Mixtures: Characteristic Properties - Phase Change (day 3)

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Objective

Students will be able to find and graph the boiling point of alcohol.

Big Idea

Boiling point is a characteristic property. Different substances have different boiling points.

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Purpose of Lesson:

 The purpose of this lesson is to let students explore boiling point and graphing in a hands on lab situation.  It also gives students the practice they need to separate water and alcohol in the next unit.

Major Strategies to Watch for:

1) Guided Graphing -  The students and I graph together.

Ready. Set. Engage!

5 minutes

Learning Goal:  Discover what happens to the temperature at the boiling point of alcohol

Opening question:  How do you think the boiling point of alcohol will be different from that of 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

Hook

5 minutes

The lab today is a little more dangerous than the lab yesterday because the alcohol fumes are flammable. I like to show this video  to show the kids that we aren't going to fool around with the fire.


It is also a great chance to talk about another characteristic property, flammability!  After the video I make sure that all students understand the fire rules.

1.  Stay in your seat when there is a lit burner at your desk.

2.  Wear goggles at all times.

3.  Make sure your desk is clear of papers.

4.  If there is a fire, be calm and call for the teacher.  Do not attempt to put it out yourself.

Finally I spray some alcohol on the table and light it on fire and show the kids that as flammable as alcohol is, when you are using a small amount of alcohol it will burn off very quickly.  

Understand and Describe the Problem

10 minutes

The purpose of this section is for students to understand the problem and get their lab reports ready.

Instructions to the students:

Yesterday we took data on the temperature of water as it boiled.  Today we are going to look alcohol.  Our lab today will be very similar to yesterday's. 

 

Students should now be ready to write the title, purpose, and hypothesis for this lab. For students who aren't ready for this independence, I use the Boiling Point of Alcohol lab report.

This lab is not as simple as the water lab. Since the fumes of alcohol are flammable, I often have little alcohol fires at the set-ups. To help with this problem, I put boiling chips inside the alcohol to even out the boiling a little, I make sure that we only use 10 ml of alcohol, and I talk to the students about safety, letting them know that if there is an alcohol fire at the desk they should just scoot their chairs back and raise their hands. There is a screencast below of what one of these alcohol fires looks like and how to handle it.

Obtain a Plan

5 minutes

Students should be fairly adept at the lab set-up since they have done a similar lab the day before.

The key to this lab is that students need to be able to take down time and temperature data over a period of time. If you are heating water in a small test tube, 10 minutes will be plenty of time. I like to have students take down data every 30 seconds. This gives the students plenty to do while the test tube is heating but doesn't overwhelm them too much.  The data table can be seen in the lab write-up.

Carry Out Plan

15 minutes

Once all students have their labs set up and goggles on, I light the alcohol burners. In my classroom the set up for the methane burners requires the students to be standing at the counters. By choosing to use alcohol burners instead I keep the students in their seats and can monitor labs easier. If you are using methane burners at lab counters be sure to keep the counters clear of materials and make sure that the counter areas isn't too crowded. In the past I have used tape to set up lab stations for kids so they have a boundary to work within.

While the test tube is heating, students are taking time and temperature data every 30 seconds.

Students mark the time that they first see the "real" boiling bubbles. I walk around the room, checking lab behavior and talking to students about what they are seeing. I am very strict when we are using fire. Any fooling around and the student is immediately asked to leave with a referral to the office.  

When students have been at the same boiling point for 5 or more times I have them blow out the alcohol burner. This time the students have been expecting the temperature to plateau and are more interested in the fact that it happens so much earlier than in the water lab. 

Evaluate / Revise the Answer

12 minutes

Now that students have their data it is time to graph.  I am using a guided graphing strategy that reminds the students of the lessons we learned in the previous lab.

I tell the students that we are going to graph this data together and that they should follow all the steps that I am doing. I explain my thinking to the students at each step and sometimes stop and ask questions, but I am NOT expecting that the students would be able to do this on their own the next time.  The point of this lesson is still to figure out what happens to the water temperature...not to become independent graphers. I explain how I would approach this task for the second day in a video at the end of this lesson.

I start by drawing the X and Y axis and and pulling in some knowledge about the purpose of graphing.  Then I title the graph, Time and Temperature of Alcohol.  I explain that this title helps the reader know what is measured and what substance is used.  I label the Y axisTemperature and the X axis Time but do not explain the idea of independent and dependent variables. That could be a lesson all on its own!  

I keep the scale of the graph simple, using a categorical scale with no increments between the numbers. Finally, I plot 3 or 4 points.  I carefully watch the students during the plotting and gauge their level of independence. When it seems to me that more than half of the class understands how to plot data points, I release to the students and then walk around and help. The students often struggle with the first point they plot on their own, but if the majority of the students have it than the rest of the kids generally catch up quickly.  

Here is a video of me explaining two finished graphs.

Once the plotting is complete, I step back into teaching mode and connect the points. The plateau becomes immediately obvious to students.  I circle the plateau and ask:

  1. What is happening to the temperature here?
  2. What was the alcohol doing at this point?
  3. Did you stop giving it heat?
  4. Did everybody's graphs plateau at the same point?
  5. Why do you think the temp stopped rising?  What is the energy doing to the alcohol?
  6. What would we call this temp? (boiling point)

 

Once the students seem to have an understanding of the data, it is time for them to write their conclusions using our sentence starters.

  • In this lab we found...
  • I know this because...
  • This makes sense because...

Closure

3 minutes

Closing Statement: "Today we did a lab that showed that boiling point is a characteristic property, that different substances have different boiling points.  Next we will be looking at how we can use boiling point to separate a mixture of Alcohol and Water."

Closing Question:  "How did our lab today show us that boiling point is a characteristic property?"

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.