Introduction to States of Matter

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Objective

SWBAT observe energy changes in a freezing-melting system.

Big Idea

To freeze milk into ice cream, students must have something to absorb thermal energy from the milk.

Background

This lesson comes as a needed break and re-engagement.  Students just worked through the two driest units of the year: Atomic Structure and then Periodic Trends and Bonding.  Throughout this time, the focus has been on the atom and invisible phenomena.  To transition into Intermolecular Forces, practice lab skills, and just have a break, we pause and make ice cream in class.

This lesson aligns with Science and Engineering Practice 3- Carrying out investigations.  There is also an alignment with Practice 6- Constructing Explanations present in the lab debriefing questions which previews the relationship between attractions in the molecules and freezing points. 

This lesson best fits Performance Expectation HS PS3-4: Plan and conduct an investigation to provide evidence that the transfer of thermal energy when two components of different temperature are combined within a closed system results in a more uniform energy distribution among the components in the system.  However, as I have an entire thermodynamics unit later in the year, I wouldn't relate this lab to that particular PE in my local district.

Materials for the lab (per student):

  • 1 gallon size baggie
  • 1 sandwich size baggie
  • 1 plastic spoon
  • 2 cups of ice
  • 6 tablespoons of salt
  • 1/2 cup of milk (I prefer whole milk as the presence of the fat helps give a creamier texture)
  • 1 tablespoon of sugar
  • 1/2 teaspoon of vanilla

Other materials (can be borrowed from your foods teacher):

  • Measuring cups
  • Measuring spoons
  • Paper Towels

 

Intro -- Test Debrief

10 minutes

When the period begins and students are seated, I begin by reviewing the class results on the Periodic Trends and Bonding Unit Exam.  While I am speaking about the overall class results, I distribute student grade reports so they can see their test score and their grade at this point in the class.  Student results varied from class to class, but some universal patterns were present in incorrectly identifying what the question asked in regards to periodic trends.  Students tended to misinterpret ordering questions such as "increasing" or "decreasing," or to misidentify which atomic property, effective nuclear charge or size of valence shell, controlled the property.  Students also did poorly on the chemical bonding objectives.  In hindsight, I would have spent an additional day or two on bonding regardless of the progress of my colleagues.

Once all students have their grade reports, I direct them to the test grade, and explain procedures if students are eligible for a re-test.  For an explanation of re-testing parameters, please see my reflection.

At this point, students can be a little down on their performance to date.  I assure them that the two hardest units of the semester are over, and that we will have more labs this unit to see the chemistry we are discussing.  I then explain that we will be kicking off our new unit with a lab, and pass out the lab sheet while I move to the lab area.

Ice Cream Lab

20 minutes

Once students have the Ice Cream Lab document, I begin to go over with them what they will do.  I ask students to decide who will be partner A and who will be partner B.

Once students have decided this, I orient them to the lab setup.  I decided to set up this lab in parallel stations, with one partner getting the ice and salt in the gallon bags and the other partner getting the milk, sugar, and vanilla in the sandwich bags.  I made this decision for two reasons: first, my students have done better when given specific tasks to the betterment of their lab group.  Secondly, a lack of time to prep out separate coolers, milk, vanilla and sugar makes this a more efficient setup, and one that is easier to change rooms with three times today.

I show the students how the ice cream lab stations directions are cut out at each station for them to follow.  I encourage them to help each other measure and pour, and to clean up as they go.  I explain that it is important that they follow directions exactly, because if they screw up, I did not buy enough materials for them to try again.

I warn students about the slight frostbite safety concern of improperly handling the ice bags with the salt.  I show them how to hold by the zipper when shaking the bags.  I then encourage them to come back to the lab.

While students are measuring and mixing, I circulate the room and make sure things are going well, prompting cleanups where appropriate.  I encourage students to record their initial observations before adding the small baggie to the larger baggie.

Some students will make procedural errors, and pay the price of not having any ice cream.  Despite the salt being across the room from the milk-- students will add salt to the milk.  Others will dump the small bag's contents into the large bag.  This becomes a comedy of errors at times, but these failures are important as it helps me to identify the students I will need to help or pair with a different partner for our upcoming wet labs.

Closing- Lab Questions

12 minutes

As students' ice cream freezes and solidifies, I direct the students to the spoons at the front of the room to eat their ice cream.  I remind them of procedures to dispose of their salted ice water and make sure they write their three observations/descriptions of their ice cream prior to tasting it.

Once all students are seated and eating their ice cream, I direct them and their partner to answer question 3 on the lab sheet: "Whenever something gets colder, it is losing heat.  Where did the heat from the milk go?"  Student responses here vary.  Approximately 75% stated that the heat goes into the ice.  The other 25% stated it went into their hands, the air, the tabletop, or even the sugar and vanilla.

Once students complete question 3 and move onto question 4, they begin to self-regulate.  Question 4 asks: "What visual evidence did you have from the lab that supports your argument in number 3?"  The desired answer to this question is that the ice melted.  Students who inferred that the heat left the milk for the ice can use the melting of the ice as the example of it gaining heat.  Students who did not get question three correct stop and think about what they saw.  This year, about half of the students who originally got question 3 incorrect went back and fixed it when pressed for visual evidence.

As students finished, I asked them to turn in their papers, and to clean up after themselves.  I explain that this unit will be focused on how we can tell the differences between attractions based on observable characteristics such as the melting point of a substance.  I inform them that tomorrow we will be working on the computers to see what the particles in their milk were doing en route to becoming ice cream.