Density Using Water Displacement to Measure Irregular Solids of the Same Matter

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SWBAT use the water displacement method to measure the volume needed to calculate density of irregular solids and determine the meaning of density as a "characteristic property of matter."

Big Idea

Water displacement is a speedy way to measure volume of objects that are not prisms.

Lesson Overview

I want to challenge my students understanding of density that will then be tested and modified by the end of class, based on the new evidence that they collect. Up until now students know that different types of matter have specific densities associated with them, based on the cube lab performed yesterday in class.  

Today, I want students to see if changing the volume or shape of the same type of matter affects density. They will see that regardless of the size or shape, the density doesn't change.  In doing so, students are deepening their understanding of MS-PS1-1 and developing SEPs as they carry out investigations.

From the American Chemical Society:

"Students may reason that since the mass of each rod is the same, the volume of each rod must have something to do with its density. Some may go so far as to say that the rod with the smallest volume must have the highest density, because the same mass is packed into the smallest volume. Or that the rod with the largest volume must have the lowest density, because the same mass is spread out over the largest volume."

I then tell students that they are going to perform the same process as the cube activity, only this time they aren't dealing with objects that have different shapes and sizes.

During the Do Now, I refresh their memory of water displacement by having them recall this lesson.

This video discusses the materials required for this lesson:

Do Now

5 minutes

I ask students to recall how scientists measure the volume of irregular objects, ones that are not prisms. I want students to turn back to their notebooks if they do not remember.  

Specifically, I ask them to determine how to measure the volume of a sphere. I hold a sphere in the air as I introduce the Do Now to the class.

Reviewing Water Displacement and Lab Procedure

5 minutes

I want my students to measure accurately so that they calculate accurate densities of each object.  We must first review water displacement.

During the review process, I make sure that students are discussing the following:

I remind students to fill the water to a certain level--round numbers make this process easier. They should slide the object down the side of the graduated cylinder so that no water is lost. They should use the "peace sign" method to hold the graduated cylinder at the base, so that they don't cover the graduations on the side of the cylinder. They should read the volume from the bottom of the meniscus.

To check for understanding, I write a sample water displacement problem on the board and have students raise their hand when they have an answer. I find that performing this quick check helps avoid confusion and leads to more accurate density calculations.  

Since they will be sharing objects that were used in other groups, I ask why they should dry the rod off before using it.

Students' responses:

"If we don't dry it off then it will affect the accuracy of our measurements  Water has mass and takes up space so our density calculation will be off if the rod is not dry."  

Students Perform Lab Activity

30 minutes

Cookie-cutter labs with predetermined outcomes are not an acceptable practice in the NGSS. As educators, we probably all have our standby lessons that have survived the test of time, but I'm going to break that mold.  

The general structure of this lab is inspired by a cookie-cutter lab, but is modified to be more student-centered and rigorous.  The video below overviews the main takeaways from the lesson.

In a typical density lab, students are asked to measure mass and volume, divide and then rank each object from lowest to highest density. The difference in this lab is that I am structuring the learning in a way where students' predictions and initial ideas will be challenged as they calculate the density of each object, and then try to model their results, based on their new findings.  

In other words, most students probably thought that the larger volume had to be more dense because there must be more matter in the larger volume. What they neglected to consider is that matter and volume of certain types of matter have a direct-proportional relationship.  

Reflecting and Asking Questions

15 minutes

Students now have mass and volume measurements for six different objects, comprised of three different materials. I want them to calculate the density of each object, if they haven't already and reflect on their findings. 

The continual use of our KLEWS chart provides my students with the necessary structure to effectively reflect and raise questions about what they're learning.

To guide students through the analysis of their results, I like to use the Powerful Questions to Ask Students highlighted by Edutopia:

1.) What do you think? Common question: What do you think about the results--what does the data show?

2.) Why do you think that?  Common question: Why do you think that each material had the same density, regardless of size or shape?

3.) How do you know this?  Common question: How do you know that different types of matter have specific densities?  

4.) Can you tell me more?  Common question: Can you explain more about the role of atomic or molecular structure?

5.) What questions do you still have?  Common question: What aspect of density are you still questioning or wondering about?

Density is one of the most abstract concepts that student will deal with this year. I certainly don't think that this one lesson and reflection section is going to be enough to promote deep understanding, but it's a start.  

Please feel free to follow up with other discrepant events and continue to have students model and explain how that is possible. Also, feel free to go back to this lesson and have students represent their understanding with manipulative activities. Remember, students will be testing the density of liquids soon, so it will be very interesting to see what connections they make to this activity.