The 4 Biggest Mistakes Students Make When Balancing Chemical Equations (And How to Fix Them)
- Androy
- Dec 5, 2021
- 5 min read
Updated: Mar 7
Does This Sound Familiar?
You spend an entire class period on balancing chemical equations—breaking it down step by step, modeling the process on the board, giving students practice problems—and yet, half the class is still stuck.
Some students are randomly guessing numbers until something "looks right." Others are changing subscripts instead of using coefficients. And a few are staring blankly at the paper, completely lost.
If you've been there (and I know you have!), you're not alone.
I used to think my students struggled simply because balancing equations was hard. But after years of teaching (and some action research), I realized the real problem wasn’t balancing at all—it was the foundational misconceptions they had before even getting to that step.
What My Research Revealed
While investigating why students struggle with balancing equations, I conducted a small research study with a group of high school chemistry students (ages 14-16).
Here’s what I did:
First, I gave them word equations and asked them to write the corresponding chemical equations.
Next, I had them balance those equations (without state symbols to keep it simple).
I categorized every mistake and coded the errors into patterns.
To further validate my findings, I interviewed other chemistry teachers and dug into the research on chemistry misconceptions.
What I found was eye-opening: Nearly all student struggles fell into four major categories—each one preventing them from balancing equations successfully.
Once I started focusing on fixing these misconceptions, I saw a huge improvement in student success.
So, what are these four common mistakes? And more importantly, how can we fix them?
Mistake # 1: Students Don’t Understand Chemical Formulae
Why This Happens
Here’s the thing: If students don’t understand how chemical formulas work, they have no chance of balancing an equation correctly.
When I first started teaching, I assumed my students knew what H₂O or CO₂ meant. I was wrong. They could read a formula, but they didn’t fully understand what it represented.
A chemical formula isn’t just a bunch of letters—it tells us:
The elements present in a compound.
The ratio of atoms of each element.
The type of bonding (ionic, covalent, or metallic).
For example, some students believe H₂O means two separate hydrogen atoms and one oxygen atom, just kind of floating around together. Others think H₂O and H₂O₂ are basically the same thing. (Hydrogen peroxide in your drinking water? No thanks!)
How to Fix It
Teach chemical bonding first: Before diving into balancing, make sure students truly understand chemical bonding. If they don’t know that NaCl is an ionic lattice, or that CO₂ is a covalent molecule, they’ll struggle with writing formulas correctly.
Use visuals and models: Students need to see what a formula represents. Use molecular models, pictorial diagrams, and even real-world examples (e.g., water vs. hydrogen peroxide) to help them make connections.
Have students decode chemical formulas: Give students a variety of chemical formulas and ask:
What elements are present?
How many atoms of each?
What type of bonding holds them together?
Once students can accurately read chemical formulas, they’ll be much more prepared to balance equations.
The diagram below illustrates the chemical formula of water and its meaning by connecting the three levels of representation: macroscopic, submicroscopic, and symbolic.
Mistake # 2: Writing Incorrect Chemical Formulas for Ionic Compounds
Why This Happens
Ever ask a student to write the formula for aluminum oxide and get AlO instead of Al₂O₃? (Or something similar?)
That’s because writing ionic formulas isn’t intuitive for most students. Unlike covalent compounds (where names hint at the number of atoms, ionic compounds require students to balance charges, which they often ignore.
They either:
Guess and hope for the best.
Write elements side by side without considering charge balance.
Memorize formulas without understanding the “why.”
How to Fix It
Teach students to predict ion charges using the periodic table: Instead of handing them a massive list of ions to memorize, show them how to determine ion charges using periodic table patterns:
For Example
Group 1 metals = +1
Group 2 metals = +2
Group 16 nonmetals = -2
Group 17 nonmetals = -1
Teach balancing charges using two methods
The Balancing Method: Match the total positive and negative charges to make a neutral compound.
The Crisscross Method: Take the charge numbers and swap them as subscripts. (Although I prefer the first method since students actually understand why it works.)
Use real-world comparisons: I tell my students that forming ionic compounds is like splitting the bill at a restaurant—everyone has to contribute an equal amount to keep things fair. If sodium (+1) and oxygen (-2) go out to eat, sodium needs to bring a friend (Na₂O) to make sure oxygen isn’t covering too much of the charge.
Mistake # 3: Incorrect Use of Brackets with Polyatomic Ions
Why This Happens
Polyatomic ions are like tiny gangs—they move together, they react together, and they shouldn’t be split up when writing formulas.
But students often:
Forget to use brackets when needed (e.g., writing MgNO₃₂ instead of Mg(NO₃)₂).
Add brackets unnecessarily (e.g., Na(Cl) instead of NaCl).
So why do students do this? I have found that:
they do not truly understand what is going on with polyatomic ions,
they do not understand how they're formed and
they do not know how to treat them when writing formulae.
How to Fix It
Help students visualize polyatomic ions as single units: I literally have students circle polyatomic ions in formulas. Once they see them as one entity, they stop making careless mistakes.
Teach the bracket rule
If you need one polyatomic ion → No brackets (e.g., NaNO₃).
If you need more than one → Use brackets (e.g., Mg(NO₃)₂).
Mix up practice problems: Students should practice identifying when brackets are needed before you throw them into writing full chemical equations.
Mistake # 4: Confusing Coefficients and Subscripts When Balancing Chemical Equations
Why This Happens
Give students H₂ + O₂ → H₂O and half of them will try to balance it like this:
H₂ + O₂ → H₂O₂
Congratulations, you just made hydrogen peroxide instead of water!
Why do students do this? Because they don’t realize that coefficients and subscripts mean different things.
How to Fix It
Use a real-world analogy: I tell students subscripts are part of a person’s identity (e.g., your DNA). You can’t change them. Coefficients are like the number of people in a group. You can change that.
Use hands-on modeling: Have students physically manipulate molecule models to see the difference between changing the quantity vs. changing the structure.
Start with simple equations, then progress to complex ones: Begin with equations where only one element needs adjusting before introducing multi-step balancing.
Final Thoughts: How to Help Students Master Chemical Equations
Balancing chemical equations is hard because it builds on so many different concepts. If students are struggling, it’s usually because they missed something earlier—whether it’s chemical bonding, writing formulas, or distinguishing subscripts from coefficients.
Quick Recap:
Teach bonding and formula writing first
Reinforce ion charges and charge balancing
Use visuals for polyatomic ions
Emphasize coefficients vs. subscripts with hands-on activities
What’s the biggest mistake YOUR students make? Share in the comments!
Try a FREE SAMPLE of My Scaffolded Balancing Chemical Equations Digital Resource for Google Slides
References:
Atkinson, R. K. (2002). Optimizing learning from examples using animated pedagogical agents. Journal of Educational Psychology, 94, 416–427.
