Mixtures: Characteristic Properties - Phase Change (day 2)

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Students will be able to determine the boiling point of water and graph their observation.

Big Idea

When a substance boils, it's temperature plateaus as it changes form.

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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.

2) Misconception alert- I try to attack some of the misconceptions of boiling in the hook using a slow motion video.

Ready. Set. Engage!

5 minutes

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

Opening question:  What happens when water boils?  What are the bubbles made of?

This is a really interesting conversation to have with kids.  All kids can tell you that when you are boiling water it is going from a liquid to a steam.  However, when you probe further and ask them what the bubbles are made of the students will say AIR!  This is very common.  I actually remember the moment in school when occurred to me that the bubbles are made of steam. Getting students to understand that the bubbles are made of steam is a great step to having them truly understand phase change.

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


5 minutes

This is a great video to show on this day because it is what boiling water looks like in slow motion. I find that two biggest misconceptions students have about boiling water are

1) The bubbles are made of air.

2) The water temperature continues to go up.

This video allows you to address the first misconception. I like to prompt the kids to watch carefully to see if they can tell what is happening in the water. I stop the video several times to point out that you can see the water moving around, which points back to the previous days learning.  I also keep asking the kids what substances are in the beaker. As the bubbles get bigger and bigger, we stop and talk about what they are made of and what is happening to the water.  

Finally, at the end of the video, I preview the lab by asking the students what they think is happening to the temperature of the water while the water is boiling. The students think-pair-share and throw out some ideas for the class.

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

We've just watched a video on water boiling and talked about what might be happening to the temperature of the water while its boiling.  Now we are going to do a lab testing this.  What sort of equipment will we need? What would our purpose be?  Let's go ahead and get the lab set up for this experiment

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

When students are finished, I ask them to think about what equipment they might need. This is not an inquiry lab, students are not going to be designing it themselves, but this question does get their creativity working and helps them feel more involved in the lab process.  Students think-pair-share and the class shares out. I have the lab supplies up at the front of the room in a box. When the students give an answer I take the supply out of the box as a visible cue. This is also a great way to get students to pay attention to what they will need to collect for the lab. 

Obtain a Plan

5 minutes

Since this is the first time we have heated liquids in class, the students will need some help with the lab set up.  I like to set this up using clamps and pegboards, but if you don't have these tools in the lab there are many ways to set this up.



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 min will be plenty of time.  I like to have students take down data every 30 sec.  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 attached lab.

Carry Out Plan

15 minutes

Once all students have their labs set up and goggles on, I light the alcohol burners. Some teachers prefer to use methane burners, but I like the control of having students staying in their assigned seats. While the test tube is heating, students are taking time and temperature data every 30 sec.  I like to have 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 their are fires in the room. Any fooling around and the student is immediately asked to leave with a referral to the office. This is a great lab to norm the students on fire lab rules because there are few things that can go wrong with this simple set up.  

When students have been at the same boiling point for 5 or more times I have them blow out the alcohol burner. Many students find it mind blowing that the temperature stops going up and plateaus. This is generally a totally unforeseen event. 

Evaluate / Revise the Answer

12 minutes

Now that students have their data it is time to graph. I am using a guided graphing strategy in this lesson for two reasons:

1) This is the most data my students have collected in a lab and the first data that makes a line graph.

2) In our school, students are expected to graph independently in 8th grade, not 7th grade.  

If your students have more experience or your school has different standards you might want to have the students work more independently.

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 for 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. There is a screen cast at the end of the lesson that shows how I would approach this task.

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 Boiling Water.  I explain to the students that this title helps the reader know what you measured and what substance you used. I label the Y axis Temperature 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! Then I scale the graph simply with no breaks talking to the students about the importance of using a consistent scale. 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.  

Once the plotting is complete, I step back into teaching mode and draw the line. The plateau becomes immediately obvious to students. I circle the plateau and ask a series of questions to the class.

1.  What is happening to the temperature here?

2.  What was the water 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 water?

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.

1) In this lab we found...

2) I know this because...

3) This makes sense because...


3 minutes

Closing Statement:  " Today we boiled water and discovered that the temperature of water plateaus at the boiling point.  Tomorrow we will look at a different substance."

Closing Question: "What happened to the temperature at the boiling point?  Why do you think this happened?"

Today, I gave a student this probe to see whether they had been able to take the data from lab and make a real life connection.  Most of my students showed great understanding but this response shows that the student did not connect the lab with the situation.  This can happen when the student doesn't really understand the mathematical thinking in the lab and is continuing to hold onto a misconception.  This student will need more explicit instruction through small group interventions.  

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.