Density Part II and Review

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Objective

Students will be able to predict the identity of an unknown metal through performing a density lab.

Big Idea

Density is an intensive property which is useful for identifying unknown substances.

Introduction

This lesson serves as a review of Unit 1 in two parts.

For the first part of the lesson students perform a lab.  Students have a chance to practice the concepts that they have learned regarding the scientific method, measuring, significant figures, dimensional analysis, and density.

Because students are preforming and experiment, collecting data, and analyzing their results they use Science and Engineering Practices 3: Planning and Carrying out Investigations and 4: Analyzing and Interpreting Data.

There are several resources used for the lab.  For each group I provide:

  • 1 piece of metal (I use metal cylinders from a set that includes copper, brass, aluminum, and steel available through Flinn Scientific)
  • 1 graduated cylinder (I use 25mL graduated cylinders.  Plastic cylinders are helpful because glass cylinders can break if students aren't gentle with placing the metal in the cylinder)
  • bottle of water
  • balance

For the second part of the lesson students are completing a unit 1 exam review paper to help them study for their exam.  This paper is a review of all of the concepts from the unit.

Engage

10 minutes

To begin the lesson and refresh students' memory on density I have them do a quick review of density including how to calculate density and how relative density determines if substances float or sink in each other. 

I pass out the paper and give them about 5 minutes to work on it on their own. 

When all students are done I go over the answers using the answer key

Because we had just reviewed the homework from the previous lesson I do not take the extra time of going over in detail the rationale behind the answers.

I use examples that are related to the examples that we did in class the previous lesson (Unit 1 lesson 7).

Elaborate

60 minutes

This lab serves as a lab review of the unit and prepares students for the Performance Task portion of their exam.  In the lab students identify an unknown metal through calculating density.  This is a formal lab where students will be writing out information on a separate sheet of paper to turn in.

  • Before students begin the lab I have them quietly read over the lab paper to themselves for about 5 minutes or until most students are done. 
  • I then help students to set up their lab papers in the following steps:
  1. I have students write a title, their name and period at the top.  
  2. I instruct students to write the problem.
  3. I have students write hypothesis and then leave two lines blank because they need to wait until they see the metal at their lab station.
  4. I have students write down the materials.
  5. I go over how students will do their procedures and demonstrate an example of a flow chart.
  6. I give students time to set up their data table making sure to instruct them to leave lots of space to write their data legibly.
  • After students papers are set up I go through an example with them using some lead wire to make sure they understand how they will be doing the lab.  I make sure to tell them to not write down what I put on my table as that theirs will be different.  This image shows what I write down while going through the lab setup and example. This is a picture of the metal being measured with volume displacement.
  • I then let the students know that I will break them into cooperative groups (see reflection on cooperative groups).  I stress that once they collect the data that they should each be answering the Analysis/Conclusion questions on their own and use their group members to help with questions and to check answers.
  • Once students have their cards, I have them go up to lab group stations with paper, pens, calculators, and the lab. 
  • As students are working on the lab I walk around to make sure that they are all working well together, that they are switching roles after each trial, and answering any questions that they may have.  Many of my students have a hard time with measuring the volume of water in the graduated cylinder so I make sure to check with students as they are measuring.  Students also continue to struggle with averages and percent error so I make sure to double check their work.
  • When students complete the lab, I have them turn in and I grade using the attached rubric.
  • Here is an example of one student's completed lab. This student did a great job for the most part, but did not give a way to overcome the human error if she did the lab again.  This is a common mistake I saw on labs.  Another common mistake was students not understanding what to put down as an unavoidable error.  Some students just wrote that it had to do with the graduated cylinder but did not give details.   When I reviewed the lab with students when I passed it back I made sure to discuss the difference between avoidable and unavoidable errors with examples from this lab.
  • Some examples of unavoidable errors are the precision limits of the balance and the graduated cylinder.  Some examples of avoidable errors are water splashing out when students place the rock in it, students accuracy in reading of the graduated cylinder, and not taring the balance.

Evaluate

30 minutes

The final part of the lesson is where students are preparing for the exam.

Students are first expected to make sure that their binders are organized for their binder check which I do at the exam (see reflection on binder check)

Students are also expected to begin to work on the Unit 1 Exam Review Worksheet.  They can work either on their own or with partners.

  • I have the review divided by lecture to help students complete it. 
  • I choose questions for the review based on the most important aspects of each set of notes and questions similar to the ones on the exam.
  • Most students do not have time to complete the review in class so they complete the assignment as homework. 
  • I check the review at the next class by stamping it.
  • I then review the answers using the answer key before the exam and answer any questions students may have.
  • The most common mistakes that students make on the review are questions from lecture 3 dealing with significant figures and rounding based on decimal place values or significant figures.