The balancing with Legos formative assessment (FA) task asks students to create the product substances given the reactants. Students have to figure out what they did to make it work, and that gets them to talk about how they think about balancing chemical equations. The FA targets chemical mechanism. It provides an opportunity for students to use a hands-on model to realize that, in order to conserve matter, the atoms need to rearrange into the new compounds in specific ratios. The goal of the FA is to elicit students’ thinking about how compounds behave. The learning goal is to have students understand that only full compounds can be introduced on the reactant side for balancing in order to make complete compounds on the product side.
In this FA activity, students receive plastic baggies (“compound sets”) containing different sizes and colors of Legos. There is a sticker on the bag that provides a key for which element is represented by each type of Lego. The type coding allows students to understand which compounds are in the bag. In this 
Balancing Equations task for students, students are given the following rules:
- Only open one compound set at a time.
- Use the sticker on the bag for the key of which compounds are in your bag and which type of Lego represents each element. They may be different for each set.
- The number of bumps or holes in each Lego do not mean anything. Compounds should be put together based on their chemical formula.
- You can use as many of each compound on the reactant and product sides as you need – but you must use ALL of the compound (for example, if you use a Cl2 in your reactant, you need to use both of the chlorines in your product).
This FA task was tested with 11th and 12th grade high school chemistry students. At the point when this FA occurred, the class was just starting a unit on balancing equations. The students already had a foundational understanding that atoms cannot change and cannot be created/destroyed – and that they rearrange in a chemical reaction. Immediately prior to this FA, students had finished a unit on molecular shape, polarity, and intermolecular forces.
Teaching reflections
Reflecting on this activity, and the conversations I had with the students, I feel that this would have been a better activity to do after they had started balancing equations. The activity gave them a very good hands-on way of understanding and figuring out how equations are balanced, but may have been better after they had worked on the math part of balancing first. Students were able to articulate what they were doing, but not many of them made the connection to the idea that ratios are involved in the balancing act.
Examples of student work (names are pseudonyms)
| Part A: Reactants: CO and O2 Products: CO2 | |||||
| Name | How many CO molecules did you use? | How many O2 molecules did you use? | How many CO2 molecules did you make? | What was the one pivotal decision you had to make in order for your solution to work? | Reflecting back on what you did to make this work, is there another solution? If so, what is it? |
| Retro | 2 | 1 | 2 | Splitting an O2 | Splitting CO and putting O2 on the C, and the O on a CO |
| Nov | 2 | 2 | 2 | The amount of carbon that we got to use | I use 1 oxygen to make a O2 |
| Ace | 2 | 1 | 2 | Splitting the O2 | Split up the carbon, put the two on top of the oxygen, and afterward has been made. |
| DMoney | 2 | 1 | 1 | Put together another CO to make the other one happy | No |
| Pineapple | 2 | 1 | 1 | I had to bring another CO to make it happy | No other way |
| Part B: Reactants: Na and Cl2 Products: NaCl | |||||
| Name | How many Na atoms did you use? | How many Cl2 molecules did you use? | How many NaCl molecules did you make? | What was the one pivotal decision you had to make in order for your solution to work? | Reflecting back on what you did to make this work, is there another solution? If so, what is it? |
| Retro | 2 | 1 | 2 | Splitting the Cl2 | (blank) |
| Nov | 2 | 1 | 2 | I have use the Cl2 and Na2 | (blank) |
| Ace | 2 | 1 | 2 | Splitting the Cl2 | (blank) |
| DMoney | 2 | 1 | 2 | Add Na to make Cl happy | I had to bring in an extra Cl to make it work |
| Pineapple | 2 | 1 | 2 | Na to add to make Cl happy | Had to bring in an extra Cl to make it work |
| Part C: Reactants: Na and H2O Products: NaOH and H2 | ||||||
| Name | How many Na atoms did you use? | How many H2O molecules did you use? | How many NaOH molecules did you make? | How many H2 molecules did you make? | What was the one pivotal decision you had to make in order for your solution to work? | Reflecting back on what you did to make this work, is there another solution? If so, what is it? |
| Retro | 2 | 2 | 2 | 1 | Splitting an H2O | (blank) |
| Nov | 2 | 2 | 2 | 1 | (blank) | (blank) |
| Ace | 2 | 2 | 2 | 1 | Splitting the H2O | 2na + 2H2O → 2naOH + H2 |
| DMoney | 2 | 2 | 2 | 1 | (blank) | I had to add atoms to make them happy |
| Pineapple | 2 | 2 | 2 | 1 | I had to bring atoms till all of them is happy | Had to put to H together so it can work |
Last two questions
Write the chemical equation out for this reaction. Be sure to put the reactants before the arrow, the products after the arrow, and use the numbers of elements/molecules as coefficients in front of the chemical formulas.
| Retro | 2 Na + 2 H2O → 2 NaOH + H2 |
| Nov | element |
| Ace | 2na + 2H2O → 2HaOH + H2 |
| DMoney | Put two together to work |
| Pineapple | Put two together to work |
In your own opinion, what do you think you were supposed to understand about balancing equations from this activity?
| Retro | Matter is neither created nor destroyed in chemical reactions. |
| Nov | Break them apart |
| Ace | Split the carbon but it can’t be destroyed or erased. |
| DMoney | They work together |
| Pineapple | Understand how to count molecules and atoms |