To develop students' understanding of sinking and floating. These experiments can serve as a precursor to further exploration of density of solids and liquids. The experiments are also designed to encourage student skills in experimental design, testing simple hypotheses, and grouping objects by common characteristics.
This activity provides an opportunity for students in Grades 3-5 to develop experimental design skills in the context of a familiar event (floating and sinking) while furthering their understanding of the concepts of density and buoyancy.
The Benchmarks introduction to the 3-5 Scientific Inquiry also clearly refers to this activity: "They should be encouraged to observe more and more carefully, measure things with increasing accuracy (where the nature of the investigations involves measurement), record data clearly in logs and journals, and communicate their results in charts and simple graphs as well as in prose. Time should be provided to let students run enough trials to be confident of their results. Investigations should often be followed up with presentations to the entire class to emphasize the importance of clear communication in science. Class discussions of the procedures and findings can provide the beginnings of scientific argument and debate" (Benchmarks for Science Literacy, p. 11).
Students will first classify a group of common objects by a characteristic of their own choosing. Then they will reclassify the same group of objects by their predictions about whether each item will float or sink in water. As a group, they will design an experiment to test their predictions (hypotheses). Although students may be unable to develop a well-controlled experimental protocol, they will work to develop a consistent sequence of steps by which they test each item and record their data. According to the research summary in the Benchmarks, elementary students are able to develop a "fair comparison." ( Benchmarks for Science Literacy, p. 331)
Be sure to provide groups with feedback on their protocol and data tables before data is collected. After groups carry out their experiment, record their results, and do a preliminary analysis of their data, they should be allowed to revise their experimental procedure and repeat the experiment, if desired. Finally, students will use a variety of resources to explore how terms such as "buoyancy" and "density" are used to explain the phenomena they have observed.
The 15 items for each group should include materials such as wood, metal, plastic, and paper. Possibilities include: paper clips, toothpicks, soda bottle caps, marbles, plastic beads and cubes, sponge pieces, pencils, pieces of aluminum foil and paper, small balls, erasers, pieces of Styrofoam, and plastic eating utensils.
Additional resources about sinking, floating, and boats can be found in the following resources:
The Way Things Work, by David Macaulay, Houghton Mifflin; ISBN 0-395-42857-2; 1988 has a good section on boats and submarines.
Buoyancy Brainteasers, found on the NOVA website has activities and problems to solve on floating and sinking.
Educational Research References:
- Wollman, W. (1977a). Controlling variables: Assessing levels of understanding. Science Education, 61, 371-383.
- Wollman, W. (1977b). Controlling variables: A neo-Piagetian developmental sequence. Science Education, 61, 385-391.
- Wollman, W., & Lawson, A. (1977). Teaching the procedure of controlled experimentation: A Piagetian approach. Science Education, 61, 57-70.
Student groups (2-3 students each) will first brainstorm to develop 2-3 characteristics by which a group of common objects could be classified (e.g., color, weight, size, shape, composition, "bendability," etc.). They will try out their classification scheme on a set of common objects (those listed in Planning Ahead).
This should be a quick, fun activity to get students thinking about the properties of the materials (plastic, metals, glass, rubber, wood, etc.). Students can share their classification scheme with the class either through a group discussion, through writings or drawings.
This activity uses a phenomenon that is already familiar to most students to help them think about how and why some items float and others sink, and to help them gain skills in gathering data in systematic ways, using a consistent experimental method. These skills can be applied for other inquiry-based activities, as well.
Do not provide definitions and explanations for terms such as "buoyancy" and "density" before the hands-on activity. Rather, allow students to explore the phenomenon first; then these terms become tools to help explain what they have already observed.
Begin by discussing the different ways that students separated their pile of materials into two groups in the introductory activity. Point out that different objects can be described by a number of characteristics, including the type of material from which they are made, their size, their shape, their color, and their weight. Some objects can be characterized by their purpose; for example, buttons and paper clips are both designed to hold things together.
Follow this by discussing another characteristic that students may not have considered - whether the objects will float or sink in water. As a group, generate a list of descriptive words for objects that float and one for objects that sink. Using the list allow the class to predict whether several demonstration objects (apple, potato, paper clip or penny, and wood piece) will float or sink.
Students may have said that objects that are "heavy" will sink while those that are "light" will float. A pan balance can be used to compare, for example, a paper clip and an apple. Students may predict that, because the apple is heavier, it should sink. Demonstrate that the apple floats and the paperclip sinks. You can show several discrepant events of this type to both generate student interest and point out that there is something more to floating and sinking than just weight. Tell students that you will be exploring this idea further in this activity.
Students should continue to work in their original groups of 2-3. Each group should re-sort their pile of objects based on their predictions about whether each object will float or sink.
Distribute the data table Sink It. Students should prepare the data table by writing the name of each object on the table in the first column, with their prediction about whether the object will float or sink in the second column.
Then, using the 3" x 5" cards, they should write a procedure for testing each object, writing one step on each card. If preferred, students can use diagrams. Each card should be numbered in order of the steps. Steps should include the recording of data and preparation of the testing tank (bucket) for the next experiment. Students should do a "dry" run, following the steps exactly as they are written, then modify their procedure, if needed.
When the students have a procedure developed, review the steps and make suggestions for steps that have been omitted or need to be edited. The object is to guide students to develop a fairly detailed procedure for this experiment. This will help build skills for future, multi-step controlled experiments.
Guiding questions could include:
- How full will your bucket be for each object tested? Do you need a certain amount of water to be able to fairly test whether something floats? Should it be the same amount of water for each item?
- How will you place the object in the bucket? Will you drop it in? If so, from what height? Will you place it halfway down into the water and then let it go? Will you place it on the bottom of the bucket and then let it go? Will you put the object in the bucket and then add water?
- How will you define floating? Is anything off of the bottom floating? Does the item have to rise all the way to the top of the water?
Once their procedure has been approved, students should put the cards in order, run the string through the holes, and tie it loosely. Students can begin their data collection, testing one item at a time, using the steps written on the flip-stack of cards. The cards should guide the procedure. For purposes of cooperative grouping, one student can serve as the card reader, a second as the equipment handler, and a third as the recorder.
After the initial data has been collected, students in the group should confer to decide whether any items should be re-tested. Some items may seem to float, then sink as they become wet. Others may have densities similar to water and may float in the middle of the bucket rather than on top of the water. Students should retest these items and should add written comments about them on the data table in the "Notes" column.
Students should analyze their data by which of their predictions (hypotheses) were confirmed and which were proven incorrect. They may or may not be able to draw conclusions about why objects did and did not float.
Further explorations of density - comparing volume and weights - will certainly help students expand their understanding of this concept. However, student groups should be able to draw some conclusions about the types of materials that float and those that sink (e.g., metals, materials that feel "light," etc.). Students may need to run additional trials with other materials to further test their conclusions.
Students should present their findings to the class as a poster or an oral presentation. They should include reading their step-by-step procedure, show the items that did and did not float, and tell what conclusions they drew about what types of items do and do not float in water.
After all groups have presented their findings, lead a discussion on two topics. The first is whether results among the groups were consistent ("Did the marble sink for everyone? How about the plastic beads? The aluminum foil?") Where there are differences in their findings, ask students to speculate why this could be? How could they explore this further? Did the difference have to do with different procedures?
Then lead the discussion into drawing conclusions about how we describe objects that float or sink. Refer back to the words the students originally used to describe items that float or sink. Ask the students to look for commonalities among the items that float and those that sink. Which descriptive words would they change? Are there words they would add?
This is an appropriate time to introduce vocabulary. Students have a concept in mind - objects that float - that can be described by a new word, "buoyant." The concept of density can also be introduced at this time; a good visual demonstration is the buoyancy of a golf ball and a ping-pong ball. They have similar volumes but one is much heavier, therefore, more dense. The concept of density can be further explored, as described in the Extensions below.
Students can hypothesize whether the type of liquid makes a difference in this experiment. They can explore hot versus cold water, salt water, soapy water vegetable oil, or corn syrup.