Funny Water

In this example, Mr. B makes his plans using his knowledge and understanding of science, students, teaching, and the district science program. His understanding and ability are the results of years of studying and reflection on his own teaching. He usually introduces new topics with a demonstration to catch the students' attention. He asks questions that encourage students to develop understanding and designs activities that require students to confirm their ideas and extend them to situations within and beyond the science classroom. Mr. B encourages students to observe, test, discuss, and write by promoting individual effort as well as by forming different-sized groups of students for various activities. Immense understanding, skill, creativity, and energy are required to organize and orchestrate ideas, students, materials, and events the way Mr. B. does with apparent ease. And Mr. B. might repeat an activity five times a day, adapting it to the needs of different classes of students, or he might teach four other school subjects.

[This example highlights some components of Teaching Standards A, B, D, and E; Professional Development Standard C; 5-8 Content Standard A and B; Program Standards A, B, and D; and System Standards D.]

Mr. B. was beginning a unit that would include the development of students' understanding of the characteristic properties of substances such as boiling points, melting points, solubility, and density. He wanted students to consolidate their experiences and think about the properties of substances as a foundation for the atomic theories they would gradually come to understand in high school. He knew that the students had some vocabulary and some notions of atomicity but were likely not to have any understanding of the evidence of the particulate nature of matter or arguments that support that understanding. Mr. B. started the unit with a study of density because the concept is important and because this study allowed him to gather data on the students' current understandings about matter.

As he had done the year before, he began the study with the density of liquids. He knew that the students who had been in the district elementary schools had already done some work with liquids and that all students brought experience and knowledge from their daily lives. To clarify the knowledge, understanding, and confusion students might have, Mr. B. prepared a set of short exercises for the opening week of the unit of study.

For the first day, he prepared two density columns: using two 1-foot-high, clear plastic cylinders, he poured in layers of corn syrup, liquid detergent, colored water, vegetable oil, baby oil, and methanol. As the students arrived, they were directed into two groups to examine the columns and discuss what they saw. After 10 minutes of conversation, Mr. B. asked the students to take out their notebooks and jot down observations and thoughts about why the different liquids separated.

When the writing ceased, Mr. B. asked, "What did you observe? Do you have any explanations for what you see? What do you think is happening?" He took care to explain, "There are no right answers, and silence is OK. You need to think." Silence was followed by a few comments, and finally, a lively discussion ensued.

It's pretty

How do you get the colors to stay apart?

Like the ones on top are lighter or something like that.

The top looks like water.

I think the bottom liquids are heavier; they sink to the bottom .

It separated into different layers because each has different densities and they sit on top of each other.

"What do you mean by density?" asked Mr. B.

It's how packed the particles are.

This one is thick so it's on the bottom. This one is thinnest.

Doesn't oil have lighter density than water?

If we put a thicker liquid in, it would go to the bottom.

There's more of this one that's on the bottom.

The atoms in some are heavier than the ones in others.

Mr. B. realized how many different ways the students explained what they saw, for example, thickness and thinness, heaviness and lightness, more and less, different densities and atoms. The discussion gave him a sense for what the students were thinking. It was clear to him that the investigations he had planned for the following weeks to focus more closely on density would be worthwhile.

Mr. B. divided the class into seven groups of four the next day. On each of the group's tables were small cylinders. Mr. B warned the students not to drink the liquid. Each group was to choose one person to be the materials manager and one to be the recorder as they proceeded to find out what they could about the same liquids used the day before (all of which were available on the supply table). Only the materials manager was to come to the supply table for the liquids, and the recorders kept track of what they did. Forty minutes later, Mr. B. asked students to clean up and gather to share their observations.

Every group identified some of the liquids. The water was easy, as was the vegetable oil. Some students knew corn syrup, others recognized the detergent. Several groups combined two and three liquids and found that some of them mixed together, and others stayed separate. Some disagreements arose about which liquid floated on which. Mr. B. suggested that interested students come back during their lunch time to try to resolve these disagreements. One group replicated the large cylinder, shook it vigorously, and was waiting to see whether the liquids would separate. Mr. B. asked that group to draw what the cylinder contents looked like now, put it on the windowsill, and check it the next day.

Mr. B. began the third day with a large density column again. This time he gave a small object to each of four students--a piece of wood, aluminum, plastic, or iron. He asked the class to predict what would happen when each of the four objects was released into the column. The students predicted and watched as some objects sank to the bottom, and others stopped somewhere in the columns.

"What do you think is going on?" asked Mr. B. "How can you explain the way these objects behaved? I don't want answers now," he went on, "I want you to try out some more things yourselves and then we'll talk." He then divided the class into four groups and gave each a large density column with the liquid layers. The students worked in their groups for 30 minutes. The discussion was animated as different objects were tried: rubber bands, a penny, a nickel, a pencil, and paper clips. Mr. B. circulated from group to group taking note of many interesting comments. With 10 minutes left in the class, he gathered the groups together and asked for some of their observations.

When we dropped something lighter in, it stopped near the top.

The rubber band is lighter than the paper clip. The paper clip is heavy so it drops down.

The rubber band has buoyancy, if you know what that means.

The nickel went all the way to the bottom because it's heavier, but the pencil wouldn't go into the last layer because it was too thick. The pencil is wood and it's lighter; the nickel is silver and it's heavier.

The nickel is denser than the pencil.

Mr. B. listened to these observations and encouraged the students to respond to one another. Occasionally he asked for a clarification--"What do you mean by that?" "How did you do that?" His primary purpose was to hear the students' ideas and encourage them to explain them to one another.

The next day he began the last of the introductory experiences. When the students came in, Mr. B. asked them to divide into their four groups and go to the tables with the density columns. Beside each column were several pieces of wood of different sizes. Students were to think and talk about what the pieces might do in the column, try them out, have more discussion, and write down some of their ideas in their science notebooks.

When enough time had passed, Mr. B. called the groups together and asked for some volunteers to read from their notebooks. Some students were struggling with what they had seen:

They stuck in the middle of the column.

The pieces are not the same weight. The bigger ones are heavier. I don't know why they all stopped in the middle.

Others seemed to understand. One student read,

If you have a block of wood and cut it into millions of pieces, each piece would have the density of the original block. If that block of wood weighed one gram and you cut it into a million pieces the weight would change. But no matter how many times you cut something, the density will not change.

When this statement was read Mr. B. asked how many people agreed with it. Most students quickly asserted "yes." But how sure were they? Mr. B. pulled out a piece of wood larger than any of those that the students had tried. "What would happen if this piece of wood were dropped into the column?" Some students said immediately that it would stop where the smaller pieces had. Others were not quite so sure. This piece was quite a bit bigger. One student asked for a show of hands. Twelve students thought this big piece of wood would sink farther and 16 thought it would sink to the same level as the others. Mr. B. dropped it in. It stopped sinking where the others had. There were a few "yeahs," a few "what's," and some puzzled looks.

As a final teaser and check on students' understanding, Mr. B. brought out two transparent containers of colorless liquids. He asked the class to gather around, took a candle and cut two quite different-sized pieces from it. The students were asked to predict what would happen when the candle pieces were put in the liquids. Mr. B. dropped the pieces into the columns: In one container the big piece sank to the bottom; in the other, the small one floated on the top. Some students had predicted this result, saying that the bigger one was heavier and therefore would sink. Others were perplexed. The two pieces were made of the same wax so they shouldn't be different. Something was wrong. Were the two liquids really the same? Mr. B. removed the pieces of wax from the containers and reversed them. This time the little one sank and the big one floated. "Unfair," came a chorus of voices. "The liquids aren't the same."

Mr. B. had used water and isopropyl alcohol. But he noticed several students were willing to explain the sinking of the larger piece of candle and not the smaller by the difference in the size of the piece.

Mr. B. closed the lesson by summing up. They had seen the density column and worked with the liquids themselves; they had tried floating objects in liquids; they had seen the pieces of wax in the liquids. What was the explanation for all these phenomena? For homework that night he asked them to do two things. They were to think about and write down any ideas they had about what was happening in all these experiences. He also asked them to think about and write about examples of these phenomena in their daily lives. After the students shared some of their observations from outside the classroom, Mr. B. would have the students observe as he boiled water to initiate discussion of boiling points.