Earth Science7th Grade

Density is a fundamental quality of matter, and a firm understanding of this concept is crucially important to many content areas from earth science to physics, but also provides a framework for the comprehension of mathematical concepts like inverse proportions. Despite this, many students continue to struggle with the relationship between mass, volume, and density, becoming cognitively trapped in misconceptions. Often students will continue to believe that heavy objects are inherently dense, or that heavy objects will sink in water, even after repeated lessons involving the formula for density. Seeing firsthand how this relationship plays out with different objects of various masses and volumes allows students the opportunity to discover the reasoning, and support it with evidence, behind why these misconceptions are false. This plan, aided by the PhET Simulations, allows teachers to fit hours’ worth of density explorations into one 55 minute lesson, avoiding needless mess and safety concerns, and giving students the guided freedom they need to defeat density misconceptions.


Content Area: Science

Standard: 1. Physical Science

3. Apply an understanding of atomic and molecular structure to explain the properties of matter, and predict outcomes of chemical and nuclear reactions

Inquiry Question 2: Which properties are the most useful in trying to separate mixtures of substances?

After a previous lesson and a brief review of density, seventh grade students will individually explore the PhET Density Simulator, and then, in small groups, measure the volume and mass, and calculate the density of three mineral samples to within a twenty percent accuracy.

Learning Goals
Students will be able to:
  • Explain the relationship between volume, mass, and density
  • Describe how changes to either volume or mass affect density

Learning Plan
Critical Question:What is the relationship between volume, mass, and density?
Engage: 5 minutes
Begin by reviewing with the students the various characteristics of minerals we use for categorization. Review how one measures mass and volume, and what density is and how it is found.
Explore: 15 minutes
Allow the students to work through the simulation, seeing for themselves how the densities of the various blocks play out. Encourage them to play with the mystery setting and rank the blocks by density. Ask them how they will find the volumes for the blocks (displacement).
Explain: 10 minutes
Discuss the results of the mystery blocks with the class. If you change the mass of a block and keep the volume the same, what happens to the density? What would one have to do to keep the density from changing? Can you tell what the density of a block will be by looking at the volume of the block? Or just the weight?
Explore (Again): 20 minutes
Have the students work through the measuring and calculating of the mineral densities in small groups or pairs. When each group is finished have them compare results with other groups.
Elaborate: 5 minutes
Bring it back to the critical question. See if the students can describe the relationship. Is density a quality that will change for a mineral over time, or is it consistent? Would we expect every sample of a given mineral to have the same density?
Students able to calculate mineral densities accurate to within 20 percent of true values will be considered to have successfully met the objective.

About PhET Simulations
The PhET simulations are designed by a group at the University of Colorado Boulder through grants from the National Science Foundation, William and Flora Hewlett Foundation, and many others. They started as simple educational models for physics; hence the name from Physics (Ph) Education (E) Technology (T), or PhET, but quickly branched out to include Biology, Math, Chemistry, and more. There are over one hundred unique simulations grouped by content or age level which model many complex and hard to understand concepts found in science and math. All the simulations are interactive, featuring siders or objects which can be manipulated, so they are more than simple videos or clips. The simulations allow users to tweak setting to model just what they are needing to see at that time, making them flexible and powerful tools for the classroom. Many of the simulations also come with teaching guides, lesson plans, and classroom materials designed to work in tandem with the simulations. All are run using Java and Flash and have very minimal system requirements, but do not currently run on most tablets or iPad/Android devices. It is possible to run the simulations offline or from a USB drive, but some have problems when run this way.
PhET Simulations Homepage
Density Simulation
JAVA Download
Flash Player Download

What is the added value of the technology?
Exploring the density of objects is an activity that any science classroom could manage logistically; it should technically be possible to replicate the same sort of simulation that the PhET Simulation accomplishes. However, doing so on an individual basis would take a restrictively large amount of time, so much so that few curricula could afford to fit it in. Furthermore, such an activity would bring added restrictions on classroom space, health and safety concerns, as well as doubts concerning learning outcomes from an unguided exploration. In addition, the effectiveness of such activities in breaking down student misconceptions, and the ability of students to complete them accurately, frequently scale with the size of objects utilized; many small objects do not allow students to conceptualize the volume aspect as fully as educators would like. The PhET Density Simulation has none of these weaknesses, and, indeed, has the added benefits of being completable in less than a half hour, requires as its only materials computers, and allows students the freedom to customize settings to focus outcomes on what they are curious or confused about. Like the great many simulations offered by PhET, the density simulation provides a quick and effective exploration of a hard-to-conceptualize idea, one that many students fail to comprehend without the use of flexible models like these.

What I learned
This lesson was originally designed and delivered by me to four different periods of seventh graders without the PhET Simulation. Reflection of the lesson’s success afterwards, and analysis of the post-assessment utilized, revealed a persistence of certain misconceptions in the way students continued to think about density. Despite various lessons and labs covering the concept, formula, and repeated usage of density, many students remained preoccupied by either weight or volume of objects as an overriding signal of the object’s density. After consulting with the other science teachers in the department, we decided to alter the lesson to give the students more work with density in a way that focused on letting them see for themselves how two objects of identical volume might have very different densities. The problem this raised was one of time; there simply was not enough time allotted within the unit to allow for an added activity of this magnitude. Therefore I suggested trying to use a PhET simulation which had the flexibility for students to reach the same conclusions without having to commit to a full day’s additional lab. After trying the simulation for themselves the teachers all seemed very positive about this change to the lesson plan. Despite the fact that the “improved” lesson will not likely be tested until next school year, I am confident the simulation’s addition will assist in the students’ learning of density.