In this lesson students grapple with the composition of matter, and they use evidence to justify their claims. This is another introductory lesson for the standard:
HS-PS1-3. Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.
The lesson relates to the standard because it provides an additional framework for students to analyze matter. In this case, they will focused on how mater is comprised. In this lesson, students turn their attention to the building blocks of matter, and as such apply the Crosscutting Concept of Structure and function. Students have to use definitions to justify why they believe different substances are either atoms, compounds, or mixtures, and so this lesson is also a good way to introduce the Science Practice of Engaging in argument from evidence.
While no prior knowledge is necessary for this lesson, students may need help in understanding what the various substances are in the Application section of the lesson.
I say attempt because I do not expect them to have all of the answers. I believe, however, that this exercise will get them thinking about the composition of matter, and how we know. I am not so concerned with correct answers at this point in the lesson; my goal is to get students thinking and conversing about matter.
After about 5 minutes, students compare their answers, first in small groups and then report out to the whole class. I project the Types of Matter Table Answer Key to aid in the discussion. These student notes on the types of matter are typical of what students took away from this part of the lesson. I was satisfied that students got the information they needed to understand today's lesson.
Lesson: Students need some help thinking about the differences between the mixtures, compounds, and elements. It may be helpful to review the slides from the Powers of Ten video that I showed yesterday, especially the slides from the leaf down to the atom. There are examples of mixtures (the leaf), as well as compounds (DNA) and atoms (carbon). I also pass around examples of each of these substances. Carbon, water, some green water, and a leaf are easily accessible examples. At this point in the year I am really trying to reinforce the idea of scale, and how different the composition looks like at different scales.
I then do a think aloud, which is when I talk about what I am thinking while I am reading. You can watch a Think Aloud video here. I begin to complete the first couple of examples on the worksheet. I am sure to reference the definitions that were established earlier in the lesson in the Types of Matter Table Answer Key and the handout A Dichotomous Key to the Classification of Matter. Note: These resources are adapted from the Tennessee Curriculum Center A Dichotomous Key to the Classification of Matter.
Guided Practice: Students do the first two examples, and then as a whole class we discuss student answers. If it appears that students understand what they need to do, I let them get to work. If not, I reteach or answer questions that arise. One helpful hint I can give students who are really struggling is the idea of a chemical formula. If a substance is made up of one chemical formula, it is a compound. If it is made up of substances with more than one type of compound it is a mixture.
Students complete the Classification Worksheet while I walk around the classroom, answering questions and discussing incorrect answers with students. Common incorrect answers provide opportunity for whole class discussion. After students complete the worksheet they compare answers with a partner up and discuss discrepancies.
A common challenge that students may during this exercise is a lack of familiarity with some of the substances. One way around this is to have the various substances on hand so that students can look at them. The water and bromothymol blue solution would be a good candidate for this, but I have everything on my lab bench and students are welcome to get a closer look at any of them.
However, I also encourage students to read the label if there is one, or research it on the Internet. By approaching the lesson in this way, I help them to employ the Practice of the Scientist Asking Questions. The questions they should ask are questions that are embedded in the Dichotomous Key.
Students share answers with the whole class. I really focus on the use of evidence during this section. Students may have a different answer than me or other classmates. For example, sand from the beach is not homogeneous; its composition varies. I explain to students that scientists use evidence to justify answers. This is also a great time to focus on how students thought one thing at first but then changed their mind after doing some research.
I check for understanding using the Classification of Matter Worksheet Answers and individual student work. I notice that there is still some confusion around this topic that has to do with not always understanding how the definitions relate to the examples.
We discuss why some of the student answers are incorrect and students change their answers, as evidenced in this student work. In some cases students are not familiar with the periodic table and so she failed to identify an element. In another case the student did not look at the example I had of the actual substance and instead used their own preconceived idea of the substance. When they say milk of magnesia, for example, they understood why it was homogeneous.