Ice Cream Lab

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Objective

Student will be able to explain how solutes effect the freezing point of a substance through performing an ice cream lab.

Big Idea

When solutes are added to a solvent they decrease the freezing point of the solvent.

Introduction

In this lesson students learn about the general idea of freezing point depression while reviewing solutions.

  • This lesson does not align with any specific performance expectations with the Next Generation Science Standards; however, it is imperative that students understand solutions before learning other concepts later in the year.
  • This lesson aligns with the Next Generation Science and Engineering Practice 3: Planning and Carrying Out Investigations.  It does so because students are performing a lab activity.
  • This lesson also aligns with the Next Generation Science and Engineering Practice 5: Using Mathematics and Computational Thinking.  It does so because students are using data from their lab to calculate the concentration of their solutions.
  • For this lesson there are several resources needed including:
    • 2 ziploc bags per student (freezer kind works best)
    • whole milk (1/2 cup per student)
    • salt (3 tablespoons per student)
    • sugar (1 tablespoon per student)
    • vanilla (1/2 teaspoon per student)
    • ice (2 cups per student)
    • oven mitts or towels
    • thermometers

Engage

20 minutes

To begin the lesson I have student review what they learned in the previous lesson related to quantitative solution calculations.  I do this by having students do whiteboard practice.

This is the PowerPoint that I use with students. 

For details on how whiteboard practice works see my reflection on Partner Whiteboards in my Metallic and Covalent Bonds lesson.

Some of the common mistakes that students make while doing the whiteboards include: 

  • using the correct formula to solve for what they want
  • placing units in their answers
  • using solute, solvent, and solution appropriately.  For example in the second example, " What is the percentage methanol by mass of a solution that contains 30g of methanol in 70g of water?" students must use the methanol as the solute and then add the methanol and the water for the solution.

Explain

10 minutes

In this section of the lesson I explain to students how they will be doing the lab.  

I first give students several minutes to read the lab to themselves.

I then go over the procedures with students.  I make sure to stress that when they are making the ice cream to seal the first bag really well.  To help students see what they will be doing I show them an i-movie that my students made several years ago which goes over the procedures.

I let students know that we will be doing the lab at the back stations where it will be assembly-line fashioned.

Finally I tell students that once they are done with the prelab questions than they can come to the back to start the lab.

Elaborate

45 minutes

This is the part of the lesson where students are doing the lab.

Once they have their prelab questions complete they come to the back of the classroom where they make their ice cream solution in one bag and then put it into the salt/ice solution in a second bag.  

I have this set up in the back of my classroom so that I can be in charge of pouring the milk/vanilla which helps to prevent spilling.  This video explains how I do this in my classroom.  At the first station I have students do everything for the first bag and then seal the bag before going to the next table.  This is a picture of the setup at the first station. 

At the second station, students put the salt/ice in the second bag.  I have a temperature probe in the ice so that they can see the initial temperature of the ice.  Here is a picture of the setup for the second station.

When students are done with making their two bags I have them go outside of my classroom to shake the bags together.  On there way out of the door I have a stack of towels so that they can grab a towel to put around the plastic bags so that there hands don't freeze.  I have them do this outside because the bags can leak salt-water which is very messy for my floors.  After about 5 minutes when students' ice cream starts to solidify I have them come back into the classroom where I have a second temperature probe set up for them to find the final temperature of their ice-salt solution and then dump into a bucket.  This is a picture of the setup at the final station.

Students then get a spoon and some chocolate chips and are able to enjoy their ice cream.  As students are eating I encourage them to complete the questions on their labs.  Here is a picture of what the final ice cream looks like.

Evaluate

When students are done with their labs I have them turn into my basket.  I then grade their labs using the rubric.

This first graded example of a student lab shows how she earned most of her points, but did not put units on her data.  Also if you notice she calculated the percent composition of both the salt and the water in the salt-water solution.  Finally, she did a good job with her conclusion noting that she must have added too much salt because the final temperature was lower than what was expected.  

In this second graded example the student also does not put units on his data.  This student also does a good job with his conclusion and points out that the heat of his hands could have warmed the bag/ice to give him a higher temperature than expected. 

In this third graded example the student misses points with his analysis for percent error.  This was a common mistake where students used the percent yield equation instead of percent error.