram on the wall (Fig. 2). These diagrams will probably suggest the idea that the water is hanging suspended in an elastic bag, and that the bag breaks or is torn away when there is too great a weight for it to carry. It is true there is no bag at all really, but yet the drops take a shape which suggests an elastic bag. To show you that this is no fancy, I have supported by a tripod a large ring of wood over which a thin sheet of india-rubber has been stretched, and now on allowing water to pour in from this pipe you will see the rubber slowly stretching under the increasing weight, and, what I especially want you to notice, it always assumes a form like those on the diagram. As the weight of water increases the bag stretches, and now that there is about a pailful of water in it, it is getting to a state which indicates that it cannot last much longer; it is like the water-drop just before it falls away, and now suddenly it changes its shape (Fig. 3) [...] Now in this case we clearly have a heavy liquid in an elastic bag, whereas in the drop of water we have the same liquid but no bag that is visible. As the two drops behave in almost exactly the same way, we should naturally be led to expect that their form and movements are due to the same cause, and that the small water-drop has something holding it together like the india-rubber you now see.

   1This description tells what a short science video shows. The video was filmed at 1,000 frames per second and is played back at 25 frames per second, so it runs 40 times slower than real life. A ruler marked in millimeters stands behind the tip of the dropper.

   2Part 1, plain water. A drop swells at the tip of a glass dropper held straight down. As it grows, its bottom rounds out and its top narrows into a neck. The drop hangs and grows slowly for most of the clip. Then the neck stretches into a thin thread and snaps. The snapping takes about 10 frames, which is one-hundredth of a second in real life. A second, much smaller droplet forms from the broken thread and falls just behind the main drop. The water left on the tip springs back up into a small rounded cap. The falling drop wobbles as it goes, turning wider than tall, then taller than wide, several times before it leaves the picture. Each main drop that falls is the same size as the one before: about 4.5 mm across, measured against the ruler.

   3Part 2, soapy water. The dropper is refilled with tap water mixed with a little dish soap. These drops hang for a shorter time and break away while they are smaller, about 3.5 mm across. They snap off and wobble in the same way.

   4What the video does not show. The narrator never says why the drops take these shapes, and the video gives no measurement of any force. It shows one dropper, filmed from one side, and each liquid for only a few drops.

   • Compare: what both sources show

     What information about the drops do Source 1 and Source 5 share?

     Result: Both describe a drop that grows at the tip, hangs, and then falls at a fixed size. Boys says "every time this happens the drop is always exactly the same size and shape," and the video description says "Each main drop that falls is the same size as the one before." Two very different sources, a lecture from 1890 and a modern camera, agree on this point.

   • Contrast: only the text

     What does Source 1 give that the video in Source 5 does not?

     Result: An explanation and a model. Boys suggests the drop behaves as if it hung "in an elastic bag," and he tests the idea with a sheet of rubber that fills with water and takes the same shapes. He also points to Mr. Worthington's magnified drawings of drops of different sizes. The video description says plainly that the narrator "never says why the drops take these shapes."

   • Contrast: only the video

     What does Source 5 show that Source 1 does not?

     Result: Details that happen too fast to see: the neck "stretches into a thin thread and snaps," a much smaller second droplet falls behind the main drop, the water left on the tip springs back, and the falling drop wobbles. The ruler also gives a size, about 4.5 mm across, which Boys does not give for his drops.

   • A seeming disagreement: "suddenly"

     Boys says the drop "suddenly falls away." The video shows the neck thinning and snapping over several frames. Is one source wrong?

     Result: No. The snapping takes about 10 frames at 1,000 frames per second, which is one-hundredth of a second. To Boys's audience, watching at normal speed, that is sudden. The camera slows the event down 40 times (1,000 ÷ 25 = 40), so it can show steps the eye cannot. The difference comes from what each source could detect, not from a mistake.

   • Why the sources differ

     Finish the comparison with one sentence about why each source gives what it gives.

     Result: Boys is a scientist explaining to an audience, so he gives reasons and a model he can show on a stage; the video is a record of what a fast camera saw from one side, so it gives precise, fast details and a size, but no reasons and no forces. A reader who uses both knows what happens and has an idea of why.

   Teacher tip. Write each finding in the right column of the chart as you say it, and say which source and which words it came from. Push students past differences in wording: "Boys says drop and the video says droplet" is not a difference in information, but "only the video gives a size in millimeters" is.

3. Guided Practice15 minutes

   Pairs read Source 2, more of Boys's first lecture, and Source 4, a write-up of a penny drop experiment with invented class results, and fill a blank copy of the three-column chart. Words to know for Source 2: tethered (tied down), evident (easy to see), water-colours (watercolor paints; British spelling), ox-gall (bile from cattle, once sold to painters to help paint spread), ether (a liquid that evaporates very fast; Boys used its vapor in a part of the lecture not printed here). Source 2 uses the words "surface tension," the scientific name for the pull of the water's skin. Circulate and hear from each pair at least once.

   1Let us therefore try another experiment to see whether in other ways water behaves as if it had an elastic skin.

   2I have here a plain wire frame fixed to a stem with a weight at the bottom, and a hollow glass globe fastened to it with sealing-wax. The globe is large enough to make the whole thing float in water with the frame up in the air. I can of course press it down so that the frame touches the water. To make the movement of the frame more evident there is fixed to it a paper flag.

   3Now if water behaves as if the surface were an elastic skin, then it should resist the upward passage of the frame which I am now holding below the surface. I let go, and instead of bobbing up as it would do if there were no such action, it remains tethered down by this skin of the water. If I disturb the water so as to let the frame out at one corner, then, as you see, it dances up immediately (Fig. 4). You can see that the skin of the water must have been fairly strong, because a weight of about one quarter of an ounce placed upon the frame is only just sufficient to make the whole thing sink.

   4This apparatus which was originally described by Van der Mensbrugghe I shall make use of again in a few minutes.

   [...]

   5If you are painting in water-colours on greasy paper or certain shiny surfaces the paint will not lie smoothly on the paper, but runs together in the well-known way; a very little ox-gall, however, makes it lie perfectly, because ox-gall so reduces the strength of the skin of water that it will wet surfaces that pure water will not wet. This reduction of the surface tension you can see if I use the same wire frame a third time. The ether has now evaporated, and I can again make it rest against the surface of the water, but very soon after I touch the water with a brush containing ox-gall the frame jumps up as suddenly as before.

   6It is quite unnecessary that I should any further insist upon the fact that the outside of a liquid acts as if it were a perfectly elastic skin stretched with a certain definite force.

   1Question. How many drops of water fit on a penny before the water spills over the edge, and does a little dish soap change the number?

   2Materials. For each pair: two clean, dry pennies; two small plastic pipettes (droppers); a cup of tap water; a cup of tap water with one drop of dish soap stirred in; paper towels.

   3Procedure. Lay a penny heads up on a dry paper towel. Hold a pipette straight up, about 1 cm above the penny, and squeeze out one drop at a time, counting each drop out loud. Watch from the side: the water rises above the penny into a dome before it spills. Stop when water runs over the edge, and do not count the drop that made it spill. Dry the penny and repeat with the soapy water, using the second pipette so that no soap gets into the plain water.

   4Results (invented class data). Six pairs tested each liquid once. Plain water: 27, 31, 24, 29, 33 and 26 drops. Soapy water: 12, 9, 14, 11, 10 and 15 drops. The plain-water domes stood tall and rounded before they spilled. The soapy-water domes stayed low and flat, and the water crept over the edge early.

   5Conclusion. Every pair fit fewer drops of soapy water than of plain water. Our results support the idea that soap weakens whatever holds the water together in a dome. The test does not show what that is, and it does not measure how strong it is; it only counts drops. The size of a drop also depends on the pipette, so our counts can be compared only with tests that use the same kind of pipette.

   1. Both. Prompt: find one idea Sources 2 and 4 share, and quote each. Teacher note: something added to water weakens its skin. Boys writes that ox-gall "so reduces the strength of the skin of water," and the penny report says the results "support the idea that soap weakens whatever holds the water together in a dome."
   2. Only the text. Prompt: what does Source 2 tell you that the penny test cannot? Teacher note: a name and a size for the force. Boys calls the water's pull an "elastic skin" and "surface tension," and he measures its strength with a weight: "about one quarter of an ounce" is just enough to push the frame through. The penny report says its test "does not measure how strong it is."
   3. Only the experiment. Prompt: what does Source 4 tell you that Boys does not? Teacher note: numbers from six repeated trials with a household soap, how much they vary (24 to 33 drops of plain water), and what the domes looked like from the side. Boys shows each demonstration once and gives no counts.
   4. Why they differ. Prompt: explain one difference by what each source was built to do. Teacher note: Boys had special apparatus (a floating frame) and wanted to show a force; the class had pennies and pipettes and could only count. The penny report also warns that its counts depend on the pipette, so they fit only tests with the same kind of pipette.

   Close by asking which surprised pairs more: that soap made the penny hold fewer drops, or that ox-gall made Boys's frame jump up. Both come from a weaker skin: in Source 2 the skin was holding the floating frame down, and on the penny it was holding the dome of water together. Activity 1 lets the class run the penny test for real.

4. Independent Practice15 minutes

   Each student reads Source 6, a written description of a computer simulation of water, and compares it on their own with Sources 1 and 2, using a blank three-column chart. Words to know: molecule (the smallest piece of water that is still water), attract (pull toward), three-dimensional (having length, width and depth, like a real object). The quiz uses Sources 1, 2, 4, 5 and 6; students keep all five open.

   1This description tells what students see in a computer simulation of water. It is a model, not a picture. Each water molecule is drawn as a blue ball about the size of a pea, in a flat box. Real water molecules are far too small to see: a single drop holds more than a thousand billion billion of them.

   2On the screen, the balls jiggle and slide past one another. Thin lines join balls that are close together, to show that neighboring molecules attract, or pull on, one another. Click on a ball deep inside the water and arrows appear around it: it is pulled about equally in every direction, so the pulls cancel. Click on a ball at the surface and the arrows point only sideways and down, because there are no water molecules above it. The surface balls are pulled inward and held close together, so the surface acts like a stretched sheet.

   3A "surface pull" meter in the corner reads 72 for pure water at room temperature. A slider adds soap. Soap molecules, drawn as orange balls with long tails, crowd to the surface and push in between the water balls there. As more soap is added, the meter falls, to about 30 at the highest setting. A second slider warms the water: the balls jiggle faster and the meter falls a little.

   4The help page of the simulation gives a warning. The balls are drawn much farther apart than real molecules, the model is flat instead of three-dimensional, and the arrows show which way each pull acts but not its true strength.

5. Closure10 minutes

   Exit ticket: On an index card, (1) write one thing the simulation in Source 6 shows that Boys's lectures do not, and (2) one thing Boys's lectures give that the simulation does not. Quote a few words from each source. Sort the cards into "compares and contrasts with quotations," "names differences without quotations" and "summarizes one source only" to plan the next lesson.

   Teacher note on the science and the language. Boys's main idea holds up: the surface of a liquid pulls inward and acts like a stretched skin, and soap, alcohol and ox-gall all weaken it. His figure for the strength of water's skin, "three and a quarter grains to the inch" (Source 3), is about 12% higher than today's value for clean water at room temperature, about 0.073 newton per meter, or about 2.9 grains per inch. The passages keep Boys's British spellings and older words (water-colours, india-rubber) exactly as printed in 1896. The ether in Source 2 is mentioned only as part of his stage demonstration; it is not for classroom use. Every video, simulation and web page in this lesson is described in words and written for this page; you may also show a real slow-motion video or simulation of your own choosing and have students compare it with the descriptions.

   For homework: students read Source 3, from later in Boys's first lecture, and Source 7, a written description of a museum web page. Words to know: grain (an old unit of weight, about 0.065 gram), milliliter (one-thousandth of a liter, about 20 drops of water), rubbing alcohol (a drugstore liquid that is mostly isopropyl alcohol mixed with water).

   1So far I have given you no idea what force is exerted by this elastic skin of water. Measurements made with narrow tubes, with drops, and in other ways, all show that it is almost exactly equal to the weight of three and a quarter grains to the inch. We have, moreover, not yet seen whether other liquids act in the same way, and if so whether in other cases the strength of the elastic skin is the same.

   2You now see a second tube identical with that from which drops of water were formed, but in this case the liquid is alcohol. Now that drops are forming, you see at once that while alcohol makes drops which have a definite size and shape when they fall away, the alcohol drops are not by any means so large as the drops of water which are falling by their side. Two possible reasons might be given to explain this. Either alcohol is a heavier liquid than water, which would account for the smaller drop if the skin in each liquid had the same strength, or else if alcohol is not heavier than water its skin must be weaker than the skin of water. As a matter of fact alcohol is a lighter liquid than water, and so still more must the skin of alcohol be weaker than that of water.

   1This description tells what is on one page of a science museum's online exhibit. The page combines photos, a chart, a short animation and two sentences of text. The museum, its page and its measurements are invented for this lesson.

   2Photos. A row of three close-up photos shows a drop hanging from the same dropper just before it falls. The liquids are tap water, tap water with a little dish soap, and rubbing alcohol from a drugstore. A millimeter grid behind the drops shows that the water drop is the largest and the rubbing-alcohol drop is the smallest.

   3Chart. A bar chart titled "Drops to Fill 1 Milliliter" shows 20 drops of tap water, 46 drops of soapy water and 52 drops of rubbing alcohol, all from the same dropper. The caption says that each bar is the average of five tries.

   4Animation. A 20-second animation zooms in on the neck of a hanging drop. Small arrows around the neck show the surface pulling up. One large arrow at the bottom shows the weight of the drop pulling down, and it grows as the drop grows. When the weight arrow becomes longer than the surface arrows together, the drop falls. For soapy water and rubbing alcohol the surface arrows are drawn shorter, so the drop falls while it is still small.

   5Text. Two sentences sit under the animation: "Soap and alcohol both weaken the pull at the surface of a liquid. A weaker surface can hold up only a smaller drop."

Differentiation Strategies

Assessment Guidance

1. Question 1 of 20 · Multiple Choice

   What does Source 6 (the simulation) show that neither Source 1 nor Source 2 shows?

   AWhy molecules at the surface are pulled differently from those inside BA falling drop that wobbles several times before it leaves the picture CSoap making a penny hold fewer drops of water before they spill DA rubber sheet that fills with water and takes a drop's shape

2. Question 2 of 20 · Multiple Choice

   Which idea do Source 2 and Source 6 share?

   AWarm water has a stronger skin than cold water does BSoap molecules crowd to the surface of the water CThe surface of water acts like something stretched DA floating frame can be held down under the water

3. Question 3 of 20 · Multiple Choice

   Source 6 says the "surface pull" meter reads 72 for pure water and about 30 at the highest soap setting. What does this give a reader that Source 2 does not?

   AProof that ox-gall and dish soap are the same substance BA number for how much soap weakens the pull of the surface CThe weight needed to push Boys's floating frame under water DThe true size of the pull on one single water molecule

4. Question 4 of 20 · Multiple Choice

   Source 4 concludes that its penny test "does not show what that is," meaning what holds the water together. Source 6 says soap molecules "crowd to the surface and push in between the water balls there." What does Source 6 add?

   AProof that the penny test in Source 4 was done wrongly BA count of how many soap molecules fit on one penny CA measurement showing the domes in Source 4 were flat DA picture of how soap could weaken the pull at the surface

5. Question 5 of 20 · Multiple Choice

   Source 6 says a ball at the surface is pulled "only sideways and down." How does this detail relate to Boys's elastic-bag idea in Source 1?

   AIt proves that a real bag of rubber surrounds every drop BIt shows that Boys was wrong to compare a drop to a bag CIt suggests a reason the surface can act like a bag DIt gives the size of the drops falling from Boys's pipe

6. Question 6 of 20 · Multiple Choice

   Boys writes, "It is true there is no bag at all really." The help page of Source 6 warns that the balls "are drawn much farther apart than real molecules." What do the two warnings have in common?

   AEach tells the reader that its picture of water is a model BEach says that drops of water have no definite size CEach admits that its measurements of force were wrong DEach says that soap destroys the skin of the water

7. Question 7 of 20 · Multiple Choice

   Source 6 has a slider that warms the water, and the meter "falls a little." Which of these could test that result with real water?

   ARereading Source 1 to find what Boys says about warm water BWatching the slow-motion video in Source 5 one more time CMoving the heat slider in Source 6 higher and higher DA penny test with cool and warm tap water, same pipette

8. Question 8 of 20 · Multiple Choice

   In Source 6 the meter falls from 72 for pure water to about 30 at the highest soap setting. The soapy reading is about what fraction of the reading for pure water?

   AAbout 0.30, the number shown on the meter BAbout 0.42, a little less than one-half CAbout 2.4, found by dividing 72 by 30 DAbout 42, the drop from 72 down to 30

9. Question 9 of 20 · Multiple Choice

   Source 4 says the soapy-water domes "stayed low and flat," and Source 6 shows the soap slider lowering the meter. Which statement best contrasts the information in the two sources?

   ASource 4 reports real water on real pennies; Source 6 models why, at a tiny scale BSource 4 explains what molecules do; Source 6 counts the drops that fit on a penny CBoth give the same numbers for soapy water, so either source could be used alone DSource 6 can be trusted more because it runs on a computer and never makes errors

10. Question 10 of 20 · Multiple Choice

    Source 5 says soapy-water drops break away "while they are smaller, about 3.5 mm across," compared with 4.5 mm for plain water. How does this detail affect the result in Source 4?

    AIt explains the low counts: soapy drops were larger, so fewer fit BIt makes the result stronger: smaller drops, yet fewer fit on the penny CIt shows that the pipettes used in Source 4 must have been broken DIt proves the class used the same dropper as the one in the video

11. Question 11 of 20 · Multiple Choice

    Source 5 ends: "It shows one dropper, filmed from one side, and each liquid for only a few drops." Why is this sentence useful to a reader comparing sources?

    AIt proves that everything the video shows is mistaken BIt explains why the falling drops wobble as they fall CIt shows that the video disagrees with Boys's lecture DIt tells how far the video's findings can be applied

12. Question 12 of 20 · Multiple Choice

    In Source 1, Boys says the stretched rubber "always assumes a form like those on the diagram." What kind of information is the reader getting?

    AFirsthand data from an experiment the reader has done BThe output of a computer simulation of water and rubber CA report in a text of an experiment the reader did not see DA measurement, in numbers, of the force on the rubber

13. Question 13 of 20 · Multiple Choice

    Source 4 warns that its counts "can be compared only with tests that use the same kind of pipette." Which comparison is fair?

    ASource 4's penny counts and the drop sizes from Source 5's dropper BSource 4's plain-water counts and its own soapy-water counts CSource 4's counts and a class that used much larger eyedroppers DSource 4's counts and the quarter-ounce weight in Source 2

14. Question 14 of 20 · Multiple Choice

    A student says, "Source 6 disagrees with Source 1, because Boys never mentions molecules." Which response is best?

    AThe student is right: Boys must have been wrong about the skin BThe student is right: a simulation is always better than a text CBoys did mention molecules, in the paragraph printed as Source 1 DLeaving something out is not disagreeing; Source 6 adds detail

15. Question 15 of 20 · Short Answer

    Fill a three-column chart for Source 2 (Boys) and Source 6 (the simulation): one entry for "Only in Source 2," one for "In both" and one for "Only in Source 6." Quote each source at least once.

    Show model answer

16. Question 16 of 20 · Short Answer

    Source 6 says the balls at the surface are "pulled inward and held close together." Use this idea to explain one thing described in Source 4 or Source 5, for example why the plain-water dome on the penny stays rounded, or why the water left on the dropper tip "springs back up." Name the source you explain.

    Show model answer

17. Question 17 of 20 · Short Answer

    Which source or sources in the lesson would you use to answer the question "Is a drop of water round while it falls?" Explain what the source you choose contributes, and name two sources that could not answer it and why.

    Show model answer

18. Question 18 of 20 · Short Answer

    Write a paragraph of 4-6 sentences that compares and contrasts what a reader learns about the surface of water from Boys's lecture (Source 1) and from the simulation (Source 6). Include one thing both give, one thing only each gives, and why the two kinds of source give different information.

    Show model answer

19. Question 19 of 20 · Short Answer

    Name one question about the skin of water that none of Sources 1, 2, 4, 5 and 6 answers. Describe a safe experiment, video or simulation that could answer it, and say what that source could and could not tell you.

    Show model answer

20. Question 20 of 20 · Short Answer

    Source 4 says its test "does not show what that is," meaning what holds the water together in a dome. Which source in the lesson comes closest to showing it, and what limit does that source admit? Quote both sources.

    Show model answer

These standards connect to RST.6-8.9: prerequisites to review first, parallel standards at the same level, and next steps that build on it.

Standard text: Common Core State Standards for English Language Arts & Literacy. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.