8th Grade Physical Science Curriculum Unit 1: Density and Buoyancy Lesson 1, part c. Magic Time! Objectives: Students will apply their skill of predicting to a new challenge, employing their understanding of the behavior of an object in liquids of different densities. Key Questions: see parts a and b Outline: This demonstration provides a startling and beautiful climax to the sequence, and allows for an informal assessment of student understanding of the previous experiment results. Time Required: 20 to 30 minutes Materials: For the Teacher 200 ml of alcohol 200 ml of water Yellow and blue food coloring Plastic wrap Ice cubes OUSD/California Science Content Standards: 1a; conducting investigations (Calif. #8.9a), 5d, floating and sinking (Calif. # 8.8d) Procedure Review student responses to the question posed for homework. Ask which liquid they think is lighter, and explore their reasons. Then ask if they have any idea how to test the most widely held hypothesis. (Standard 1a) Someone is likely to suggest combining the liquids. This is a good demonstration, but must be done carefully by the teacher. Also, since the liquids are identical to the eye, they need to be colored so they can be differentiated. This is easily accomplished with yellow and blue food coloring. Color alcohol one color, water the other color. Drape the plastic wrap in the beaker like a loose-fitting cover, dipping down to the surface of the water. Slowly pour the alcohol into the plastic wrap, then gently slide the plastic wrap out, so the alcohol is left on top of the water. The effect is quite dazzling. Assessment: To finish the demonstration, ask the students to record in their notes a prediction of where the ice cube will go in this two-layer combination. If they have grasped the previous experiment results, they will predict that it will go in the middle, as it will sink through the alcohol and float on the water. (OUSD Standard 5d) At this point, if you ask the students why the ice cube sank in the alcohol, they are just as likely to say "because it is alcohol" as to say anything about weight, let alone density. Most certainly haven't really even a glimmer about density yet -- which is why we need several more experiences. Unit 1 Home