Density: The Wax Mystery

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Objective

Students will be able to plan and carry out investigations to find possible explanations for a wax "mystery".

Big Idea

Students have dealt with solids and placing them in water, but they haven't looked at the density of liquids. This lesson has students think about the factors that lead to various phenomena, like separation and floating in different liquids.

Overview

I want my students to deepen their understanding of density by looking at how different liquids act when mixed together or when an object with the same density is placed in different liquids Ultimately, students (hopefully) determine that, like solids, all liquids have their own unique densities. We will hold off on thermal energy affecting the density of liquids for now.  

Students first experience a discrepant event, where a piece of wax is placed in two unknown, clear liquids (water and rubbing alcohol). They look alike, and their viscosity appears the same. But wax will float on water and sink in alcohol. Students then try to think of what they need to know in order to solve the mystery.

Students then participate in a predict, conduct and explain activity that involves pouring unknown liquids into the same graduated cylinder, and then attempt to explain the resulting phenomenon. This is done by developing an investigation that gives them the information needed to draw conclusions.

Students next model their understanding, using evidence in the form of density calculations, to determine why different liquids separate in a graduated cylinder.

Students reflect on their experiences to explain how the wax floats in one beaker but sinks in another. Their conclusion (hopefully) is that density of each liquid is different, resulting in the wax sinking or floating, based on its relative density to each liquid.  

Do Now

5 minutes

Students are asked to predict what will happen when I place a piece of wax in two different beakers of unknown liquids. Up until now they have been dealing with solids, but haven't considered the effect of the liquid's density when determining if solids sink or float.

I want to see if they can:

1) Realize that if the wax pieces are made of the same matter that their density is the same. 

2) Explain the role that liquid density has in determining if solids float or sink in liquids.

Once students write down predictions in their science notebooks, they share their ideas within their group. I then perform the demonstration and students record the results.

The results: Students see that the wax floats in one clear liquid but sinks in the other clear liquid.

Developing Questions to Investigate

5 minutes

As teachers, it is important that we provide the necessary structure for our students to be successful, but also must create authentic opportunities for students to test and investigate their ideas. We essentially need to know when it is time to get out of the way and let kids take the helm of their own learning, and this is that time.  

I ask students to generate questions that can be investigated and we then share the questions with the class. As students are sharing, I'm typing their questions into a Google Spreadsheet and projecting them onto the board. This empowered students to feel comfortable with coming up with testable questions that drive the inquiry process.

 

Student questions about wax experiment

Student questions logged on Google Spreadsheet

Planning and Carrying Out Investigations to Solve the Wax Mystery

25 minutes

Planning and Carrying Out Investigations (SP3) doesn't just happen--students need necessary structures in place to be successful. I give each group these questions to help start the conversation about how to plan investigations.  

Now students plan an investigation and make a list of the materials needed to carrying it out.  The point of this activity is for students to develop their ability to Carry Out Investigations (SP3), while deepening their understanding of density. For example, if students decide they need to find the density of each liquid, they are applying prior core knowledge from other investigations.

Additionally they are determining what instruments to use to measure mass and volume, which helps them feel more proficient at conducting investigations. In order to conduct an experiment, students must have a deep understanding of how to calculate density using mass and volume. They can then measure each one and calculate the density of each liquid.

They must also confirm that the wax pieces have the same density, in order to see that it's the difference of the two liquids densities, and not the density of the wax, that is causing the phenomenon to occur.  

It may take some time for students to plan an investigation that will result in the needed measurements. Even if they don't come up with all aspects needed to explain the cause of the demonstration, you should allow students to figure this out on their own. For example, let's say that they want to measure the density of each liquid, but they leave out measuring each piece of wax --you could circulate around that group and ask, "How can you be sure that it's not the wax that is causing the outcome?"  Students can then discuss what they need to do and plan accordingly. This is drastically different from telling kids that they are missing a key point, because you are facilitating (coaching) their growth, not steering them to a neat outcome.  Resist the urge to guide too much!

Note: For safety, I didn't let my 6th graders experiment with the rubbing alcohol (liquid B) on their own. I had groups come up to the lab table area and I supervised students as they tested their questions.

Student Arguments--Explaining the Wax Mystery

10 minutes

You have options with this lesson--it can either be 1 day or 2 days.

In order to make it a one day lesson, groups will have to test different questions and share their results.  It may be good if 2 groups investigate the same question to encourage some form of replication.  Groups can then share out their results and you can then ask students to explain the wax mystery.  When students are sharing out their data and observations, I have them stand up and write their results on the board for others to see.  Alternatively, you could have student share their results in a Google Document.

If you wish to promote true, deep inquiry then you should allocate 2 days for students to perform investigations and then use their information to explain the wax mystery.  

Getting kids started:

Some classes may be able to use their mathematical and computational thinking (SP5) to construct explanations (for science) (SP6) without much assistance.  However, if students need some more structure to get started, I draw the beakers on the board and ask students to create a list of "what we know."

Students look at their results and say that they know:

1) The density of each piece of wax is the same. (Most kids, with rounding, get .8 g/mL.)

2) The density of each liquid is different. (Liquid A: 1.0 g/mL and Liquid B: .786 g/ML, 91% rubbing alcohol.)

3) The wax floated in liquid A but sank in liquid B.

Students then use their information to explain this mystery.  They are asked to write a paragraph that explains in detail what is occurring.  During this process they are mainly using CCCs: Cause and Effect and Patterns.

We then share our explanations with the class during a class discussion.

I want to see my students' abilities to use their understanding of density, established in this lesson and this lesson, to apply to solving a mystery, similar to a challenge that a scientist would have to address in real-life.