C3.2 · Explain equation balancing through atoms and conservation of mass

Ontario Grade 10 Science

Chemistry: Chemical Reactions

How atoms and conservation of mass explain the numbers in a reaction

What you will learn

• Explain why a chemical equation must have the same number of each kind of atom on both sides.
• Connect balanced equations to the conservation of mass.
• Balance simple chemical equations by changing coefficients, not subscripts.
• Check a balanced equation by counting atoms.

1. From a familiar idea to a reaction equation

• Reactants are written before the arrow; products are written after it.
• A chemical formula shows which atoms are present and how many are in one particle.
• An equation is balanced when each kind of atom has the same count on each side.

2. A model for conservation of mass

• Conservation of mass is represented in an equation by matching atom counts on both sides.
• Count each element separately.
• A mismatch means the written equation needs balancing.

3. How to balance without changing the substances

• Coefficients change how many of a substance are represented.
• Subscripts are part of a formula and must not be changed to balance an equation.
• Recount every element after changing a coefficient.

4. Evidence, models, and safe practice

• A model predicts what should be true about atom counts.
• A measurement must account for the matter involved to make a fair comparison.
• Use teacher-approved procedures for any classroom investigation.

Atom-count check for hydrogen forming water

Worked example

Balance a simple formation equation

1. Count the starting atoms
   The reactants contain two hydrogen atoms in one hydrogen molecule and two oxygen atoms in one oxygen molecule. The product has two hydrogen atoms and one oxygen atom in one water molecule.
   $$\mathrm{H_2} + \mathrm{O_2} \rightarrow \mathrm{H_2O}$$
2. Make oxygen counts match
   There are two oxygen atoms on the left but only one on the right. Put a coefficient of two before water. This represents two water molecules and gives two oxygen atoms on the product side.
   $$\mathrm{H_2} + \mathrm{O_2} \rightarrow 2\mathrm{H_2O}$$
3. Match hydrogen and check
   Two water molecules contain four hydrogen atoms, so place a coefficient of two before hydrogen. Both sides now have four hydrogen atoms and two oxygen atoms.
   $$2\mathrm{H_2} + \mathrm{O_2} \rightarrow 2\mathrm{H_2O}$$

Worked example

Balance a reaction with two products

1. Start with the given formulas
   The reactants are methane and oxygen. The products are carbon dioxide and water. Count carbon, hydrogen, and oxygen atoms from the subscripts in each formula.
   $$\mathrm{CH_4} + \mathrm{O_2} \rightarrow \mathrm{CO_2} + \mathrm{H_2O}$$
2. Match carbon and hydrogen
   There is one carbon atom on each side already. Four hydrogen atoms on the left require two water molecules on the right, because each water molecule contains two hydrogen atoms.
   $$\mathrm{CH_4} + \mathrm{O_2} \rightarrow \mathrm{CO_2} + 2\mathrm{H_2O}$$
3. Match oxygen and verify
   The products now contain four oxygen atoms in total: two in carbon dioxide and two in the water molecules. Two oxygen molecules provide four oxygen atoms on the reactant side. The counts match for all three elements.
   $$\mathrm{CH_4} + 2\mathrm{O_2} \rightarrow \mathrm{CO_2} + 2\mathrm{H_2O}$$

Worked example

Spot and correct an invalid change

1. Keep each formula unchanged
   Changing a subscript changes the number of atoms in the formula and therefore represents a different substance. Balancing must preserve the formulas provided for the reactants and products.
   $$\mathrm{H_2O} \neq \mathrm{H_2O_2}$$
2. Adjust the amounts instead
   Use coefficients to change how many molecules are represented. Two water molecules contain four hydrogen atoms and two oxygen atoms, so two hydrogen molecules and one oxygen molecule match those counts.
   $$2\mathrm{H_2} + \mathrm{O_2} \rightarrow 2\mathrm{H_2O}$$

Common mistakes and how to avoid them

Lesson summary

• A chemical equation represents reactants changing into products.
• Atoms are rearranged in a reaction, so each kind of atom must have the same count on both sides.
• Matching atom counts represent conservation of mass.
• Balance equations by changing coefficients, not subscripts.
• Verify a result by counting every element on both sides.

Check your understanding

Key terms

Atom

A small particle that makes up matter. Each element has its own kind of atom.

Chemical formula

A set of symbols and numbers that shows which atoms are in a substance and how many of each are represented.

Chemical equation

A written model showing reactants changing into products.

Reactant

A starting substance in a chemical reaction.

Product

A substance formed in a chemical reaction.

Coefficient

A number placed before a formula to show how many of that substance are represented.

Subscript

A small number within a chemical formula that shows how many atoms of an element are in one particle of the substance.

Conservation of mass

The idea that total mass stays the same during a reaction when all the matter involved is accounted for.

Continue through SNC2D

View the complete SNC2D Ontario Grade 10 Science curriculum and lessons

• C2.6 · Classify everyday substances as acids, bases, or neutral
• C3.3 · Describe evidence that a chemical change has occurred
• C1.1 · Investigate safety and environmental risks of chemical reactions
• C1.2 · Apply reaction knowledge to environmental challenges
• C2.1 · Use reactant, product, compound, and related terminology
• C2.2 · Model and diagram molecules in simple reactions

About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 10 Science (SNC2D), expectation C3.2. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

Official curriculum reference

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