Mixtures: Characteristic Properties - Solid Identification

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Objective

Students will be able to use density to identify which substances are the same.

Big Idea

Density is a characteristic property.

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Purpose of Lesson:

The purpose of this lesson is to introduce density as a characteristic property and give students the experience of using density to identify substances.  To do this lab you will need a set of density cubes or cylinders. These have equal volume but different mass

5 minutes

Learning Goal: Discover how density can be used to identify substances.

Opening Question:  Remember the fake crown from a few days ago,  how was density used in this situation to tell the difference between the real gold and the fake gold.

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.

Pre-assessment

5 minutes

I start each unit with a pre-assessment to open up the student thinking and help uncover misconceptions. This allows me to modify my teaching, compact the curriculum, form appropriate groups, and measure growth. By far the best formative probes I've found are by Page Keeley.  She's written several books such as Uncovering Student Ideas in Science Volume 1, 2, 3 and 4. In this probe, Floating Logs, the students are demonstrating their thinking about density as a characteristic property. Specifically, this probe gets to the misconception that density changes as the size of the object changes.

In the student response, the student shows clearly that they are still equating MASS with DENSITY. The student is ready for this lesson and will hopefully exhibit a shift in thinking through conversations and writing.

Hook

10 minutes

For the hook today we are going back to the reading from yesterday but just to read one specific part, the section on density.  As well, I put the density cylinder sets on the table. My cylinder sets contain four cylinders that are made of aluminum and one that is made of brass. I ask the students to compare the cylinders and predict which one is brass. (All the cylinders are painted so the students cannot use appearance.)  I let students pick up  the cylinders during the explore time and generally this means that they have a decent prediction of which cylinder is made of brass.  You can also hold off on let students be hand-on until it is time to mass.  This can lead to a bigger surprise.

We re-read the section of the reading together and then discuss using the evidence of our observations to predict which substances are the same.

Discovery Activity

30 minutes

Now, I quickly teach the students how to find Density. BEWARE. Teachers can end up spending weeks and weeks on density.  The purpose of this lesson is to use density to identify a substance. The method I use to have the kids calculate density is quick and easy.

First I have the students write the formula down in their lab book.

Density = Mass/volume

Then I show the kids how to use displacement to find the volume of the cylinders and use the electronic balances to find the mass.  (I tend to use electronic balances instead of 4-beam balances any more.  There is more information on this in the reflection.)

I tell them that in today's lab we are going to be trying to figure out which of the cylinders could be made out of the same substance. I pass out the graduated cylinders and then let the students go to work. I walk around and help students with their calculations. Since the volume of all of my cylinders is 1 ml, it is easy to tell when students have made a mistake and I simply ask them to redo it.

This lab can be used as a write-up for student work.  Sometimes I print them out for students to save class time, while other times I have students create their own labs to turn in.

Once the students have the data, I show them how to use the histogram on the lab write-up to graph their data. This is a scaffolded graph, because I am not expecting the students to make their own today. Again, the point of this lab is to use density to identify substances.

In this screencast I explain how to make a histogram, what some of the benefits and problems of histograms.

Finally, the students write their conclusions.

Closure

2 minutes

Closing Statement: "Today we used density as a characteristic property.  We were able to do this because different substances have different densities.  Tomorrow we will look at the densities of liquids."

Closing Question: "How were we able to tell our two substances apart? Would the size of the sample change the density of the sample?"

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.