On Day 1 this multi-day lesson focuses on mixtures. Students develop and carry out a procedure for separating a mixture of sand, salt, iron filings, and marshmallows by physical means. On Day 2, 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).
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.)
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.
In a previous lesson, students identified criteria required for an effective procedure by sorting student writing samples from procedures written in a previous lab. Check out this Mini Lesson in the Density Test Tube Challenge Lab to get more information about where my students developed the criteria they are about to brainstorm.
In groups, have students brainstorm criteria they feel is important for effective procedures. While my students can brainstorm many of these criteria, I provide them with a procedure that a student wrote in the Density Test Tube Challenge Lab. (This procedure is included in the resource section.) Providing them with a student example helps students specifically identify and put words to criteria.
Have each group share and create a class list on the board or on poster paper. Students typically brainstorm the following:
Let students know that they will be writing a procedure to separate a mixture by physical means and that they should keep the brainstormed criteria from the mini lesson in mind when writing. Explain that each group will be given a mixture of sand, salt, iron filings, and mini marshmallows. Show students the mixture and let them know the tools that they have available to them. These materials include: strainer, ring stand, Bunsen burner, matches, beakers, cup with holes in the bottom, a tray, a magnet, and goggles.
Express to the students that each member of the group must be an active member of creating the procedures. Say something such as, “Once your procedure is approved, only then can you get your materials and begin working. Although you must only complete one procedure per group, each member of the group needs to provide input and ask questions as a part of the appropriate group discourse we have worked on all year. When you bring me your procedure, I will be asking follow up questions and I will be asking random members of your group for insight into your work. Answering my questions with, “I don’t know. John wrote that.” will not be accepted. You all need to take ownership in your group’s procedures!”
I find that it is important to show the students the materials that they will have available to them. Physically being able to see and touch the materials can spark better ideas for their uses. Below is a picture of a complete set lab station set up. One other important note is that it typically takes students 40 or more minutes to create an approved procedure.
I also find it is important to have the students note the size of the holes in the strainer and cup with holes. The key element to understand is that sand can travel through the strainer, but cannot pass through the cup with holes. I have plastic cups made with holes; however, you can simply poke holes in the bottom of a plastic cup if you do not have access to the type of cups I have.
I prepare the mixtures in Dixie cups.
In the following video, I look at a student procedure and note the important aspects of the procedure that I look for as well as the typical things that students have challenges with.
Students then begin carrying out the procedure they wrote on the first day. Below are a series of pictures that takes you through the typical path students take to separate their mixtures.
First, students use the strainer and catch the sand, salt, and iron filings in a tray.
Then, students use a magnet to take out the iron filings.
Students then place the sand in the cup with holes and set it on top of a beaker. They then pour water through the mixture and catch it in the beaker below.
Finally, students pour the salt water into a crucible and place it over a Bunsen burner until the salt is left behind.
Due to time constraints, I only have them evaporate one crucible full of the salt water instead of the entire volume of salt water.
A separated mixture!
Ask students to recall that there are two types of mixtures, heterogeneous and homogeneous. Ask students what type of mixture they worked with today. Students should explain that the mixture they separated was heterogeneous as the mixture was not uniform throughout. Ask students to work as a group at their table to use "Shape Vocabulary" to create a model for heterogeneous and homogeneous mixtures. Students should write those vocabulary words in the shape of their definition in a way that shows both the similarities and differences between the two. Students may include pictures in their "Shape Vocabulary", but the word itself has to be written in the shape of its definition. In other words, the model should not just be a picture with a label. Ask groups or individual students to share their reasoning for their models.
The group below explained that the homogeneous mixture was evenly spread out and they had tried to make it appear to be in water to show that the molecules were dissolving as they knew that dissolving can create homogeneous mixtures. The heterogeneous mixture this group drew was based on Chex Mix. With each letter of the word heterogeneous being written in different types of matter, the students explained that these letters were not mixed evenly throughout.
Understanding the difference between these two types of mixtures will play a role in their understanding of the next lesson, Mixtures Labs: Separating Mixtures and Rates of Dissolving Day 2.