Mixtures Labs Day 2: Separating Mixtures and Rates of Dissolving

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Students will be able to write a detailed procedure and model various methods that mixtures can be separated by physical means.

Big Idea

Students develop their own procedures to separate a mixture and then carry out their plan using lab tools of their choice!

Introduction and Connection to the NGSS and Common Core

On Day 1 of this lesson we focused on mixtures. Students developed and carried out a procedure for separating a mixture of sand, salt, iron filings, and marshmallows by physical means.  On this day, Day 2 (sometimes carrying into a third day), students develop ideas about factors that can increase the rate of dissolving.  Finally, students work on "sounding like a scientist" as they write evidence based scientific paragraphs.

This lesson is designed to connect to 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.6-8.1   Cite specific textual evidence to support analysis of science and technical texts.

CCSS.ELA-LITERACY.WHST.6-8.2   Write informative/explanatory texts, including the narration of historical events, scientific procedures/ experiments, or technical processes.

CCSS.ELA-LITERACY.WHST.6-8.2.D   Use precise language and domain-specific vocabulary to inform about or explain the topic.

CCSS.ELA-LITERACY.WHST.6-8.2.E   Establish and maintain a formal style and objective tone.


Science and Engineering Practices:

When students write about the relationship between factors that affect the rate of dissolving, students construct, use and present written argument supported by empirical evidence and scientific reasoning to support their claim (SP7).  This also ties to the Science Practice Constructing Explanations as students include qualitative and quantitative relationships between variables that predict(s) how factors affect the rate of dissolving (SP6).

Crosscutting Concepts:

Students experiment with temperature, surface area and shaking/stirring and collect data to determine their effect on the rate of dissolving. (CCC - Cause and Effect:Events have causes, sometimes simple, sometimes multifaceted. Deciphering causal relationships, and the mechanisms by which they are mediated, is a major activity of science and engineering.)  

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

5 minutes

Ask students, "What are you going to learn today?".  Students should respond with finding the answer to the Essential Question, "How do particles combine into new substances?  What evidence can show how the physical and chemical properties of the substances change?".  I write this EQ on the board and provide it for the students on their Chemistry Unit Plan.  (The unit plan as well as the notes sheet students have relating to identifying mixtures are included in the resource section.)

I then let the students know that specifically we are going to look at mixtures over the next few days.  I explain that in the past lesson, Marshmallow Molecules, we looked at how mixtures are not combined chemically and can be separated by physical means.  In addition, we read about how when solutes are dissolved in a solvent, a mixture is created.  Let students know that their focus today is to determine a procedure for separating a mixture by physical means and investigating factors that could affect the rate of dissolving of a solute in a solvent.

Day 2 Mini Lesson: Pulling Information From Text

10 minutes

In my class, students have been working on interacting with text in meaningful way.  We have been implementing a strategy called the "Ladder of Discourse" in which students write/mark up the text to document their thought process as they read.  Students work their way up the "ladder" as they work towards 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).  

During this second day of the lesson, I explain that there will be two portions to this lab about mixtures, one portion focusing on the separation of heterogeneous mixtures and one portion on homogeneous mixtures.  First, they will separate their heterogeneous mixtures using the procedures they wrote in the previous class period.  Then, they will be exploring factors that affect the rates of dissolving in solutions.  Before beginning, explain that students need to obtain the information necessary to understand these factors. 

Provide students with the "Solutes and Dissolving" reading and allow them time to "talk to the text" as they try to connect with each rung of the ladder on the "Ladder of Discourse".

In the student example above, the "Tweet" could be where the student wrote "Yum!" next to the word "Kool-Aid" in the first line.  The "Huh?" could be where she circled "Solutes" and put a question mark or where she asked "Why is water in a faucet a solution?".  The "Found It!" could be where she found the definition to solute or where she makes the connection to what we have been learning in connecting that a "solution" is homogeneous.  The "Discourse" could be considered where the student asks, "How does temperature affect this?".  Temperature is not mentioned in the text, but when the student read that it dealt with collisions between molecules, she recognized that temperature can affect this and therefore could affect the dissolving rate.  

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 Transformations, How does heat move?, Thermal Protection Systems

Ladder of Discourse:  Refracting Telescopes, Engineering Earthquake Structures

Day 2: Rates Of Dissolving Stations

25 minutes

Once students have completed the separation of their mixture, they follow procedures to complete Rates of Dissolving Lab stations dealing with factors that affect the rate of dissolving.  One thing to note about the student document is that the data tables provided already include quantitative numbers.  The procedures only ask for students to make qualitative observations about which dissolves fastest. Because of this, I often only provide the students with the actual student document after they have performed these three stations.  The purpose of this student write up is to have them practice citing evidence and data.  Precisely timing when the solutes completely dissolve can be hard to accomplish by a 7th grader in a class period.  Thus, I have them simply identify which trial group dissolved the "fastest".  Then, I have them use the exact data from the tables already provided in the student document so they can practice using quantitative data to prove a point.  

Below are the procedures as well as pictures of the set up for each station.  

Does temperature affect the rate of dissolving?

  1. Fill a beaker with 200 ml of cold water, and then another with 200 ml of hot water.
  2. At the same time, drop a sugar cube in each.
  3. Observe the beakers for two minutes.  Record which temperature water is dissolving its sugar cube the fastest.

Does surface area affect the rate of dissolving?

  1. Crush a sugar cube into fine granules using a mortar and pestle.
  2. Fill two beakers with 200 ml of water.
  3. Simultaneously (at the same time) drop the crushed sugar cube and an intact sugar cube into the separate beakers.
  4. Observe for two minutes.  Record which sugar cube was dissolving the fastest.

Does Shaking affect the rate of dissolving?

  1. Place several teaspoons of sugar in each of two jars nearly full of water.
  2. Shake one jar vigorously for one minute.  Allow the other to remain undisturbed.
  3. Compare the amount of dissolved sugar in each jar.

After completing these lab stations, students write well developed paragraphs backed by evidence from the text and the lab.  In my class, we use a format called "ABCDE" paragraphs.  For a more detailed look at student work and this format, see the "A Look at Student Work" section.

Day 3 Mini Lesson: Sound Like a Scientist, Sorting Student Work

15 minutes

Provide students with the following student samples (also included in the resources) cut into slips of paper.  Ask students to sort these in order of "Sounds Like a 7th Grader" to "Sounds Like a Scientist".  Once they have ordered the student samples, students must share the reasoning for the order that they choose.

I explain that this was a pre-writing prediction that I had my students complete a couple years ago before they completed the temperature and dissolving lab that my current students just completed yesterday.  

I explain that to the students that the question the students were answering was, "How would you predict that temperature affects the rate of dissolving?".  I also emphasize that this was a pre-write and merely a prediction.  Thus, commenting on the structure of a paragraph is not what we are looking at here.  We are looking merely at these students ability to "Sound Like a Scientist" in what they wrote.

Below are the typical explanations and comments students typically make about each sample:

Student Sample 1

Feedback:  Students share that this "sounds like a 7th grader."  Anyone who was not this student’s science teacher would have a hard time understanding this prediction.  Here is what this student meant – Adding heat (it) creates more movement in the (molecules).  The student uses “it” for adding energy and does not mention what has more movement.  The student does not give any kind of relationship statement to connect the vocab in the questions.  There is no mention of temperature or of dissolving.  Thus, the student needs to use stronger vocabulary and make sure that they have actually answered the question asked.

 Student Sample 2:

Student Sample 3:

Feedback for Samples 2 and 3:  Students state that these samples "sound like a 7th grader" and often provide similar feedback for both samples.  These students are making a relationship between heat and dissolving.  However, the students are not using the vocabulary from the question and do not back up the relationship with any science vocabulary.  “Hot” could be “increased temperature” and “faster” could be “an increased rate of dissolving”.  This students do not mention their reasoning at all and therefore have left out any mention of molecules.  In Sample #3, the word “stuff” should be “molecules” or “solute”.

Student Sample 4:

Feedback:  Students explain that this "sounds like a scientist".  They note the effective use of vocabulary for a pre-write.  The student uses vocabulary like temperature, molecules, dissolve, etc.  In addition, the writer takes these terms and relates them to a real world example to make their meaning clear.  Students still may state that there are areas that this student could improve in.  For example, this student makes a strong attempt at writing in third person by saying “someone” in the second section; however, the student does use “they” after that.  Students may also note that the word “huge effect” might sound more like a 7th grader than a scientist.

Day 3: Writing Explanations

45 minutes

Provide students time to complete their Rates of Dissolving Lab Sheet.  Encourage students to "sound like a scientist" as they write their conclusions.  Emphasize that they should use strong vocabulary ad write in the third person in order to maintain a formal tone.

For these responses, I ask students to write in an ABCDE format.  I go through a quick review of the requirements for this type of writing:

A - Assertion - Write a claim.  In this case, a relationship statement.

B - Background - Provide the reader with information they will need to understand your explanation.  Often, this is letting the reader know what was done in the lab.

C - Citation - Pull evidence from a text to support your claim.  Cite this text by title.  Pull evidence from data tables or graphs.  Cite these by title and be sure to compare two data points with units when including data.

D - Discussion - Discuss how the lab supports the claim by making connections to the information that was cited in the "C" section.

E - End - Write a conclusion sentence that ties back to the claim.

A tip for you -- the ABCDE paragraph is one of the most useful tools I have found to help students organize their scientific writing.  A Quick Guide to the ABCDE Paragraph is included in the resources section.  In addition, the "A Look at Student Work" section includes a video of me looking more closely at the ABCDE paragraph.

Here is a look at student work samples and the ABCDE paragraph format.  The student work looked at in this video is included in the resource section. 


Closure: Separating Mixtures Writing "Put Your Finger On It"

5 minutes

Once students have completed their ABCDE paragraphs, have students conclude the lesson by peer editing their responses.  When peer editing, I use a process I call Put Your Finger On It.  I provide the students with a checklist of items they should be able find in their peer's writing.  For each item, they have to physically touch it with their finger.  If they cannot physically touch it, they provide their peer with feedback about what they are missing so that student can make corrections.  

It is important to emphasize and follow through that the students are actually touching the paper with their fingers.  In my experience, students just check things off the list.  As students peer edit, I ask them to show me where they actually found items on the list.   Physically touching the paper to find the important items increases the student feedback.  

Allow students to have time to make the revisions necessary based on their peer's feedback.