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C3.1 · Identify synthesis, decomposition, displacement, and combustion reactions

Learn to identify synthesis, decomposition, displacement, and combustion reactions through clear examples and targeted practice.

Ontario Grade 11 Chemistry

Chemical Reactions

Ontario Grade 11 Chemistry — study topic C3.1

A burning material may give off light or heat and form new substances. A metal placed in a suitable solution may become coated as the solution changes. Chemists describe these changes with reaction equations. A reaction type is a name for a recognizable pattern in how reactants become products. In this lesson, you will learn to identify four such patterns by reading the complete equation.

What you will learn

  • Identify synthesis, decomposition, displacement, and combustion reactions from their reactants and products.
  • Use particle-level changes and equation patterns to explain each identification.
  • Recognize that one reaction can fit more than one description.

1. Read the equation before naming the reaction

A chemical reaction changes starting substances, called reactants, into new substances, called products. Reactants are written to the left of the arrow, and products are written to the right. The arrow means “forms” or “produces.”
A chemical formula uses element symbols and subscripts to show which atoms, and how many, are in a substance. For example, each water molecule contains two hydrogen atoms and one oxygen atom. A coefficient is a number in front of a formula. It counts particles or formula units. It does not change the substance’s formula.
At the particle level, atoms are rearranged during a chemical reaction. They are not created or destroyed. A balanced equation has the same number of each kind of atom on both sides. Balance an equation by changing coefficients, not subscripts. State symbols can show whether a substance is solid, liquid, gas, or dissolved in water.
reactants→products\text{reactants} \rightarrow \text{products}
  • Reactants are on the left; products are on the right.
  • Identify a reaction from the full pattern across the arrow.
  • A balanced equation has the same count of each kind of atom on both sides.

2. Synthesis and decomposition: combining or separating

In a synthesis reaction, two or more reactants combine to form one product. At the particle level, atoms or groups of atoms from the starting substances are rearranged into a single product. For example, magnesium reacts with oxygen to form solid magnesium oxide. The equation shows two reactants forming one product.
In a decomposition reaction, one reactant forms two or more products. The atoms in the starting substance are rearranged into multiple substances. For example, hydrogen peroxide can decompose into water and oxygen gas.
These patterns focus on the number of reactants and products. The general forms are shown below. In these patterns, letters stand for substances; they are not element symbols or complete chemical formulas.
2Mg(s)+O2(g)→2MgO(s)2H2O2(aq)→2H2O(l)+O2(g)\mathrm{2Mg(s) + O_2(g) \rightarrow 2MgO(s)}\qquad\mathrm{2H_2O_2(aq) \rightarrow 2H_2O(l) + O_2(g)}
  • Synthesis: multiple reactants form one product.
  • Decomposition: one reactant forms multiple products.
  • Check both sides of the arrow; the number of atoms in a formula is not the reaction pattern.

3. Displacement: replacing or exchanging

In a single displacement reaction, an element replaces another element in a compound. A compound is a substance made of more than one element chemically joined. The general pattern is an element plus a compound forming a different compound and a different element.
For example, iron can replace copper in copper(II) sulfate solution. At the particle level, iron atoms take copper’s place in the compound, and copper forms solid metal. The equation shows iron and copper changing partners across the arrow.
A double displacement reaction has two compounds exchanging partners. For example, aqueous silver nitrate and sodium chloride form solid silver chloride and aqueous sodium nitrate. In this lesson, displacement includes both the single-element replacement pattern and the two-compound exchange pattern. Do not label a reaction displacement just because a metal appears; look for the replacement or exchange.
Fe(s)+CuSO4(aq)→FeSO4(aq)+Cu(s)\mathrm{Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)}
  • Single displacement: an element takes another element’s place in a compound.
  • Double displacement: two compounds exchange partners.
  • Read the formulas on both sides to see what changed.

4. Combustion: reacting with oxygen

Combustion is a reaction in which a substance reacts with oxygen and releases energy, often as heat or light. Burning is a familiar observable example. The presence of oxygen as a reactant is an important clue, but oxygen alone does not establish that a reaction is combustion. Read the whole equation and consider whether it represents a combustion reaction.
For example, magnesium burns in oxygen to form magnesium oxide. This reaction is combustion because magnesium reacts with oxygen and releases energy. It also fits the synthesis pattern because two reactants form one product. Reaction-type descriptions can overlap: one describes a particular feature, while another describes the overall number pattern.
When asked to identify a reaction type, state the pattern that the equation supports. If more than one description fits, name both when the question allows it.
2Mg(s)+O2(g)→2MgO(s)\mathrm{2Mg(s) + O_2(g) \rightarrow 2MgO(s)}
  • Combustion involves a substance reacting with oxygen and releasing energy.
  • Do not identify combustion from the mere appearance of oxygen somewhere in an equation.
  • A reaction may be both combustion and synthesis.

Worked example

Classify three reaction equations

Identify the reaction pattern or patterns shown by each equation: potassium chlorate forms potassium chloride and oxygen; zinc reacts with hydrochloric acid to form zinc chloride and hydrogen; carbon reacts with oxygen to form carbon dioxide.
  1. Count the starting and ending substances
    The first equation has one reactant and two products. That pattern is decomposition. The coefficients show that potassium, chlorine, and oxygen atoms are conserved.
    2KClO3(s)→2KCl(s)+3O2(g)\mathrm{2KClO_3(s) \rightarrow 2KCl(s) + 3O_2(g)}
  2. Look for an element replacing part of a compound
    In the second equation, zinc is an element and hydrochloric acid is a compound. Zinc takes hydrogen’s place, forming zinc chloride and hydrogen gas. This is single displacement.
    Zn(s)+2HCl(aq)→ZnCl2(aq)+H2(g)\mathrm{Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)}
  3. Check for combustion and other matching patterns
    In the third equation, carbon reacts with oxygen and releases energy, so it is combustion. It also has the synthesis pattern: two reactants form one product.
    C(s)+O2(g)→CO2(g)\mathrm{C(s) + O_2(g) \rightarrow CO_2(g)}
Answer: The first reaction is decomposition. The second is single displacement. The third is combustion and also has the synthesis pattern.
Check: All three equations are balanced. The first has 2 potassium, 2 chlorine, and 6 oxygen atoms on each side. The second has 1 zinc, 2 hydrogen, and 2 chlorine atoms on each side. The third has 1 carbon and 2 oxygen atoms on each side.

Common mistakes and how to avoid them

Changing a subscript to balance an equation.
Correction: Change coefficients only. A subscript is part of a substance’s formula, so changing it changes the substance.
Calling any reaction with two reactants synthesis.
Correction: Check the products too. Synthesis has multiple reactants forming one product.
Calling every reaction that contains oxygen combustion.
Correction: Combustion involves a substance reacting with oxygen and releasing energy. Check the whole equation; oxygen among the products is not enough.
Confusing single displacement with double displacement.
Correction: In single displacement, an element replaces another in a compound. In double displacement, two compounds exchange partners.

Lesson summary

  • Synthesis combines multiple reactants to form one product.
  • Decomposition changes one reactant into multiple products.
  • Displacement involves an element replacing another in a compound, or two compounds exchanging partners.
  • Combustion involves a substance reacting with oxygen and releasing energy.
  • Read the whole equation. A reaction can fit more than one pattern.

Check your understanding

Question 1

Which pattern best describes this reaction: sodium reacts with chlorine to form sodium chloride?
  1. Synthesis
  2. Decomposition
  3. Single displacement
  4. Double displacement
Show answer and explanation
Synthesis
Two reactants combine to form one product, so the pattern is synthesis.

Question 2

Which pattern best describes calcium carbonate forming calcium oxide and carbon dioxide?
  1. Combustion
  2. Decomposition
  3. Single displacement
  4. Synthesis
Show answer and explanation
Decomposition
One reactant forms two products, so the pattern is decomposition.

Question 3

Which pattern best describes aluminum replacing copper in copper(II) chloride to form aluminum chloride and copper?
  1. Decomposition
  2. Double displacement
  3. Single displacement
  4. Combustion
Show answer and explanation
Single displacement
Aluminum is an element that takes copper’s place in a compound. The balanced equation is 2Al(s)+3CuCl2(aq)→2AlCl3(aq)+3Cu(s)\mathrm{2Al(s) + 3CuCl_2(aq) \rightarrow 2AlCl_3(aq) + 3Cu(s)}.

Key terms

Reactant
A starting substance in a chemical reaction.
Product
A substance formed in a chemical reaction.
Coefficient
A number in front of a formula that counts particles or formula units.
Synthesis
A reaction pattern in which multiple reactants form one product.
Decomposition
A reaction pattern in which one reactant forms multiple products.
Displacement
A reaction pattern in which an element replaces another in a compound, or two compounds exchange partners.
Combustion
A reaction in which a substance reacts with oxygen and releases energy.

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 11 Chemistry (SCH3U), expectation C3.1. 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.

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