8th Grade Physical Science Curriculum Unit 1: Density and Buoyancy Lesson 1, part a. Engage -- Heavy Ice? Objectives: After completing the lesson, students will be able to distinguish between an observation and an inference. Students will also be provoked to question why objects float or sink. Key Questions: · What is the difference between an observation and an inference? · Why does an object float or sink? · How could an object float in one liquid and sink in another? Outline: Students are used to seeing ice float in water. Because alcohol is less dense than ice, the ice will sink. This demonstration present s them with a discrepent event; a phenomenon that violates their expectations. We do this to provoke questions and get them to open their minds to the need for a new way of looking at floating and sinking. We also take advantage of the fact that they make incorrect inferences to point out the difference between observations and inferences. The most important part of this activity is actually the observation --> question --> hypothesis --> experiment --> new question cycle. As you go through the steps, you can teach the students a version of the "scientific method," which we term the inquiry cycle. Along the way we are challenging their misconceptions about floating and sinking. In particular, the use of a single material, ice, with a constant density, shows that in fact there must be something different about the liquids. This will lead into the following activity, where we discover the different densities of the liquids. Time: 30 to 45 minutes Materials; For the Teacher: Two clear glass 250 or 500 ml beakers, labeled A and B Two+ ice cubes (you should have extras) 200 ml of alcohol, isopropyl or ethanol 200 ml of water Assessment: Student comments in the discussion will give you a sense of the experiences they have had with sinking and floating, and of their awareness of the concept of density. Textbook Reference/homework: pages 82 - 88 OUSD/California Science Content Standards: 1a; conducting investigations (Calif. #8.9a) Procedure Step One: Begin by telling students: The most important tool a scientist has is his or her powers of observation. So today, we are going to stretch and build those powers. Reveal to the students two identical beakers, one containing about 200 ml of alcohol, the other containing the same amount of clear water. Ask them to record in writing as many detailed observations as they can. They can also include a sketch. You can walk around the room holding the beakers out before the students, but do not let them sniff them, as the odor will spoil the surprise later. When students are done writing, ask for their observations, and record them on the board or on the overhead. Step Two: Show them the two ice cubes. Pose the question; "What will happen if the ice is placed in the beakers?" Ask for predictions and hypotheses, and record them on the board. Have students write down their personal prediction. Point out that our next step is a test. The ice is placed in each liquid. It sinks in alcohol, and floats in water. Circulate in the room, asking them to sketch what they see, and record detailed observations. Step Three: What did you "observe"? Review the initial observations the students made. Point out the distinction between true observations, such as "There is clear liquid in each beaker" and statements such as "there is water in each beaker." The second statement is an inference, which means it is a judgement we have made, drawn from our observations. One of the things scientists do is to make inferences, but we need to check those inferences, which is one reason we do experiments. (Note: Astute observers may pick up on some other details that are quite informative. For example, the ice in alcohol will, after a few minutes, begin to melt, and actually float up off the bottom on a layer of water. This layer will be colder than the alcohol above, so students may notice condensation on the bottom part of the beaker only. These observations may help the students later.) Unit 1 Home