September/October 2017 – Vol. 30 No. 1

Destroying Water: A Classic Lab Rejuvenated for NGSS

Posted: Monday, June 20th, 2016

by Rich Hedman and Lisa Hegdahl

After nearly 15 years teaching the 1998 CA Science Standards, many science educators have file cabinets and hard drives full of activities. The activities themselves are valuable in that they clearly illustrate scientific concepts and phenomena. However, in the past, they were often used only to verify information already presented in class. One of the many challenges of implementing the Next Generation of Science Standards (NGSS) is to move towards three dimensional learning and still utilize activities from the past. How can teachers modify labs that used to be just recipes for verification and turn them into experiences that engage students in the process of scientific discovery?

Electrolysis of water is a classic chemistry lab used as a way to confirm that water is made of 2-parts hydrogen to 1-part oxygen— in other words, that the chemical formula, H2O, is actually based on the proportion of atoms in a water molecule.  Teachers tell students that the chemical formula of water is H2O, and that during the experiment, they will be breaking water into hydrogen and oxygen gases.  Ion-rich water is electrified with direct current (DC), and gas bubbles form at the positive and negative terminals in the solution.  The gases are collected in tubes, and the volume of gas present in each tube is compared.  It turns out that twice as much of one gas is collected compared to the other gas. Teachers frequently use a splint and flame test (very carefully; following all safety protocols) to identify which gas is which (oxygen relights a splint, hydrogen pops loudly) and to verify that the elements that make up water have different properties than the water itself. Students see that there is twice as much hydrogen as oxygen, which verifies the chemical formula of water, and the lesson is completed in one class period.

How does this lesson align with the performance expectations (PEs) of NGSS? Where did students use the 3-dimensions of NGSS (science and engineering practices, crosscutting concepts, and disciplinary core ideas) to make sense of phenomenon? We asked ourselves these questions and then set out to “NGSS-ize” the activity.  We began by examining the performance expectations and disciplinary core ideas (DCIs) related to this lesson.

The most closely aligned performance expectation is:

  • MS-PS1-5Develop and use a model to describe how the total number of atoms does not change in a chemical reaction and thus mass is conserved.

The disciplinary core idea connected to this PE is –

  • PS1.B Chemical Reactions: the total number of each type of atom is conserved* and thus mass does not change.  

(*Note: we focus on conservation of the particles in this lesson, not on the mass.)

So, the underlying science ideas were indeed connected to an NGSS PE and DCI.  How about the science and engineering practices (SEPs) and the crosscutting concepts (CCCs)?  From the description of the classic electrolysis activity above, it is clear that students are not engaged in science and engineering practices.  The teacher is asking the questions, the teacher is designing the investigation, the teacher is constructing the explanations, and the teacher is communicating the information.  So, the lesson, as described, completely fails the SEP test.  Regarding the CCCs, from the description above, it is difficult to determine whether or not the teacher would draw attention to the related crosscutting concepts, such as patterns, systems and system models, or energy and matter in systems.  An NGSS lesson plan would need to explicitly include those connections.

Three-Dimensional Alignment

The first step in ‘NGSS-izing’ the classic electrolysis lesson was to figure out a way for student groups to engage in sense-making during the investigation.  Instead of telling students the answer (that water is H2O and the number of particles is conserved before and after the reaction), we wanted the students to figure that out for themselves, based on the patterns (CCC-patterns) they detect in their data.  What follows is a brief description of what we came up with after much thoughtful collaboration.  We tried to focus our description here on the NGSS shifts in the lesson and not so much on detailed procedures, which you can access by contacting either one of us.

Part l

Photo by Lisa Hegdal

Photo by Lisa Hegdahl

Students obtain a condiment cup that contains ionized water and record their observations of its properties in their science notebooks.  Students place a 3-ounce condiment cup over a 9 volt battery and mark the location of the battery terminals.  At the terminal marks, students insert metal tacks into the condiment cup so that the tacks stick up into the cup and place the condiment cup on top of a battery with the tack heads touching the battery terminals.  In their science notebooks, students write observations of what they see. Students should see bubbles coming up from both tacks, with the (-) terminal producing more bubbles. Students write an initial explanation (SEP – constructing explanations) for what they observe.  As a class, they discuss the groups’ observations and their explanations for the phenomenon.  Students are then directed to think about how they might capture the gases.  Eventually the teacher will guide students to the possibility of placing test tubes over the tacks.

Part II

Photo by Lisa Hegdahl

Photo by Lisa Hegdahl

Students fill 2 labeled (+ and -) test tubes up to the top with the ionized water. Placing their thumb over the top of the test tube, they turn the test tube upside down over the tack making sure not to take their thumb off the test tube until it is under water.  This is to prevent air from getting into the test tube.  The (+) test tube should be over the tack that is in contact with the (+) terminal and the (-) test tube should be over the tack that is in contact with the (-) terminal.  Each of the test tubes should begin to fill with gas.  The class discusses what kind of quantitative data they can take to illustrate what is happening in the test tubes.  Through thoughtful questioning, the teacher leads this discussion to the realization that the amount of gas can be approximated by measuring the height of the gas in each test tube. After running the reaction for 20 minutes, the student groups record their data on a class data table.

Students share with the class what they think is inside the test tubes above the liquid. Typical student answers include the following: “nothing”, “air”, “water vapor”, “hydrogen”, and “oxygen”.  The teacher records the student answers and asks students if they can think of ways that any of these possibilities can be tested (SEP – planning and carrying out investigations). The teacher will probably need to discuss with students how a splint flame test can be used to test two of their ideas; for example, oxygen will relight a splint, and hydrogen will pop loudly when exposed to the splint.  The teacher works with a group of students to conduct these tests in front of the class.  The results are that the lit splint held over the (-) test tube will ‘pop’, and a glowing flint held over the (+) test tube will reignite.  At this point you may want individual students to construct claim, evidence, reasoning statements for what they observed and how they identified the gases (SEP – Engaging in Argument from Evidence).

Part III

Now that the students have identified the gases in each test tube, students are tasked with analyzing the class data table and discussing with their table groups the patterns (CCC-Patterns) they see.  The goal is for students to realize that just about every group obtained a 2:1 ratio of hydrogen to oxygen. This is not as easy as it might seem, as students have to identify the 2:1 ratio from measurements that are not always exactly 2:1.  If some groups have data that does not fit the pattern, discuss possibilities for why that might be true.  Once the students identify that electrolysis of water is producing twice as much hydrogen as oxygen, the teacher should ask the students: “WHY? What causes this pattern?” Student groups should develop answers to this question, and then share their ideas (SEP-Obtaining, Evaluating, and Communicating Information). Usually several groups will have an “aha” moment and say that there is twice as much hydrogen as oxygen produced because a water molecule itself is composed of 2 hydrogen atoms for every 1 oxygen atom.  Students have figured it out for themselves!

To wrap things up, we provide students with a handout summarizing the particle model of matter (SEP- Developing and Using Models).  We then ask students to apply the particle model to explain (in words and pictures) what was happening before and after the electrolysis reaction.  Our goal is for students to draw water molecules as the reactants, and separate hydrogen and oxygen atoms as the products, in such a way that the number of particles is conserved.  This task allows us to assess students related to Performance Expectation MS-PS1-5.

Conclusion

Our modifications have turned a typical lab, which was designed to confirm something students were told, into an experience where students figure out a concept on their own.  In the process, students are engaged in several of the NGSS science and engineering practices and crosscutting concepts and are able to make progress towards mastering a performance expectation. Modifying classic experiments to align with NGSS is not an easy task, but it will provide students with valuable experiences in making sense of the world around them.

Please don’t hesitate to contact either one of us for the handouts and other useful information about this revamped activity.

Rich Hedman – Director, Sacramento Area Science Project (SASP) and CSTA member, hedmanrd@csus.edu

Lisa Hegdahl – 8th grade science teacher, McCaffrey Middle School in Galt, CA NGSS Early Implementer, President of CSTA, lhegdahl@galt.k12.ca.us

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Written by Lisa Hegdahl

Lisa Hegdahl

Lisa Hegdahl is an 8th-grade science teacher at McCaffrey Middle School in Galt, CA and is Past-President of CSTA.

2 Responses

  1. I LOVE this lab – I use it after we make stop motion movies to show why we balance equations as an application investigation for what we have learned throughout chemistry thus far. Thank you!

  2. Melissa – thank you for taking the time to let us know! Glad to hear it has been useful for you. Please feel free to share on FB, Pinterest, and Twitter – or whatever way you prefer to share resources with other teachers.

    -Jessica Sawko, CSTA

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