Mixtures: Characteristic Properties - Liquid 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 to prepare five different colored liquids.  Mine are:

  • Water - blue
  • Alcohol - green
  • Weak salt water - yellow
  • Strong salt water - red
  • Second container of water - orange

Ready. Set. Engage!

5 minutes

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

Opening Question: Think back to the demonstration we did yesterday.  What if we had had a mystery substance. How could we use density to identify the mystery liquid?

Students record their opening question on their learning goal sheet and are ready to start class 3 minutes 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

Hook

10 minutes

I went to visit friends in northern Washington and took these pictures of lavender oil distillation at their lavender farm.

I use this presentation with students to show them how everyday people use characteristic properties.

Discovery Activity

30 minutes

The purpose of this lab is for students to get to see again that density can be used to identify which substances might be the same, this time with liquids.  I set up my liquids as below.

  • Water - blue
  • Alcohol - green
  • Weak salt water - yellow
  • Strong salt water - red
  • Second container of water - orange

The salt water is simply epsom salts dissolved in water. I make the yellow solution fairly weak, but I check the density to make sure the it is distinguishable from water. I saturate the water with the epsom salts to make the red solution.  

The students can use one graduated cylinder. They mass the cylinder first and then get the first liquid. They record the volume, measure the mass and then subtract the mass of the cylinder to find the mass of the liquid. Finally, they calculate the density.   I do NOT let my students replace the liquid when they are done, because invariably they put it in the wrong container. Instead I ask the students to simply pour it down the sink. They repeat this procedure with the other liquids and then make a histogram and a conclusion.  This histogram actually works better with class data.  If you have enough time to let students collect the class data and graph it I highly suggest it because it can lead to really interesting conversations.  

In this video, I discuss the class data histogram and the essential points I would make sure that students understand.

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This lab is a culminating lab about density as a characteristic property.  This is an important opportunity to make sure that students have understand the mind idea behind density as a characteristic property.  I do this through a whole class Q &A using random name sticks.  Read more about this practice in the reflection.

 

These are the questions that I asked students. 

1. Why couldn't you identify the substances by appearance?

2.  What does density measure?

3.  Does the amount of liquid change the density?  Why or why not?

4.  What did the density data show us that helped us identify which substances might be the same?

5.  Are you sure that these two liquids are the same?

6.  What tests would you further do to prove that these substances are the same?

7.  What sources of error were there in this lab?

Independent

10 minutes

The purpose of this section is to give students a chance to process their lab through a writing.  In my class, students have a Writing to Think Notebook that we use to store our thinking.  I do not grade the writings because it's important to me that students are writing for themselves rather than for me.  

When I first started this practice I was concerned that students might not really be writing about the topic without the accountability of grade, however, when I collected a sample I found that students were writing on topic.   I was pleased that students were taking this opportunity to expand their thinking.  

Today's prompt was, "Using the data from today's lab, how does Density allow us to identify substances."

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. "

Closing Question:  "How were we able to tell our two substances apart?  How could this process be used in real life?"

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.