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C2.3 · Investigate and represent synthesis, decomposition, and displacement

Learn to investigate and represent synthesis, decomposition, and displacement through clear examples and targeted practice.

Ontario Grade 10 Science

Chemistry: Chemical Reactions

Investigating synthesis, decomposition, and displacement

A bicycle left in damp air may develop rust. A fizzy tablet in water may release bubbles. These observations can suggest that a change has happened, but they do not tell us everything about it. In a chemical reaction, starting substances change into different substances. The starting substances are reactants, and the substances formed are products. This lesson focuses on three ways to describe reaction patterns: synthesis, decomposition, and displacement.

What you will learn

  • Describe the patterns of synthesis, decomposition, and displacement.
  • Represent these reaction patterns with words, simple models, and chemical equations.
  • Use observations as evidence while following safe, supervised investigation practices.

1. Grade 9 bridge: substances and representations

An element contains one kind of atom. A compound contains atoms of different elements joined together. A chemical formula uses element symbols and numbers to represent a substance. For example, each water particle is represented by H2O\mathrm{H_2O}. The small 2 means that each particle has two hydrogen atoms. No small number after O means there is one oxygen atom.
A chemical equation is a short model of a reaction. It shows reactants on the left and products on the right. The arrow means “forms” or “changes into.” A plus sign separates different substances on the same side. A word equation uses substance names. A chemical equation uses formulas.
A particle model is a simplified drawing or description of particles before and after a reaction. It can help show how a reaction pattern works. It is a model, not a direct view of atoms during a reaction. In this lesson, focus on what substances join, break down, or replace one another.
reactants→products\mathrm{reactants \rightarrow products}
  • Reactants are the starting substances; products are the substances formed.
  • Formulas and equations are ways to represent substances and reactions.
  • A particle model can show the pattern of change.

2. Three patterns to recognize

Synthesis means putting together. In a synthesis reaction, two or more simpler substances combine to form one compound. Iron and sulfur can form iron sulfide. A simple model is A+B→AB\mathrm{A + B \rightarrow AB}. The letters stand for substances. This pattern has multiple reactants and one compound product.
Decomposition means breaking down. In a decomposition reaction, one compound forms two or more products. A simple model is AB→A+B\mathrm{AB \rightarrow A + B}. The pattern has one compound reactant and multiple products. The letters show the pattern; they do not name specific substances.
Displacement occurs when one element takes the place of another element in a compound. For example, magnesium can take the place of copper in copper(II) sulfate. The products are magnesium sulfate and copper. A simple model is A+BC→AC+B\mathrm{A + BC \rightarrow AC + B}. In this model, A and B stand for elements, while BC and AC stand for compounds. The key idea is replacement.
A+B→AB;AB→A+B;A+BC→AC+B\mathrm{A + B \rightarrow AB;\quad AB \rightarrow A + B;\quad A + BC \rightarrow AC + B}
  • Synthesis: simpler substances join to form one compound.
  • Decomposition: one compound forms multiple products.
  • Displacement: one element replaces another element in a compound.

3. From observations to a model

An observation is a description of what you noticed. Examples include bubbles, a colour change, a temperature change, or a new solid. An interpretation is an explanation of what an observation may mean. “Bubbles appeared” is an observation. “A gas may have formed” is an interpretation.
Observations are evidence, but one observation may not identify a product or reaction pattern. Bubbles can suggest that a gas formed, but they do not identify which gas. A colour change can signal a change, but it does not name the new substance. Use information about the reactants and products as well as observations.
When the substances are known, represent the change with words, a particle model, or formulas. Then compare the starting and ending patterns. Did simpler substances join into one compound? Did one compound form several products? Did one element take the place of another in a compound? These questions help identify the reaction pattern without guessing from appearance alone.
  • Keep observations separate from interpretations.
  • Use the known substances and the pattern of change to identify a reaction type.
  • A model is useful when it matches the information available.

4. Evidence and safety in an investigation

In a supervised classroom investigation, record what you can see before, during, and after the activity. Note changes such as bubbles, colour, temperature, or the appearance of a solid. These records provide evidence for an interpretation. They do not always reveal exactly which products formed.
Do not identify an unknown product from appearance alone. Different substances can look alike, and a reaction may occur without an obvious visible change. If the products are not known, describe what you observed and say what remains uncertain. A careful conclusion is stronger than a guess.
Follow the teacher’s procedure and school laboratory safety rules. Use only the materials and amounts provided, and wear the protective equipment required for the activity. Do not taste chemicals or try chemical reactions at home. Tell the teacher about spills, breakage, or unexpected changes. Investigations should be supervised.
  • Record evidence carefully before interpreting it.
  • Do not claim that an observation identifies a product unless the evidence supports that claim.
  • Carry out chemical investigations only as directed in a supervised laboratory.

Compare the three reaction patterns

PatternSimple modelWhat changes
SynthesisA+B→AB\mathrm{A + B \rightarrow AB}Two or more simpler substances form one compound.
DecompositionAB→A+B\mathrm{AB \rightarrow A + B}One compound forms multiple products.
DisplacementA+BC→AC+B\mathrm{A + BC \rightarrow AC + B}One element replaces another element in a compound.

Worked example

Synthesis: iron and sulfur

Iron and sulfur form iron sulfide. Represent the reaction with a word equation and a chemical equation, then identify its pattern.
  1. Compare the starting and ending substances
    The reactants are two elements, iron and sulfur. They form one compound, iron sulfide. That change matches the synthesis pattern.
  2. Represent the reaction
    Use substance names for a word equation. Use element symbols and the compound formula for a chemical equation.
    iron+sulfur→iron sulfide\mathrm{iron + sulfur \rightarrow iron\ sulfide}
Answer: The reaction is synthesis. Its word equation is iron + sulfur → iron sulfide, and its chemical equation is Fe+S→FeS\mathrm{Fe + S \rightarrow FeS}.
Check: Two simpler substances form one compound, which is the defining pattern of synthesis.

Worked example

Decomposition: calcium carbonate

When calcium carbonate breaks down, the stated products are calcium oxide and carbon dioxide. Represent the change and identify its pattern.
  1. Identify the pattern
    The starting substance is one compound, calcium carbonate. It forms two products, calcium oxide and carbon dioxide. One compound forming multiple products is decomposition.
  2. Write the representations
    The word equation names the substances. The chemical equation uses their formulas to represent the same stated change.
    calcium carbonate→calcium oxide+carbon dioxide\mathrm{calcium\ carbonate \rightarrow calcium\ oxide + carbon\ dioxide}
Answer: The reaction is decomposition. Its chemical equation is CaCO3→CaO+CO2\mathrm{CaCO_3 \rightarrow CaO + CO_2}.
Check: The representation begins with one compound and shows two products.

Worked example

Displacement: magnesium and copper(II) sulfate

Magnesium reacts with copper(II) sulfate to form magnesium sulfate and copper. Represent the reaction and identify its pattern.
  1. Track the change
    Magnesium is an element, and copper(II) sulfate is a compound. In the stated reaction, magnesium takes the place of copper in that compound. This matches displacement.
  2. Represent the reaction
    Write the substances by name or use their chemical formulas. The representation shows magnesium sulfate and copper as the products.
    Mg+CuSO4→MgSO4+Cu\mathrm{Mg + CuSO_4 \rightarrow MgSO_4 + Cu}
Answer: The reaction is displacement. Magnesium takes the place of copper in copper(II) sulfate.
Check: One element replaces another element in a compound, which is the displacement pattern.

Common mistakes and how to avoid them

Calling every reaction with two reactants synthesis.
Correction: Look at the products and the change. Synthesis forms one compound. Displacement involves an element replacing another element in a compound.
Treating bubbles or a colour change as proof of a particular product.
Correction: Record the observation accurately. Use the known substances and other investigation information before making a claim about products.
Assuming that a particle model is a direct picture of atoms.
Correction: A particle model is a simplified representation. It helps show a pattern but is not a direct view of particles during a reaction.

Lesson summary

  • Synthesis joins simpler substances to form one compound.
  • Decomposition begins with one compound and forms multiple products.
  • Displacement occurs when one element replaces another in a compound.
  • Words, particle models, and chemical formulas can represent these patterns.
  • Observations support investigations, but they do not always identify products.
  • Use safe, supervised procedures for chemical investigations.

Check your understanding

Question 1

One compound forms two products. Which reaction pattern fits?
  1. Synthesis
  2. Decomposition
  3. Displacement
  4. No reaction pattern can fit
Show answer and explanation
Decomposition
Decomposition has one compound reactant and multiple products.

Question 2

A student observes bubbles during a supervised reaction. What can the student conclude from that observation alone?
  1. A gas may have formed, but the bubbles do not identify it.
  2. The gas must be oxygen.
  3. The reaction must be displacement.
  4. The bubbles prove that the products are known.
Show answer and explanation
A gas may have formed, but the bubbles do not identify it.
Bubbles can be evidence that a gas formed, but appearance alone does not identify the gas or the reaction pattern.

Question 3

In the model A+BC→AC+B\mathrm{A + BC \rightarrow AC + B}, what kind of change does the pattern represent?
  1. A compound separates into products.
  2. Two simpler substances form one compound.
  3. One element takes the place of another in a compound.
  4. A product changes into a reactant without a reaction.
Show answer and explanation
One element takes the place of another in a compound.
The model shows A joining with part of the compound while B is released. It represents displacement.

Key terms

Atom
A tiny unit of an element.
Chemical equation
A symbolic representation showing reactants changing into products.
Compound
A substance made of atoms of different elements joined together.
Decomposition
A reaction in which one compound forms two or more products.
Displacement
A reaction in which one element takes the place of another element in a compound.
Observation
A description of what was noticed.
Product
A substance formed in a chemical reaction.
Reactant
A starting substance in a chemical reaction.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 10 Science (SNC2D), expectation C2.3. 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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