Introduction to Density
8th grade Science
Background: Students will have previously learned about mass and volume, and their physical interpretations. Students will also have been responsible for knowing how to read a triple-beam balance and understanding how to use a triple-beam balance to measure mass (including how to zero the instrument and what constitutes an accurate measurement. Students will come into this lesson with the basic knowledge of the volume calculation for basic geometric shapes (rectangular prism, cylinder, sphere) and calculating volume by water displacement. Understanding will be checked by a homework assignment due before the lesson begins.
Objective: Students will be able to calculate the density of objects. Students will understand that an equal mass of a more and less dense objects requires more volume of less dense objects. Conversely, students will understand that of two equally-sized containers, the one filled with the more dense object will contain more mass. Students will record the mass and volume of unknown samples and calculate their densities.
Estimated Time: 55 minutes
Essential Questions: How do we calculate density? What is its physical meaning?
Knowledge Objectives: Students should know: density = mass / volume, more density means more mass for a given volume.
Materials: Laptop, projector, PowerPoint, student lab notebooks, hershey kisses, reese’s peanut butter cups, hershey nuggets, krackel, mr. goodbars, hershey dark chocolate, hershey milk chocolate, triple-beam balance, water, graduated cylinder, rulers
“Draw My Science: Mass, Volume, and Density” on YouTube – https://www.youtube.com/watch?v=n-pQf71TI-w
Vocabulary: Mass, Volume (already discussed), Density – The amount of mass that a substance has in a given amount of space.
Prerequisite knowledge/skills: Students will know the definitions of mass, volume, and density. Students will know how to calculate volume of regular shapes and how to measure mass.
1.) Do Now: Students will hand in their assignments regarding how to measure mass (how to read a triple-beam balance), and how to calculate volume (both by calculation and volume displacement). Homework papers will be handed back to a different student to grade and answers will be discussed in class. (10 min)
2.) Show students “Draw My Science” video (no guided notes). (12 min)
3.) Students will have been given a “density triangle” showing the relationship between density, mass, and volume and a brief physical description of each quantity. To calculate any one of the three, they cover that variable and the remaining 2 vertices of the triangle will be shown with the appropriate relationship. This tool will be explained so that students can use it when preparing for their exams. In addition, students will take notes on 2 slides that describe how to calculate density for regularly and irregularly-shaped objects. (5 min)
4.) Students will be given rulers, a triple-beam-balance, and each table of 4 students will be given a different candy to measure. A data table will be projected onto the main board and students will have to fill in length, width, and height (for regular-shaped objects) and then volume (irregularly-shaped objects’ volume is found by water displacement). Then, students will measure the mass of the objects by triple-beam balance and write their answers on the board, along with the calculated density. (20 min)
5.) In the remaining 5 minutes of class, students will discuss the results of the data collection and determine the best value of chocolate (i.e. which chocolate bar gives you the most chocolate for your money, if all of them are identically priced). (5 min)
Discovery-based learning activity: If time permits, students will be shown a simple demonstration of density and asked to explain the results of the demonstration in the next class. The demonstration involves a large tub of water and cans of regular coke and diet coke. Students will be given a worksheet with three sections that is due the next day. Part 1: Which can will float and which can will sink? Students will be asked to make a prediction and write 1 sentence backing up their prediction. Part 2: What actually happened? Students will be asked to identify which can floats and which can sinks. Part 3: Why do you think this happened? Students will have to use the newly introduced concept of density to explain that regular coke is more dense than diet coke, so it sinks to the bottom of the tub.