Density Test Tube Challenge

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Students will be able write a clear scientific procedure and use their knowledge of density to create a density column.

Big Idea

Students are given salt, water, and food coloring and are asked to problem solve to create a density column!

Introduction and Connection to the NGSS and Common Core

In this two day lesson, students use water, salt, and food coloring to create a density column in a test tube that includes layers of colored water.  Students write their own procedures to complete this challenge which asks them to put their problem solving to the test!

This lesson is designed to address the following NGSS and Common Core Standards:

MS-PS1-2  Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.  

CCSS.ELA-LITERACY.WHST.6-8.2   Write informative/explanatory texts, including the narration of historical events, scientific procedures/ experiments, or technical processes.

CCSS.ELA-LITERACY.RST.6-8.3   Follow precisely a multistep procedure when carrying out experiments, taking measurements, or performing technical tasks.

Science and Engineering Practices:

In this lesson, students analyze and interpret data to create a model that predicts how adding different amounts of salt to water will change the density of the solution (SP4).  In addition, students plan and carry out their investigation as they write their own procedure to create their density columns (SP3).

Cross Cutting Concepts:

Relating to the NGSS, students use their data of mass, volume, and density to create a model that predicts how adding different amounts of salt to water will change the density of the solution.  Thus, this connects to the Cross Cutting Concept of “Patterns” which explains that macroscopic patterns are related to the nature of microscopic and atomic-level structure.

Warm Up: Formative Assessment

5 minutes

Provide students with the Density Formative Assessment (Density Column).  Have the students complete this independently.  Again, I provide feedback on these formative assessments and then sort the student work into groups of similar learners.  Then, in a future lesson, I conference with these groups of learners and meet with them about their similar needs.

The student has correctly ordered the objects in the beaker.  There are two specific aspects of this student's work that are important to look for.  First, the drawing of the toy truck shows the truck sitting right on the border, which is accurate.  Watch for students drawing objects half way "hovering" in the middle of the liquids.  Also, in the written response, the student uses data to support her reasoning.  She compares the density of the toy truck using actual numbers for the densities of the vegetable oil and corn syrup.  Students can tend to forget to use data or use generalities such as "the density is more than the vegetable oil but less than the corn syrup".  Students need to recognize that scientists use specific data to support their claims.

As I write feedback on these formative assessments, I sort them into groups of similar learners that I meet with in small conference groups in a future lesson to provide individualized instructions to meet their needs.  The groups typically include:

1.  Students that have "hovering" objects in the beaker.

2.  Students that misplace objects in the beaker.

3.  Students that do not use data in their reasoning for the toy truck.

4.  Students that use generalizations as opposed to data in their reasoning for the toy truck.

5.  Students that include the density of the truck but do not compare to the densities of the vegetable oil and corn syrup.

Connecting to the Essential Question: What are you going to learn today?

5 minutes

Ask the students, “What are you going to learn today?”.  Students should respond with the Essential Question, “How can the arrangement and motion of the particles in a substance affect its physical properties and how can they be measured?”  I keep this EQ posted on the board and students also have a copy of it in their Density Unit Plan.

In the previous two lessons (Density Cylinders and Density Column), students have been introduced to the following skills and have self-assessed their level of mastery on a scale from 1 to 4 (4 being mastery).  As students to take some time to re-assess their level of mastery and change any number that they feel is appropriate to change.

D.2  I can provide evidence explaining how a substance’s state of matter affects the arrangement of its particles.

D.3   I can identify the physical properties that are affected by the arrangement of the particles in a substance.

D.4  I can effectively use laboratory tools in order to measure temperature, mass, volume, and density.

Mini Lesson: Writing Procedures and Sorting Student Work

25 minutes

Cut the Procedures: Sorting Student Work document and provide each pair of students with each of the procedures.  These procedures are referencing a lab that my students had performed earlier in the year in which they tested a variety of brands of diapers to determine which was the most absorbent.

Have students compare the two procedures and determine which “sounds like a scientist” and which “sounds like a 7th grader”.  Upon completing this, have students determine the aspects of the “sounds like a scientist” procedure that makes it more effective.  As a class, have the students share the criteria they determined and create a list either on the board or chart paper of these aspects.

Students share that Procedure #1 “sounds like a 7th grader” while Procedure #2 “sounds like a scientist”.  The criteria that students typically cite include (evidence students cite from the procedures are in parenthesis after the criteria):

  • Includes specific measurements.  (100 mL)
  • Names the lab tools by name.   (beaker)
  • Provides details of “how” to complete the step.  (hold the diaper in a “U” shape)
  • Recording data is mentioned. (record data)
  • Uses the science vocabulary for what to mention. (measure volume)
  • Separates each trial group as different steps. (Repeat steps 2 – 7 for Huggies.)
  • Is a numbered list. (1., 2., 3., etc)
  • Someone else who wasn't in the class could follow it.

Explain to the students that they will be writing their own procedures today and that they need to consider these criteria as they write.  Leave the criteria list posted in the front of the room where it is visible for the students.

Density Test Tube Challenge

75 minutes

Provide students with the Density Test Tube Challenge Student Document. (Students in my class already have a background in density prior to this lesson.  A resource sheet that they have access to is included in the resources.)

Ask the students, “What are two ways you could define density?”.  Students should respond with “how tightly packed the molecules are” and “mass divided by volume”. 

Ask the students, “What are two ways that you could manipulate the variables to increase the density?”.  Students should respond “increase the mass” or “decrease the volume”.

Explain the Density Test Tube Challenge to the students.  Tell them they can follow along with your instructions on their student document.  As I explain this, I get really into the role of Mission Impossible.  I may be corny, but middle school students eat it up!

Say, “Your challenge, if you choose to accept it, is to create a density column inside a test tube using only water, salt, and food coloring.  There must be three layers, each a different color.  It should be clear where one layer stops and the next begins.

The materials provided for the challenge include test tube, ring stand, test tube holder, salt, food coloring, water, three beakers, eye dropper, scale, and calculator.

As your leader, I do want to share some ‘Super Secret Tips for Success’:

  1. When you suspend your test tube on the ring stand, tilt it slightly at an angle!
  2. When you are placing your solutions into the test tube, do one drop at a time!  If you squeeze all of the liquid out fast, the layers will mix!
  3. Before making any solution, place masking tape on the beakers and label them #1, #2, and #3.  Then, measure the masses of the three beakers and write them down somewhere!
  4. Measure the mass of the beaker with the solution in it BEFORE putting any into the test tube!  Remember to record this data somewhere!

Before you can begin to make your density test tube, your leader (me) must approve your plan.  Your group must complete a detailed procedure that meets the criteria we determined today.  You procedure should include directions for how to create the density column AND calculate the density of each of the layers.

Here are some things that must be included:

How much water is in each beaker?

What color is in each beaker?

How much of the liquid do you put in at a time?

How do you find the volume?

How do you find the mass?

How do you find the density?

Which order (of colors) will you put in the test tube?

Other than these tips, I do not tell them how to complete the challenge.  It is important that students problem solve this on their own!  Do NOT lead them to the idea that they need to vary the amounts of salt for each layer in order to be successful.  The learning here comes in the students independent application of density principles.

Once your procedure is approved, only then can you get your materials and begin working.  Although you must only complete one procedure per group, each member of the group needs to provide input and ask questions as a part of the appropriate group discourse we have worked on all year.  When you bring me your procedure, I will be asking follow up questions and I will be asking random members of your group for insight into your work.  Answering my questions with, “I don’t know.  John wrote that.” will not be accepted.  You all need to take ownership in your group’s procedures!

Typically, students will take one day to write the procedure and one day to complete the challenge.

Below are videos of students completing the challenge.  Notice how they touch the eyedropper to the side of the test tube and go very slowly.  In addition, note that the test tube is clamped at an angle.  This aids in success. 

 Completed Test Tube Challenge

Density Test Tube Challenge: Set Up and Teacher Tips

Set Up:

Around the room, set up as many stations as you will have groups.  Each station will require:

  • Container with salt
  • 1/8 or 1/4 tsp measuring spoons
  • 3 beakers of the same size
  • Stirring rod
  • 3 eyedroppers
  • Ring Stand
  • Test tube/biuret clamp
  • Scales must also be available.

Teacher Tips:

  • Food coloring is messy and students tend to overuse it.  Either you should be the one that adds the food coloring to their beakers (which can be time consuming), or you need to specifically show how you expect them to add only a few drops.
  • Food coloring stains clothes.  Make students aware of this and warn them about leaning on countertops as there might be food coloring on the tables.
  • Food coloring will also stain wood. I have black lab counters that don’t stain.  If you don’t, cover your tables!
  • When students share their procedures, adjust their number of scoops of salt that they include if needed to avoid saturation.  For example, if students say they are going to include 20, 10, and 5 scoops of salt, not only will they waste salt, but the layers will mix as two will be saturated.  Students are not aware of how many scoops are appropriate so I typically share this with them.  I find that in 100 mL of water 5, 2, and 0 scoops of salt is a great place to start!  Do NOT share this with them before they bring their procedures to you.  If you do, you will be telling them how to solve the problem.  They need to develop the idea of adding salt to increase density on their own!
  • If students are unsuccessful, take the opportunity to share with them the idea of saturation and that if they have more than one beaker that has salt that won’t dissolve and has built up at the bottom of the beaker, that those beakers have the same density and they will mix.  This is a common mistake!
  • There are a couple of other factors that cause students to not succeed in the challenge.  If they squirt the eyedropper too fast for the second and third layers, they mix.  Show these groups how they can touch the eyedropper to the side of the test tube and squeeze very slowly.  Second, students often forget how many scoops they put in each beaker.  Even though they write it in their procedure, if students are struggling, they may be putting the layers in the test tube in the wrong order.
  • If there are groups that finish early, have them complete the “Double Challenge” from the lab document.  They should create a column that has 5 layers!  I have actually had students create 14 layers!

A Look at Student Work

In this video, I go through a student procedure and provide feedback for some expectations and areas of struggle that I see in many student's work.

Student Lab Document:

This student has recognized that density is how tightly packed the particles in a substance are and that it can be calculated by dividing mass by volume.  In addition, the student recognizes that to increase density by manipulating the variables in the density equation, one could increase the mass or decrease the volume.

While this student has calculated the densities correctly, the student has forgotten units throughout the data tables.  A couple points of interest about the densities of the liquids.  First, the plain water will not be exactly 1.0 g/ml.  For students that are aware of this, it is a great opportunity to explain that tap water has other particles dissolved in it and that it is not pure water.  In addition, students need to have a mathematical foundation to notice the differences in densities.  For example, students might not notice that 1.16 is greater than 1.1.  

Closure: Exit Slip

5 minutes

On a sheet of paper, have students answer the following question:

If you were to repeat this challenge, but I added the constraint that you had to use 5 scoops of salt in each beaker, how would your procedure change to create the Density Test Tube Challenge?

(Students should note that they would then use different amounts of water.  The beaker with the lowest amount of water would be the most dense and the beaker with the most water would be the least dense.)