DoAssignment.ca

DoAssignment study guide

C3.4 · Write word equations and balance simple chemical equations

Learn to write word equations and balance simple chemical equations through clear examples and targeted practice.

Ontario Grade 10 Science

Chemistry: Chemical Reactions

Grade 10 Science — C3.4

When a chemical change happens, the starting substances change into new substances. A word equation describes that change in words. A chemical equation uses formulas. A balanced chemical equation shows the same number of each kind of atom before and after the change. In this lesson, you will practise writing and balancing simple equations. The examples are models for learning, not instructions to carry out reactions.

What you will learn

  • Write a word equation that names the reactants and products.
  • Use chemical formulas to represent a simple chemical equation.
  • Balance an equation by changing coefficients, not subscripts.
  • Check that each kind of atom is present in equal numbers on both sides.

1. Grade 9 bridge: substances and atoms

A substance is a kind of matter with its own name and properties. In a chemical change, atoms are rearranged to form different substances. The atoms are not lost or made in the change. This idea helps explain why a chemical equation must balance.
A chemical formula shows which atoms make up a substance. For example, water has the formula H2O\mathrm{H_2O}. The small number written low after a symbol is a subscript. It tells how many atoms of that element are in one particle of the substance. If there is no subscript, the count is one. So one water particle contains two hydrogen atoms and one oxygen atom.
An element is a substance made of one kind of atom. Its chemical symbol is a short form of its name. For example, H\mathrm{H} stands for hydrogen and O\mathrm{O} stands for oxygen. Learn a formula as a whole: a subscript belongs to the symbol just before it.
  • Atoms are rearranged in a chemical change.
  • A formula tells which kinds of atoms are present and how many are in one particle.
  • A subscript is part of a formula and must not be changed when balancing.

2. From an observable change to a word equation

In a chemical change, evidence may include a colour change, bubbles, a temperature change, or a new solid. One observation alone may not prove which substances formed. The equation describes the reaction being considered; it does not replace careful observation.
A word equation names the starting substances and the substances formed. The starting substances are called reactants. The substances formed are called products. Write reactants on the left and products on the right, with an arrow between them. Read the arrow as “yields” or “produces.”
For example, imagine a classroom model of hydrogen reacting with oxygen to form water. The word equation is hydrogen + oxygen → water. The plus sign separates substances that react together. It does not mean that the substances are simply being added like numbers.
When more than one reactant or product is present, use a plus sign between their names. Keep the names accurate. A word equation is not balanced by counting letters in the names; it is a description of the chemical change. To check atom counts, use formulas.
  • Reactants are the starting substances; products are the substances formed.
  • The arrow points from reactants to products.
  • Word equations describe a reaction using substance names.

3. Build and balance a simple chemical equation

A chemical equation uses formulas in place of substance names. To balance it, count each kind of atom on both sides. If the counts differ, place a whole-number coefficient in front of a formula. A coefficient tells how many particles of that substance are represented. If no coefficient is written, it is understood to be one.
The coefficient multiplies the entire formula. For example, 2H2O2\mathrm{H_2O} represents two water particles: four hydrogen atoms and two oxygen atoms. The subscript still belongs to the formula. A coefficient changes the number of particles; a subscript changes the substance itself.
A useful method is to write the correct formulas first, then count atoms on each side. Adjust coefficients until the counts match. Recount after every change, because a coefficient affects every atom in that formula. At the end, check each element separately and use the smallest whole-number coefficients when possible.
Do not try to fix an unequal count by changing a formula’s subscripts. That would describe a different substance. For instance, changing water from H2O\mathrm{H_2O} to H2O2\mathrm{H_2O_2} does not balance water; it gives a different formula. Balancing preserves the identities of the substances and adjusts only how many particles are represented.
  • Balance by changing coefficients in front of formulas.
  • Never change a subscript to balance an equation.
  • Check the number of each kind of atom on both sides.

4. Check the model and use it safely

A balanced equation is an atom-count model of a chemical change. It shows that the same kinds and numbers of atoms are represented before and after the reaction. It does not say that the substances are identical on both sides: their atoms may be joined in different arrangements, which is why new substances can form.
In class, a teacher may use observations, diagrams, or safe demonstrations to connect a reaction with its equation. Follow teacher instructions and classroom safety rules. Do not attempt chemical reactions at home based on an equation. Some substances and reactions can be hazardous, even when the written equation looks simple.
A hypothetical equation is useful for practising atom counts, but it is not a measured result. When describing an actual investigation, report only observations that were made. Do not claim that a colour change or bubbles identify a particular product unless the evidence supports that conclusion.
  • A balanced equation represents equal atom counts, not identical substances.
  • Equations are models and do not replace observations.
  • Use chemical reactions only in a supervised, appropriate setting.

Worked example

Hydrogen and chlorine form hydrogen chloride

Write the word equation, then balance the formula equation for hydrogen reacting with chlorine to form hydrogen chloride. Treat this as a paper model, not an instruction to perform the reaction.
  1. Name the change
    Put the starting substances before the arrow and the formed substance after it. This gives a word equation before any atom counting is needed.
    hydrogen+chlorine→hydrogen chloride\text{hydrogen} + \text{chlorine} \rightarrow \text{hydrogen chloride}
  2. Write formulas
    Hydrogen and chlorine each have two atoms in one molecule, so their formulas are H2\mathrm{H_2} and Cl2\mathrm{Cl_2}. Hydrogen chloride has one atom of each kind in its formula, HCl\mathrm{HCl}.
    H2+Cl2→HCl\mathrm{H_2 + Cl_2 \rightarrow HCl}
  3. Match the atom counts
    There are two hydrogen atoms and two chlorine atoms on the left, but only one of each on the right. Put a coefficient of two before hydrogen chloride. This changes the particle count without changing its formula.
    H2+Cl2→2HCl\mathrm{H_2 + Cl_2 \rightarrow 2HCl}
Answer: The balanced equation is H2+Cl2→2HCl\mathrm{H_2 + Cl_2 \rightarrow 2HCl}. The word equation is hydrogen + chlorine → hydrogen chloride.
Check: The left has two hydrogen atoms and two chlorine atoms. The right has two hydrogen atoms and two chlorine atoms.

Worked example

Aluminum and oxygen form aluminum oxide

Write the word equation, then balance the formula equation for aluminum reacting with oxygen to form aluminum oxide. Use atom counts to explain the coefficients.
  1. Write the word equation
    The reactants are aluminum and oxygen. The product is aluminum oxide.
    aluminum+oxygen→aluminum oxide\text{aluminum} + \text{oxygen} \rightarrow \text{aluminum oxide}
  2. Write the formula equation
    The formulas are Al\mathrm{Al} for aluminum, O2\mathrm{O_2} for oxygen, and Al2O3\mathrm{Al_2O_3} for aluminum oxide. These formulas must stay unchanged while balancing.
    Al+O2→Al2O3\mathrm{Al + O_2 \rightarrow Al_2O_3}
  3. Balance oxygen, then aluminum
    The product formula contains three oxygen atoms, while oxygen enters in groups of two. Six oxygen atoms can be counted on both sides by using three oxygen molecules and two aluminum oxide units. Those two product units contain four aluminum atoms, so place four before aluminum.
    4Al+3O2→2Al2O3\mathrm{4Al + 3O_2 \rightarrow 2Al_2O_3}
Answer: The balanced equation is 4Al+3O2→2Al2O3\mathrm{4Al + 3O_2 \rightarrow 2Al_2O_3}. The word equation is aluminum + oxygen → aluminum oxide.
Check: Each side has four aluminum atoms and six oxygen atoms.

Worked example

Sodium and chlorine form sodium chloride

A worksheet gives the word description sodium reacts with chlorine to form sodium chloride. Write the word equation and balance its formula equation.
  1. Set up the word equation
    Write the named reactants to the left and the named product to the right. This keeps the direction of the change clear.
     sodium+chlorine→sodium chloride\text{ sodium} + \text{chlorine} \rightarrow \text{sodium chloride}
  2. Replace names with formulas
    Use Na\mathrm{Na} for sodium, Cl2\mathrm{Cl_2} for chlorine, and NaCl\mathrm{NaCl} for sodium chloride. Chlorine’s formula has two chlorine atoms, so count both before balancing.
    Na+Cl2→NaCl\mathrm{Na + Cl_2 \rightarrow NaCl}
  3. Balance both elements
    The left has two chlorine atoms, so place two before sodium chloride. That gives two sodium atoms on the right, so place two before sodium on the left.
    2Na+Cl2→2NaCl\mathrm{2Na + Cl_2 \rightarrow 2NaCl}
Answer: The balanced equation is 2Na+Cl2→2NaCl\mathrm{2Na + Cl_2 \rightarrow 2NaCl}. The word equation is sodium + chlorine → sodium chloride.
Check: Each side has two sodium atoms and two chlorine atoms.

Common mistakes and how to avoid them

Changing a subscript to make the atom counts match.
Correction: Keep each substance’s formula fixed. Change coefficients in front of formulas instead.
Forgetting that a coefficient applies to every atom in a formula.
Correction: Multiply the coefficient by each subscript when counting. For example, two water particles contain four hydrogen atoms and two oxygen atoms.
Counting a formula with no written coefficient as zero.
Correction: An unwritten coefficient means one particle of that substance.
Writing a chemical equation without first identifying the reactants and products.
Correction: Use the word equation to establish which substances start and which substance or substances form.

Lesson summary

  • A word equation names reactants and products, with an arrow showing the direction of the change.
  • A chemical equation uses formulas to represent those substances.
  • Balance by adjusting coefficients so each kind of atom has an equal count on both sides.
  • Never change subscripts to balance an equation.

Check your understanding

Question 1

Which word equation correctly describes carbon reacting with oxygen to form carbon dioxide?
  1. carbon dioxide → carbon + oxygen
  2. carbon + oxygen → carbon dioxide
  3. carbon + carbon dioxide → oxygen
  4. oxygen → carbon + carbon dioxide
Show answer and explanation
carbon + oxygen → carbon dioxide
Carbon and oxygen are the starting substances, so they are reactants on the left. Carbon dioxide is the product on the right.

Question 2

Which equation is balanced for nitrogen reacting with hydrogen to form ammonia?
  1. N2+H2→NH3\mathrm{N_2 + H_2 \rightarrow NH_3}
  2. N2+3H2→2NH3\mathrm{N_2 + 3H_2 \rightarrow 2NH_3}
  3. 2N2+H2→2NH3\mathrm{2N_2 + H_2 \rightarrow 2NH_3}
  4. N2+2H2→NH3\mathrm{N_2 + 2H_2 \rightarrow NH_3}
Show answer and explanation
N2+3H2→2NH3\mathrm{N_2 + 3H_2 \rightarrow 2NH_3}
The second option has two nitrogen atoms and six hydrogen atoms on each side.

Question 3

When balancing an equation, which part should you change?
  1. A formula’s subscript
  2. The element symbols
  3. A coefficient in front of a formula
  4. The names of the products
Show answer and explanation
A coefficient in front of a formula
Coefficients change how many particles are represented while keeping each substance’s formula the same.

Key terms

Atom
A tiny unit that makes up an element and is counted in a chemical formula.
Chemical formula
A set of element symbols and subscripts that represents a substance.
Chemical equation
A symbolic representation of a chemical change, using formulas and an arrow.
Coefficient
A number placed before a formula to show how many particles of that substance are represented.
Reactant
A starting substance in a chemical change.
Product
A substance formed in a chemical change.
Subscript
A small number in a formula that shows how many atoms of an element are in one particle.
Word equation
A description of a chemical change using the names of the reactants and products.

Continue through SNC2D

View the complete SNC2D Ontario Grade 10 Science curriculum and lessons

About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 10 Science (SNC2D), expectation C3.4. 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

Report a correction or ask a question