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Chemistry · Grades 7–10

Balance equations without changing formulas

Use coefficients to conserve atom counts.

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Learn the idea

Conservation

A chemical equation must preserve each element and net charge.

Coefficients

A coefficient multiplies the whole formula that follows it.

Subscripts

Changing a subscript changes a chemical formula rather than balancing it.

Check

Count every element separately on each side; use the smallest whole-number coefficients when requested.

Worked examples

In 2H₂ + O₂ → 2H₂O, how many hydrogen atoms are represented on the product side?

The coefficient 2 multiplies the subscript 2, giving four H atoms.

4

Balance __ H₂ + O₂ → __ H₂O using the smallest whole numbers. What is the H₂ coefficient?

2H₂ + O₂ → 2H₂O balances four H atoms and two O atoms.

2

Check your understanding

  1. Why cannot H₂O be changed to H₂O₂ merely to balance oxygen?

    1. That would change the substance.
    2. It only changes the font.
    3. Subscripts never mean anything.

  2. In 3CO₂, how many oxygen atoms are represented?

  3. For N₂ + 3H₂ → 2NH₃, which atom count is equal on both sides?

    1. One hydrogen atom.
    2. Three nitrogen atoms.
    3. Six hydrogen atoms.

Answers and marking notes

Answers and marking notes
  1. That would change the substance.

    Water and hydrogen peroxide have different compositions.

  2. 6

    Three molecules, each with two O atoms, represent six O atoms.

  3. Six hydrogen atoms.

    3H₂ has six H atoms and 2NH₃ has six H atoms.

Further reading and review references

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Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.