Density of Solids and Liquids
Lesson 2 of 11
Objective: Students will be able to measure the density of solids (regular and irregular shapes) and liquids.
By 8th grade, students should have already been introduced to measuring volume, mass and density of solids and liquids. This lesson serves to review and reinforce these concepts. This lab does not include measuring the density of gases. That can be found in another of my lessons.
I include this lesson here even though it is not an explicitly addressed in the NGSS Middle School standards as there are still many students in grades 6-8 who will not have mastered measuring density of matter.
This lesson should be able to completed in one 50 minute class period. A list of materials needed is included in the resources.
Place one set of materials on each tray. If using salt water, make sure to prepare this ahead of time and have in 30 ml beakers.
While this lab does not involve any dangerous chemicals, I always encourage teachers to have students get into the habit of using safety goggles with all labs.
Begin with an elicit activity.
Ask students to open their science journals and respond to the following questions:
What is mass? What is volume? What is density?
Give them 5 minutes to write down their ideas then turn and talk to their table group, sharing their responses. I use this time to listen and monitor group discussions. Should you notice that certain students are not speaking, ask them what their thoughts are in order to pull them into the discussion.
Ask each table group to come up with a working definition that they all agree on for mass, volume and density. The goal here is not a textbook definition but a working definition that makes sense to your students.
To help those students who might have limited English language skills or need more support, consider providing a scaffold. Perhaps a visual image or partial definition along with images to help them put these ideas together.
Bring the whole group back together and ask each table to share their working definitions.
Guide the class into drafting a working definition for each term and record this on chart paper or on a word wall.
If using science journals with indexes, add the words to the index at this time if you like.
Show students a marble, a die and a screw and ask them how they would determine the mass and volume for each. Most of them will now to use a scale for the mass but measuring the volume may produce some varied responses.
If your students have not learned about calculating the volume of a geometric solid, this is good place to discuss the formulas used to measure the volumes of spheres, cubes and cylinders. Here is a lesson you may consider from fellow BetterLesson Master Teacher Erin Doughty on Making Sense of the Formula for determining volume.
For the screw, since it is an irregularly shaped solid, another method is needed, water immersion. Commonly refereed to as Archimedes' Principle, where the volume of the submerged portion equals the volume of fluid it displaces. This is great place for a demonstration. You will need a 100 ml graduated cylinder filled with 50 ml of water and an irregularly shaped object that will fit down into the cylinder.
Here is a YouTube video on water displacement to show your students. You may want to turn off the volume -- there is no narration but there is music. It would be important to pause the video as it pays to check on student understanding.
Note: if you are using a glass cylinder, you should teach about measuring the volume by finding the meniscus, however many of the Nalgene® polymethylpentene graduated cylinders nowadays do not have a meniscus.
With the students reading along, review the Density Lab Student Sheet to be sure everyone understands the expectations of the lab. I like read through the introduction and directions as a class to help ensure that my students understand what they are being asked to do and why. I teach this lesson at the beginning of the year and have found that the more time I take to instill good habits, like reading the lab first, helps prevent procedural errors later.
With your lab trays all set up and your students all having read the lab and asked their clarifying questions, it is time to start the lab.
While the students are working on the lab, I like to circulate around the class to check in with each group to be sure they are on track and help any students who might need additional assistance.
I find it to be a good practice to have students share their data to the class. You might create a data table on the whiteboard, poster paper or project it from a laptop then have students come and fill in their data as they finish each section of the lab. Once complete, I like to discuss the data and what patterns we see.
Assuming everyone measured the same or similar objects, is the data consistent from group to group or not and why? When determining the density of these objects, can we determine an average? How would we do that?
In the student work below you can see where their average densities for the same objects are all different. By collecting and averaging a larger sample we get averages closer to the actual densities of the metals.
Students need to practice the skill of analyzing data and discussing both in small group and as a class. While not as glamorous as the lab's themselves, data analysis is a key skill that students must develop and use when making claims based on evidence. The analysis will aid them in valuing the importance of accuracy and precision of data as they learn the rigor required to be consistent when collecting data during labs and investigations, a skill that is still very much in development at the middle grades.