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Grade 9 · lesson 1 of 4

Check whether an equation conserves atoms

Count each type of atom on both sides of a chemical equation.

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Supplied materials

  • Paper-only model; do not obtain, mix or ignite chemicals.
  • Symbols: H is hydrogen, O is oxygen. H₂ contains 2 H atoms; O₂ contains 2 O atoms; H₂O contains 2 H and 1 O. A coefficient multiplies the entire formula.

Learn the idea

During an ordinary chemical reaction, atoms are rearranged into different combinations. A balanced equation has the same count of each element before and after the reaction.

A subscript describes one molecule or formula unit; a coefficient counts how many of that unit are represented. To balance an equation, change coefficients rather than the subscripts that identify the substances.

  1. For 2 H₂ + O₂, count H atoms and O atoms separately.
  2. Count the same elements in 2 H₂O.
  3. Check that each element’s count matches.
  4. Use the counts to choose a balanced equation below.
Worked example

See the method in action

In 2 H₂ + O₂, hydrogen count is 2 × 2 = 4 and oxygen count is 2. In 2 H₂O, hydrogen count is 2 × 2 = 4 and oxygen count is 2 × 1 = 2. Both element counts match.

Your turn

Check your reasoning

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Which equation has equal H and O atom counts on both sides?

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See the answer and explanation

2 H₂ + O₂ → 2 H₂O

2 H₂ + O₂ → 2 H₂O has four H and two O atoms on each side. The other choices have unequal hydrogen or oxygen counts; changing a coefficient affects every atom in that formula.

Take it further

For N₂ + H₂ → NH₃, try coefficients 1, 3 and 2. Check: N₂ + 3 H₂ → 2 NH₃ gives two N and six H atoms on each side.

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Longer courses for learners ages 14+

These related courses have their own preparation and projects. They are not early-grade assignments.

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