Reviewing Counting Atoms and Balancing

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SWBAT model, identify and balance chemical reactions demonstrating the Law of Conservation of Mass.

Big Idea

Students observe two demonstrations and have to justify claims based on evidence whether each demo represents a physical or chemical change.

NGSS Standard Alignment

Once a week classes are shortened from 50 minutes to 42 minutes due to teacher meetings (resource day) in the morning.  These days require a specific planning that sometimes cause me to leave typical aspects out of my lessons.  This lesson is heavy in review which will allow me to assess student mastery of some old content (counting atoms) and assess where students are with balancing equations.

Performance Expectation (PE)/Disciplinary Core Idea (DCI)

This lesson is aligned with HS-PS1-7, the uses of mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction; and DCI-PS1.B, the fact that atoms are conserved, together with knowledge of the chemical properties of the elements involved, can be used to describe and predict chemical reactions. Students continue to develop an understanding of how to count atoms and balance equations. Balancing equations is difficult for many students and typically requires at least two lessons for student mastery.   

Science and Engineering Practices (SP)

HS-PS1-7 is one of the few high school Performance Expectations that focuses on the use of mathematics to describe a concept.  In the process of mastering this PE students will be required to balance chemical equation, exhibiting proficiency of the Science and Engineering Practice of Mathematics and Computational Thinking. 

Crosscutting Concepts (XC)

During this lesson students will recognize that energy and matter is an underlying crosscutting concept involved in all chemical reactions.  This is not the main focus of this lesson, but as students continue to develop an understanding of chemical reactions throughout the unit they will build on the idea that matter and energy are part of chemical reactions.


10 minutes

Shortened periods like today are a great day to break out the white boards and assess students.  I find that on our resource day I need to engage students more than I normally do, so using white boards is a very interactive way to assess students.

As students enter the class I tell them to take out the previous day’s assignment so that we can go over it.  I have them do it at the beginning of the class because it is routine, even though it won't be used till the next part of the lesson.  This assignment came from the previous day were I had a substitute for the day.  So I only have an attachment, not a lesson to link to.  The assignment was the first time they had to balance an equation using pencil and paper and not a simulation (ExploreLearning).

Once students have out their papers I ask them to grab a white board and marker and prepare for a brief exercise to show me you know how to count atoms. 


The following is a picture of how I have white boards ready to go.  It’s very straightforward and they are out of the way, so students are not messing with them before the review starts.

The following video is short clip of my students engaging in white board review. As I am highlighting each chemical formula (as seen in the previous Screencast video of the counting atoms worksheet) students are counting the atoms and holding up their boards.  Each one takes about 1 ½ minutes and really lets me see who knows the material.  If the majority of students struggle, I will show them how to get the answer…this will stretch it out to about 2 minutes.



20 minutes

After the white board review I have them put all the boards in a nice pile out of the way so that we can focus on the homework assignment from the previous day.  Since they already have it out it makes for a smooth transition.

The previous day’s assignment starts with some counting of atoms and labeling the parts of a chemical equation, such as reactant, products, subscript, yields and coefficient.  Since we counted atoms already I skip this part and briefly cover the parts of a chemical equation.  Most students have no problem with this part.

Next I go over how to balance an equation by showing them a step by step method.  I have been using a worksheet that I have used for years that came from made by T. Trimpe 2006.  It’s called balancing act and does an excellent job of providing the students with 4 steps to balancing a chemical equation.  I find if students follow these steps, balancing becomes much easier and less time needs to be spent reteaching the concept.

After walking through the steps (3-5 minutes) I move on to see how students did on it without out any formal teaching.  My expectations for student’s mastery is very low due to the fact that they have had little exposure to it.  However, after this lesson I anticipate students will be much better at balancing due to a formal explanation.

After the step by step instruction and going through several of the problems we can move on to reinforcing what they learned during the formal instruction and guide practice.  I spend about 20 minutes on this

I have found that their are a variety of excellent resources at that can help students with balancing chemical equations. If you have not visited it, I highly recommend doing so.

Elaborate (homework)

The final part of this shortened period is students performing a Team Mole Balancing Race.  They have not been exposed to the mole yet but I decided to call it the mole because it builds curiosity which can be used later when stochiometry is introduced.

In the mole challenge (mole challenge equationactivity students work in groups of two and try to balance as many equations correctly till the bell rings.  This original idea for this activity comes from, but this has been modified to introduce students to the mole and how it relates to balancing equations and the conservation of mass.

I first assign students into groups of two or three students and handout the worksheet that they will put their answers on.  Next I read the instructions so the know how to play the game. 

Team Mole Balancing Race


  1. Pick a card and balance the equation.  You may not peak at the card before choosing it.  You may not put the card back if you do not like the problem.
  2. You may only work on one card at a time.  Both team members must get the problem correct before moving on to the next card.  Both team members must show me the correct balanced equation before getting another card.
  3. You must stay at your table and solve the problem with your partner.

Instructions:  Write the number, equation, counted atoms (showing work as I call it!) and answer in the box below.  If your answer is correct after check in, put the card back where you got it and pick another card.  If you do not balance the equation correctly, keep trying till you get it right.

Again this is a great way to get students engaged on a shortened class period in balancing chemical equations.  Today is meant only to be an introduction to this activity the next class period will begin with the true team race.  The top 4 teams will receive a free homework pass.  Sometimes I give out food, but the next 2 units are heavy in homework and I think students will appreciate it more.