Why do some things float while other things sink?

• Students will make predictions and carry out experiments to test their predictions.
• Students will identify important properties that may affect an object's ability to sink or float.
• Students will design specific experiments to determine the effect of changing the mass or volume of an object on that object's ability to float or sink.
• Students will come to their own working definition and mathematical relationship for why an object floats or sinks before being introduced to the concept of density.

Activity Handout (Microsoft Word 28kB May7 07)

• Students are given a plastic tub filled with a variety of materials—wood of different sizes and shapes, a pencil, a marble, a styrofoam ball, a cork, a plastic spoon, a rubber band, a penny, a toothpick, a paper clip, and whatever else is handy. They are asked to predict which objects will float and which will sink when placed under the level of the water and let go.
• Students perform the experiment and compare the results to their predictions. They discuss why they thought certain objects would sink/float. The class brainstorms a list of properties of the different objects that may affect whether the object floats or sinks. Using the objects they already have, students do simple tests to rule out some of the properties (such as color, shape, etc). In the end, it should be apparent that the most important properties are mass and volume (the instructor needs to guide the class to this conclusion).
• The students are then given a second set of materials, five film canisters and at least 50 pennies, with which to test the effect of changing the mass and volume of an object (or system, in this case) on whether the object floats or not (See "Activity 2" handout). Students are asked to design and perform an experiment that only changes one of these properties (mass or volume) while keeping the other constant and then seeing how a change in the property affects whether the object sinks or floats. Before running their experiment, they make a prediction and record why they think their prediction is correct.
• They run their experiment, take data, and record their observations. They then compare their results to their prediction. Next, they go back and design a second experiment to test the other property (mass or volume), record predictions, run the experiment, make observations, and then compare their results to their prediction.
• After they have completed both experiments, they do a "Check your Understanding" question and predict how many film canisters they would need to use to float 50 pennies. After making a prediction, they test it by performing the experiment.

It is not until the very end discussion of this activity that we talk about what density is at all. We never mention the word unless a student brings it up. We want students to explore this "sinking property" on their own and come to their own understanding without being hampered by any prior ideas about what density is. We also want them to experience discovering an important scientific concept through exploration and experimentation.

The two experiments that students should design and perform are as follows:

1. Change mass and keep volume constant—Using one canister, start with zero pennies in it and see if it floats. Add one penny to the inside of the canister each time, thereby increasing the mass of the system while leaving the volume constant, and see how many pennies you need to sink the canister (answer: about 12-14).
2. 2. Change volume and keep mass constant—Using a set number of pennies that would sink one film canister, attach a second canister to the first to increase the volume but keep the mass the same. Tips: To attach two canisters together, take the lid off of the second one and force the bottom of the first canister into the mouth of the second. Have students use a balance to figure out how much mass they are adding with the second canister (about 1 penny-worth) and then remove pennies to accommodate the mass of the canister.

Students should use a ratio to determine how many canisters they need to float 50 pennies—they should not simply guess. Try to get them to talk in more mathematical terms about how increasing the mass increases the "sinking property" and increasing the volume decreases the "sinking property." We usually talk about direct and indirect relationships and we get the students to write out an equation: "sinking property" = mass/volume. When solving the Check Your Understanding question, we push students to use a ratio: 1 canister will float with 12 pennies inside, so how many canisters will float with 50 pennies inside? 1/12 = x/50

Students are assessed through a written reflective essay and lesson plan for these activities. See the Rubric for Metacognitive Reflections for information about the goals and assessment of such reflections.

Rubric for assessing metacognitive reflections (Acrobat (PDF) 146kB May7 07)