7th Grade Science : Unit #8 - Mixtures: Characteristic Properties : Lesson #6

Mixtures: Characteristic Properties - Density Tube Demonstration

Objective: Students will be able to see how density can be used as a method for separating liquids and solids.
Standards: 5-PS1-3 SP1
Subject(s): Science
60 minutes
1 Teachers' Lounge - 0 minutes

Purpose of Lesson:

The purpose of this lesson is to provide students with an interactive demonstration with liquids to show what happens when liquids of different densities are added together.  

2 Ready...Set...Engage! - 3 minutes

Learning Goal: Discover what happens when liquids of different densities are added together. 

Opening Question:  What happens when you mix oil and water?  Why?

Students record their opening question on their learning goal sheet and are ready to start class 3 min after the bell has rung.  I reward students who get started early with ROCK STAR SCIENTIST tickets.  

Follow the links to learn more about the beginning of class strategies and ROCK STAR scientist tickets

3 Hook - 5 minutes

The purpose of the hook today is to have students see density in action, practice with using the word density, and to show the particle structure of density. I put some containers of oil and water on the table and let the students observe them. I provide the students with stir rods so that they can attempt to mix the substances up.

I walk around and listen to the students discussions. Generally, students talk about the oil being "light" and the water being "heavy". This is a great time to ask them this riddle, "What weighs more a pound of feathers or a pound of bowling balls?"  I want the students to stop using the words heavy and light and start using the word dense.  

After we think about and discuss the riddle, I draw pictures of three substances on the board. These are "particle" pictures. I ask the students to observe the similarities and differences between the pictures and to relate the pictures to density.


4 Collaborative - 10 minutes

One of the biggest misconceptions students have about density is that it changes with amount. If they hold on to this misconception they will have a hard time using density to separate substances because they will continue to think that amount matters.  This very simple lab helps address that misconception. In the Density of Water Lab, students are finding the density of different amounts of water. This lab is incredible simply and can be completed and turned in in about 20 min. I generally give the students a lab handout rather than having them create their own for this lab, simply because I am saving time for the demonstration in the next section.  

Choosing Depth
Adjustments to Practice

One problem that teachers face all the time is choosing the depth to teach a topic. Now I know that depth and complexity are the big words right now...but the truth is you can't always teach to depth because teachers are on deadlines. Sometimes you have to have faith that the seeds you are planting now will be harvested in later years.

This simple lab is a great example of this. Some years, when the emphasis of the district or school is on graphing or analysis, I will spend and entire day on this lab. We take class data, have conversations about error, make histograms, and line graphs. This year, the emphasis in my district is on a simple understanding of density and reading and writing.  My curriculum guide states that my job is to show students how to use density to separate substances.  This lab does this well without deeper discussions.  It can be frustrating as a teacher to feel constricted and have to make these decisions, but you have to have faith that the scope and sequence serves the students' needs. When I have confusion or questions  about depth, I always contact my district's curriculum expert who is also a science teacher but has a district view versus a classroom view.  

5 Focus - 5 minutes

The purpose of this section to is exhibit thinking about amount and density. For this purpose I get the reading out that we have been using and re-read with the students the first paragraph. 

"A characteristic property is a physical or chemical property that is unique to a particular substance.  It can be used to identify the substance. Characteristic properties are not affected by the amount or shape of a substance: A bolt made from iron will have the same characteristic properties as the hull of an iron ship or a piece of iron railing."

The I do a think-aloud about density and this idea. I point out to the students that if I have one cup of water and then I add another cup of water, my water will have a higher mass because there is more of it.  However, even because it has a higher mass, the density stays the same.  The density of 1 cup of water is 1 g/ml and the density of 2 cups of water is 1 g/ml.  The reason this is is because density measures the compactness of the particles, and amount doesn't change the compactness.  So even if I had a swimming pool of water, it would only be 1g/ml.  

Don't rush this think-aloud. Take your time. Use images or realia. Check for understanding. Give the students an opportunity to make sense of this.

6 Guided - 10 minutes

The density tube is one of my favorite demonstrations to do.  It's not which liquids you choose to use, but it is critical to consider density. You need an assortment of liquids with different densities.  I like to use water, alcohol, soap, oil, corn syrup, and salt water. In this video I show what materials I used to make the density tube and what the finished product looks like.


To begin the lab, I call all the students to my demonstration table. I think that bringing them together adds to the engagement because they can access each other and this leads to more excitement and discussion.  

I start by putting the salt water into a large graduated cylinder. (The larger, the better, for this demonstration.) Then I get the corn syrup and ask the students what they think is going to happen when I add the corn syrup. Do they think it will stay on top? Go to the bottom?  Mix in the middle?

Once students make a prediction, I pour the liquid in and we watch to see what happens. I continue this process for all the liquids. The key to making this demonstration effective is to elicit lots of student participation through questioning and assistance.

If you have time, an extension you can use is to add solids to the tube. It is fun to put in a metal, rubber, wood, and wax object because they will descend to different levels in the tube.

I give the students a simple summary assignment at the end.  

7 Closure - 2 minutes

Closing Statement:  "Today we looked at how amount of a liquid doesn't change the density and how different liquids have different densities."

Closing Question: "How could you use the characteristic property of density to separate substances?"

Closure depends greatly on timing and is not as easy to plan in advance as opening.  You can find more information about how I manage closure here.