Balancing Chemical Equations PhET: Step-by-Step Guide + Free Worksheet

Balancing a chemical equation means placing whole-number coefficients in front of each chemical formula so that the number of atoms of every element is equal on both the reactant and product sides. The PhET Balancing Chemical Equations simulation makes this visual and interactive, showing a live atom count on both sides so you can see exactly when the equation is balanced.

If you have ever stared at an equation like H₂ + O₂ → H₂O and wondered why the numbers feel arbitrary, you are not alone. In my experience teaching chemistry, most students struggle not because the concept is hard, but because no one shows them a clear, repeatable method. This guide fixes that. I will start with a fully solved example, extract the general rules from it, and then give you a printable worksheet to practice on your own.

Start Here: A Fully Solved Example — Balancing H₂ + O₂ → H₂O

Before I explain any rules, look at this complete solution. Read it once, then I will explain every decision.

  REACTANTS                    PRODUCTS
  ---------                    --------
  2H2  +  O2                   2H2O

  H: 2x2 = 4          =        H: 2x2 = 4   [OK]
  O: 1x2 = 2          =        O: 2x1 = 2   [OK]

  Result: BALANCED

Now that you have seen a complete solution, let me explain what each step means and why it works.

What Is Balancing a Chemical Equation?

Balancing a chemical equation is the process of adjusting coefficients so that the total number of atoms of each element is identical on both sides of the reaction arrow. This reflects the law of conservation of mass: atoms are rearranged in a chemical reaction, but they are never created or destroyed.

A coefficient is the large number placed in front of a chemical formula. In 2H₂O, the coefficient is 2, meaning two molecules of water. A subscript is the small number inside a formula — the 2 in H₂O means each water molecule contains two hydrogen atoms. You must never change subscripts because doing so changes the identity of the substance.

How to Use the PhET Balancing Chemical Equations Simulation

The PhET simulation from the University of Colorado Boulder is the best free tool available for practicing equation balancing. Here is exactly how to use it.

1. Go to phet.colorado.edu/en/simulations/balancing-chemical-equations and click the green Play button. No download or account is needed.
2. Choose the “Introduction” tab to start with guided practice, or “Balancing Game” for timed challenge mode.
3. Look at the equation displayed. Below each formula, you will see an atom counter panel showing how many of each atom exist on each side.
4. Use the up/down arrows next to each formula to change its coefficient. The atom counters update in real time.
5. When all atom counts match, the simulation displays a smiley face and the word “Balanced” in green. Mismatched counts show a frowning face and red numbers.
6. Use the “Check” button in game mode to score your answer and move to the next equation.

[IMAGE: Screenshot of PhET Balancing Chemical Equations simulation showing the atom counter panel with green balanced indicators | ALT: PhET balancing chemical equations simulation interface showing live atom count feedback]

Step-by-Step Method for Balancing Any Chemical Equation

This 6-step method works for every equation you will encounter in grades 8-11. I derived it from the worked example above and from years of watching students make the same fixable errors.

1. Write the unbalanced equation with correct formulas. Do not change any subscripts.
2. Count atoms of each element on both sides. Make a small tally table — this prevents counting errors.
3. Identify the most complex molecule (most elements or largest subscripts) and treat it as your anchor with coefficient 1.
4. Balance one element at a time. For combustion reactions: balance carbon first, hydrogen second, oxygen last. For other reactions: start with elements that appear in only one reactant and one product.
5. Recount all atoms after each change. A coefficient change on one molecule affects every element in that molecule.
6. Reduce to lowest whole-number coefficients. If all coefficients are even (e.g., 2, 4, 2), divide by 2. Coefficients must be whole numbers, never fractions.

Three More Fully Worked Examples

Worked Example 2: Ammonia Synthesis (N₂ + H₂ → NH₃)

Worked Example 3: Combustion of Methane (CH₄ + O₂ → CO₂ + H₂O)

Worked Example 4: Iron Oxide Formation (Fe + O₂ → Fe₂O₃)

Common Mistakes When Balancing Equations (Wrong vs. Right)

These are the four errors I see most often. Each one has a clear fix.

The Coefficient Trap: What Most Guides Get Wrong

Practice Worksheet: 10 Balancing Problems

Use this worksheet to practice the method from the worked examples above. Try each problem on paper first, then check your answers with the key below. The problems go from easy to hard.

Instructions: Balance each chemical equation by writing the correct coefficient in front of each formula. Do not change the subscripts. Show your atom counts for each element.

1. H₂ + O₂ → H₂O
2. N₂ + H₂ → NH₃
3. Fe + O₂ → Fe₂O₃
4. CH₄ + O₂ → CO₂ + H₂O
5. Na + Cl₂ → NaCl
6. Al + HCl → AlCl₃ + H₂
7. C₃H₈ + O₂ → CO₂ + H₂O
8. KClO₃ → KCl + O₂
9. Mg + HCl → MgCl₂ + H₂
10. Fe + CuSO₄ → FeSO₄ + Cu

How to use this worksheet: Print the PDF (or work from the screen), attempt each problem without looking at the answer key, then self-check using the key. For any problem you get wrong, go back to the worked examples and identify which step you missed. One round of self-correction is worth three rounds of passive reading.