The balancing equations trap
A free TEAS® chemistry explainer with an original practice question
A balanced equation has the same number of each kind of atom on both sides, because atoms are not created or destroyed in a reaction.
The trap is changing a subscript to make the numbers work. A subscript is part of the formula: H₂O is water, H₂O₂ is hydrogen peroxide, a different substance. You balance only with coefficients, the whole numbers written in front of each formula.
Count each element on both sides, adjust one coefficient at a time, and finish by checking that the coefficients are the smallest whole numbers that work.
Try a sample question
Hydrogen gas reacts with oxygen gas to make water: H₂ + O₂ → H₂O. Which equation is balanced with the smallest whole-number coefficients?
- A.H₂ + O₂ → H₂O₂
- B.2H + O → H₂O
- C.2H₂ + O₂ → 2H₂O
- D.4H₂ + 2O₂ → 4H₂O
Show the answer
C. 2H₂ + O₂ → 2H₂O
The balanced equation is 2H₂ + O₂ → 2H₂O. To balance, you change only the coefficients, the numbers in front. Two water molecules hold 2 oxygen atoms, which matches one O₂. They also hold 4 hydrogen atoms, which matches two H₂.
Why the other answers tempt:
- A. The atoms match, but H₂O₂ is hydrogen peroxide, not water. Changing a subscript changes the substance. Only coefficients can change.
- B. Hydrogen and oxygen gases are made of molecules, H₂ and O₂. Writing them as single atoms changes the reactants.
- D. This is balanced, but every coefficient can be divided by 2. The question asks for the smallest whole numbers.
Coefficients count molecules. Subscripts are part of the name. Never change the name.
Checked against OpenStax Chemistry 2e, 4.1 Writing and Balancing Chemical Equations. Original question, not from a real test.
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