Attaining Balance

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SWBAT derive the need to balance chemical equations to satisfy the Law of Conservation of Matter from a concept attainment activity.

Big Idea

By analyzing data, students can construct explanations related to balancing equations, a central concept.


Since all the equations I've presented my students so far have been balanced, I needed to build up the understanding that some equations may need to be balanced.

Previously, students experienced this via the PhET simulation, Reactants, Products and Leftovers.  However, we did not move into actually balancing the equations at that time.

Today we will do a concept attainment activity to build a recognition of balanced vs. unbalanced chemical equations.  The activity consists of a series of examples and non-examples, with the students trying to determine what makes them different, and ultimately what the examples have in common.

Traditionally, the teacher controls all the exemplars, and students record them on paper.  However, given the complicated nature of writing out all the equations, I provided them both on the student sheet, and cut out in strips of paper for students to physically sort out.  I am hopeful that the physical strips of paper will help students categorize and recognize equations that are unbalanced.

Students will be using the mathematics of the equation to determine if they are, or are not balanced (HS-PS1-7).  Students will be interpreting data to determine the patterns present in the equation data set (SP4) and then constructing an explanation (SP6) for the reason equations are grouped together.

Opener- Returned Work

5 minutes

When class begins, I pass back their types of reaction quizzes, and any other graded work I have.  I remind students again of the ability to correct their quizzes on a separate sheet of paper for half of their missed points.  Quiz corrections provide students a chance to focus on what they misunderstood.  If it was merely that they misinterpreted the question, it is a confidence builder.  If the content was lacking, corrections provides a chance to revisit the content and clear up their misconceptions.

I then explain that today's activity may feel frustrating at times, as I will be asking them to think without much help for most of the period.  I ask them to fight through any frustrations, and keep working with me until the end of the period.  Providing acknowledgement that a concept is difficult helps build persistence in my students.  Too often, students encounter something that they think is difficult, but perceive it as a personal fault and give up.  By presenting the idea that this lesson may be a struggle, students engage and fight through the difficulties more readily.

Concept Attainment-Need to balance

35 minutes

Next I pass out the envelopes with the Balancing Reactions Concept Attainment Cut Outs and ask students to get them out and flipped right side up and in order.  While students are doing this, I pass out the Balancing Reactions Concept Attainment Activity sheet.

I explain the directions to the students, that there are 16 samples.  7 of the samples (I made an error and one was not balanced) are examples of what we are learning about today.  9 of the samples are not examples of what we are learning today.  The goal for the students is to determine the pattern they see and figure out what the positive examples have in common that makes them different from the negative examples.

I have students flip the paper over, and explain how they will record their thinking.  At any time they can make a prediction about the rule that groups things together.  They are to record what example we are on, so they can track their thinking.  Then they write down what they believe the rule is.  I point out that it is ok to be wrong, that I provided them three opportunities to make their best prediction before we talk together at the end to determine the final rule.

I ask if there are questions, and the first one is "What is the rule about?" and I respond "That's what we're figuring out.  It's like when you were little and watched Sesame Street-- one of the things was not like the other.  Today we are going to play that game on a bigger scale and try to determine how these are grouped."

We flip the paper back over and look at the first sample, which I am also displaying on the overhead projector.  I tell the class "This is an example of an equation that follows the rule, so mark it yes on your paper, and sort the strip to your yes pile."

The second equation is also a yes, but the third is not.  At this point, I ask what they know about each equation. Responses include:

  • The first and third are combustion reactions, and the second is synthesis.
  • They all have water as a product
  • Every chemical in them are gases.

I stop and process with them at this point.  "If we know that not all three are good examples of the rule, what do we know the rule can NOT be right now?"

  • "They can't just have water as a product, since #3 is a no."
  • "They can't just be all gases since #3 is a no."

"If you think you have a prediction right now about what the rule IS, flip the paper over, label it as step 3, and write your prediction."

Most students aren't willing to take a gamble at this point.  We go through the next three samples, pause, and then the next two.  After 8 samples, I ask all students who haven't made a prediction yet to make one, even if they aren't sure it is right.  This is important to get kids to commit to something, even if it might be in error.  I want the students involved and thinking, and I want to see the evidence of their thinking.  While students are writing, I move around the room.  In my 4th period class, one young lady caught the pattern very early, after only 5 examples.

Most students aren't this fast, so we continue to work through the examples.  I provide the next two.  Then with 10 samples in their data set, I begin to ask students to predict if the remainders will be a yes or a no.  I tell them to think about the rule they have in mind, and decide if the example fits their rule.  After students have a chance to predict, I tell them the actual fit to the rule.

Predicting helps some students determine the rule, and they will flip their paper over and put a new prediction.  Some students wait me out until the end.

Now, with a full data set, I ask students what rules they have decided do not work.

  • "I thought it was the chemicals with numbers in front until 5 and 6 were non-examples"
  • "Through number 10, you had an example of all five types of reactions, so I thought that might be it, but number 14 was a second synthesis"
  • "The states of matter didn't make any difference"

Now I ask students who are confident what the final rule is.

You have the same number of each type of element on both sides for the yes responses.

What is the name of our rule for that?

The law of conservation of matter.

So if we obey the law, and we have the same amount on each side, what are we called?


I stand on one leg, hold out my arms and say "If I have the same weight in each hand, what am I right now?"



I pounce on the word balanced and explain that is the difference.  "Equations that obey the law of conservation of matter are said to be balanced.  Those that do not are unbalanced."  I ask students to write down the final, definitive rule.  While they write I pass out our closing activity.  Below is an example of a student who worked their way towards the correct rule.


10 minutes

To end the lesson, I have students complete a short worksheet that I borrowed from another teacher.  I am unsure about the copyright of the source, so a completed copy by a student is provided in the next lesson's opener when we review the answers.  Students need to check 10 equations and determine if they are balanced or unbalanced.

At the end of that section, there are five matching questions on vocabulary terms and definitions.  Students complete this and turn it in at the end of the period.  When students are struggling, I refer them back to the examples from our activity today, and the rules they developed in how to determine if an equation was balanced.

We will go over the worksheet as our opener tomorrow, so students can take them home to finish if needed.