Sugar, Solutes...Satisfaction!

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Students will be able to explain how properties of reactants and products change after a reaction and explain how the motion of particles, temperature, and state of matter are affected by adding and removing thermal energy.

Big Idea

Students explore how solutes affect the freezing point of substances and how phases changes are related to the shift in thermal energy as they make ice cream!

Introduction and Connection to the NGSS and Common Core

In this lesson, explore endothermic and exothermic reactions, phase changes, and how solutes affect freezing/boiling points by making ice cream.  In preparing for this lab, students obtain information from text as they work on using some text strategies called "Talking to the Text" and the "Ladder of Discourse".  

This lesson is designed to address the following NGSS and Common Core standards:

MS-PS1-2  Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.

MS-PS1-4  Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.  

CCSS.ELA-LITERACY.RST.6-8.4  Determine the meaning of symbols, key terms, and other domain-specific words and phrases as they are used in a specific scientific or technical context relevant to grades 6-8 texts and topics.

Compare and contrast the information gained from experiments, simulations, video, or multimedia sources with that gained from reading a text on the same topic.

CCSS.ELA-LITERACY.RST.6-8.10  By the end of grade 8, read and comprehend science/technical texts in the grades 6-8 text complexity band independently and proficiently.

Introduce a topic clearly, previewing what is to follow; organize ideas, concepts, and information into broader categories as appropriate to achieving purpose; include formatting (e.g., headings), graphics (e.g., charts, tables), and multimedia when useful to aiding comprehension.

Science and Engineering Practices:

In reading and completing the "Ladder of Discourse" on their Skill 4 Notes Document, students use strategies to obtain scientific information and evidence from text (SP7).  In addition, during class discussion, students back up their explanations in the lab document with evidence from their qualitative observations (SP8).

Crosscutting Concepts:

When students discuss what they saw in the lab in comparison to what they read in the text, students can begin to see patterns in the way that adding a solute can impact the boiling and freezing points of substances. Thus, students can begin to see that macroscopic patterns are related to the nature of microscopic and atomic-level structure (Patterns).  

In creating patterns of energy transfer that occur in both phase changes and endothermic and exothermic reactions, students track how energy moves during various phase changes (Energy and Matter).

Connecting to the Essential Question: What are you supposed to learn today?

5 minutes

Ask students, "What are you going to learn today?".  Students should respond by saying that they will be answering the Essential Question, "How do particles combine into new substances? And, what evidence can show how the physical and chemical properties of the substances change?"  This EQ is posted on my board and on the student's Chemistry Unit Plan.  

Explain that today make ice cream!  Emphasize that while making ice cream is fun, they are going to focus on making connections to Skill 4 on their Chemistry Unit Plan, which states, "I can identify the physical and chemical properties of the reactants and products in a reaction."  Specifically, students are going to pay close attention to physical property changes caused by adding solutes and how energy transfers as matter changes phase.

**It is important to note that my students have already been introduced to physical and chemical properties as well as endothermic and exothermic reactions before this lab.  To see their introduction to these concepts, check out the following lessons:  Crack that Marble Labs (chemical and physical properties) and Talk About Burning Your Money! (phase changes, energy transfer, endo- and exothermic reactions)

Mini Lesson: Solutes Affecting Boiling/Freezing Points and the Ladder of Discourse

25 minutes

Ask students to turn to their Skill 4 notes page (This notes page along with a set of True/False Statements and Answer Key are included in the resources.).  In my class, at this point in the year, we have been working hard on obtaining information and meaning from text.  I tell my students that today's lesson is an opportunity for them to demonstrate independence with this skill.  In a previous lesson (Crack That Marble Labs), students already performed this task for a portion of their Skill 4 notes page.  In this lesson, I ask them to read silently from the bottom of the second page beginning with "Solutes and Dissolving" through the end of the notes document.  When reading students must "talk to the text" and climb the "Ladder of Discourse".  The levels of the ladder are "Tweets" (text to self connections), "Huh?'s"(questions or concepts they do not understand), "Found It" (finding answers to questions through context clues or finding science answers), and "Discourse" (combining ideas to think beyond the text).  

For more background on "talking to the text" and the "Ladder of Discourse" check out the following lessons.  These lessons include videos of me demonstrating these strategies and student work.

Talking to the Text:  Mystery Box Modeling:  Energy TransformationsHow does heat move?,Thermal Protection Systems

Ladder of Discourse:  Refracting TelescopesEngineering Earthquake Structures

In the student work below, notice that the student "talked to the text" to show their thought process as they read.  There are a couple of things to note in this student's thinking.  First, the student first questioned what solubility was and labeled it with a question mark.  This would be a "Huh?"; but, then, she finds the answer in the parenthesis following.  This could be a "Found it!".  "Found it!" is using context clues to attach science meaning or making science connections.  I love how this student took time to make meaning of the visual aids.  She notes that with the horizontal bar graph that with no salt, "shorter to boil, longer to cook".  The graph doesn't specifically say this in words.  She took the time to analyze what the graph was saying and make meaning of it. This is a skill of an effective reader.  She does the same thing in the cup diagrams by noting that "more arrows = more evaporation".  She looked at the picture and attached a quantitative approach to the arrows in making a connection.  Lastly, in my class we have really been working on "discourse".  Part of "discourse" is generating questions based on the Cross Cutting Concepts of the NGSS.  In this section, the student asks, "Does the structure or shape of the pasta affect how the pasta cooks?" (CCC - Structure and Function) and "Could we design an experiment to test if there is a proportional relationship between the amount of solute and the boiling point?" (CCC - Proportion, Scale, and Quantity or Cause and Effect).  As students increase their awareness of the Cross Cutting Concepts, they begin to make thoughtful connections and questions in what they read.


Making Ice Cream!

45 minutes

Set up stations with the Ice Cream Directions/Station Signs in order following the recipe along with the lab materials. 

Ice Cream Directions:

  • Place one quart sized freezer bag inside of another quart sized freezer bag.  This will prevent any unwanted salt in the mixture.
  • In the inside quart sized bag, measure .5 cup milk, .5 teaspoon of vanilla extract, .25 cup evaporated milk, and 3-6 teaspoons of sugar.  Close the bag, making sure it is well sealed.
  • Take a gallon sized freezer bag and fill it about half way with ice.
  • Add about a cup of either rock salt or any ice-melter you find at a grocery store or gas station. The salt that is sold for home water softening is relatively inexpensive, and totally pure, non-toxic salt.
  • Put the quart sized freezer bag inside of the gallon sized bag.  Seal the gallon sized bag.  BOTH BAGS SHOULD REMAIN SEALED!
  • Have students put on their winter gloves and knead the ice around the quart sized bag.  Students can work in partners and take turns at this process.
  • Students can knead from 10 – 30 minutes depending on how well they work at it and the consistency that they prefer.


Teacher Tips:

  • Remind students before this experiment that it would be beneficial for them to bring in winter gloves.  The process can make hands cold!
  • Have plenty of paper towels around!  Condensation forms on the outsides of the bags so they do get wet!
  • When sealing all of the bags, instruct students to get as much air out as possible.  
  • Don't buy the cheap freezer bags!  They tend to break during the kneading process and you will end up with salty ice cream.
  • Check ahead of time for food allergies!
  • When kneading, it is important to work the inside mixture as much as possible.  If students just touch the outside bag and squeeze around the ice, the ice cream will not freeze.  


Ice Cream and Scientific Discourse: Student Discussions

15 minutes

During this unit, my students have been working on group discussion techniques.  (For insight into the initial teaching of these strategies, check out this S'Mores Lesson.)  I have included in the resource section the sentence starters that my students use when engaging in group discussion.  In addition, it is important to note that students are also working on strengthening their claims by including both evidence in the form of observations during the lab and from text.

Student Discussion 1:  Why did we add salt to the ice in the outside bag to make the ice cream?

Notice that these students are both referencing text and observations during the lab.  One thing I love about this discussion is that it takes all three members of the group to share their ideas to get the full picture.  Without each of the student's insights, full meaning would not be formed.  By using sentence starters and established discussion practices, more members of each group play valuable roles in our science discourse.  By the end, students state that adding salt decreases the freezing point, which causes the ice to melt.  The students note that melting is endothermic so the melting ice takes energy in from the inside bag causing it to freeze.


Student Discussion 2:  Why did we add salt to the ice in the outside bag to make the ice cream?

In other groups, there may be one student that really gets it and takes the lead on the explanation.  However, in my class, we have been working on how to be an active member of the group even if you are not the one that is the leader or even if you are unsure of your understanding.  In this discussion, one student speaks to a lot of the discussion points, but then, another student finds a way to participate by saying, "Do you have any text evidence to support that?".  This prompts the students to turn to the text to back up the claims that were made.  Also, a different student recognizes she still is not clear on the concepts.  So, she asks, "What page was the endothermic/exothermic reactions on?"  and asks a follow up question that she says she needs further explanation on.  Without focusing on discussion strategies, textual evidence and sentence starters, students would have just let the first student explain everything, taken it for granted, nodded their heads and moved on.


Student Discussion 3:  Why would a chef add salt to a pot of water s/he was going to cook pasta in?

This group does a great job of including everyone and working collaboratively to eventually get to the answer.  Students mention that adding a solute increases the boiling point.  However, they almost stop there.  I find that sometimes I need to ask follow up questions that push the "how?" and "why?" of discussion topics.  Once I ask these follow up questions, the students get to the explanation.  While it is important to not interrupt student discussions, if you notice groups leaving out the "how" and "why" it is important to step in (briefly).


5 minutes

After completing this lesson, students in my class have had many opportunities to learn about physical and chemical properties and "Skill 4" from the Chemistry Unit Plan.  Thus, to close this lesson I ask students to add to their working towards mastery list and re-assess themselves on their level of mastery on their Chemistry Unit Plan.  It is important that students take time to reflect on their individual levels of mastery and needs so that they can seek the help they need.

Working Towards Mastery List:

After labs/projects/formative assessments, students plot their scores and then in the open area on the bottom, they add to their "Working Towards Mastery List".  The "Working Towards Mastery List" is a place students write the concepts or topics that they have not yet mastered or need to study more in depth.  It is a way for each individual student to organize their specific feedback.  Come summative assessment time, students have a clear record of not only their growth, but of the specific concepts that they missed along the way that should be the focus of their studies.

In my class, I score everything as a 1, 2, 3, or 4 (4 being mastery); thus, the y axis is labeled with these scores.  However, you could place your own grading scale that represents your classroom.  In addition, along the x axis, it is labeled "a, b, c, d, etc".  In my gradebook, in order to organize all of my formative assessments in chronological order by skill, I use this system.  You might include assignment names or any other numbering system that would indicate time progression on the x axis.

The student below makes notes about which phases changes are endothermic and would take in energy and which phase changes are exothermic and would release energy.


Student Self-Assessment:

Notice in the picture below that students self-assess themselves on each skill of the unit and re-assess themselves along the course of the unit.

A Look at Student Work

The student below includes well formatted diagrams that are also accurate scientifically.  She includes a title, labels, picture, and caption that all serve the purpose of the question.  

Question 1:  Draw a diagram that shows how adding salt to the ice allowed the ice cream mixture to freeze.

Question 2: Explain why a chef would add salt to a pot of boiling water if he/she wanted to cook pasta faster.

For a closer look into the successes of this student's work and a discussion on common mistakes students will make, watch the following video: