Tenth Grade · Science · week 21

Chemical equations and balancing

Balance chemical equations by adjusting coefficients so that the same number of each type of atom appears on both sides, without ever changing a subscript.

A chemical equation is a shorthand sentence describing a reaction: reactants (the starting substances) on the left, an arrow meaning "produces" or "yields," and products (the new substances formed) on the right. The law of conservation of mass says atoms are never created or destroyed in a chemical reaction, only rearranged -- so a correctly written equation must show exactly the same number of each type of atom on both sides.

Balancing an equation means adding coefficients (whole numbers placed in front of a formula) until the atom counts match on both sides. The one absolute rule: NEVER change a subscript to balance an equation -- changing a subscript changes what substance the formula represents entirely (H2O is water; H2O2 is an entirely different substance, hydrogen peroxide). Only coefficients, which show HOW MANY of that whole formula you have, are allowed to change.

A reliable method: write the correct, unchanged formulas for every reactant and product first, then balance one element at a time, usually saving the element that appears in the most different formulas for last, and double-checking every element's count on both sides at the very end.

📋 For the teacher
Teach it (10–15 min):
  • Run this as a real, hands-on lab in words: put a small amount of baking soda and vinegar in a sealed sandwich bag, weigh the sealed bag before and after the reaction happens inside it. The mass stays the same, even though a gas (carbon dioxide) is visibly produced -- direct evidence for conservation of mass, since nothing escaped the sealed bag.
  • Have them physically point to and tally each element on both sides of an equation with a pencil, writing the running count above each formula, rather than trying to balance equations purely in their head -- the visible tally catches mistakes early.
  • When they are stuck, suggest balancing the element that appears in the fewest different formulas first, and saving any element that appears in three or more formulas for last.
  • Praise the RECHECK step specifically as its own habit, separate from the balancing itself -- a balanced-looking equation that was never rechecked is exactly where careless errors survive.
Talk about it:
  • What does the law of conservation of mass require of a correctly balanced equation?
  • Why can coefficients change but subscripts cannot?
  • What is a reliable order for deciding which element to balance first?

Mastery looks like: They correctly balance chemical equations using only coefficients, and verify their answer by recounting every element on both sides.

Common stumbles: Changing a subscript to try to balance an equation; forgetting to recheck ALL elements (not just the one just adjusted) after changing a coefficient.

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