DoAssignment study guide

C3.5 · Identify reactants and products in common reaction types

Learn to identify reactants and products in common reaction types through clear examples and targeted practice.

Ontario Grade 10 Science

Chemistry: Chemical Reactions

Recognize what goes in and what forms in familiar reaction patterns

When iron reacts with oxygen, the starting materials change into a new substance: rust. In a chemical reaction, the substances present at the start are called reactants. The new substances formed are called products. This lesson uses simple reaction patterns to help you identify each one. You do not need to predict every reaction or explain every detail of how it happens.

What you will learn

Identify reactants and products in a chemical equation.
Use the position of substances and the reaction pattern to name reactants and products.
Recognize common patterns such as synthesis, decomposition, displacement, and combustion.

1. From starting materials to new substances

In Grade 9, you learned that a chemical change forms one or more new substances. A colour change, gas bubbles, or a solid forming in a liquid can be clues that a reaction may have occurred. A clue is not, by itself, proof of exactly which substances formed.

A chemical equation is a short model of a reaction. The substances written before the arrow are the reactants. The substances written after the arrow are the products. The arrow means “changes into” or “forms.” For example, in

\mathrm{A + B \rightarrow C}

, A and B are reactants and C is a product. The letters here stand for substances; they are not chemical formulas.

Reactants and products are identified by their places in the equation, not by whether they are solids, liquids, or gases. A reaction can have one reactant or several, and one product or several.

\text{reactants} \rightarrow \text{products}

Reactants are on the left side of the arrow.
Products are on the right side of the arrow.
The plus sign separates substances on the same side.

2. Common reaction patterns

A reaction pattern is a familiar arrangement of reactants and products. The pattern can help you read an equation, but it does not replace checking which substances are actually written on each side.

In a synthesis reaction, two or more reactants join to make one product. In a decomposition reaction, one reactant breaks down into two or more products. These names describe the number and arrangement of substances, not how quickly the reaction happens.

In a single-displacement reaction, one element reacts with a compound and takes the place of one element in that compound. In a double-displacement reaction, two compounds exchange parts and form two new compounds. These labels describe common patterns in equations.

In a combustion reaction, a fuel reacts with oxygen. In the common pattern for a fuel containing carbon and hydrogen, the products include carbon dioxide and water. Always identify the products shown in the equation rather than assuming that every reaction has the same products.

\mathrm{A + B \rightarrow AB}

Synthesis: several reactants form one product.
Decomposition: one reactant forms several products.
Single displacement: an element and a compound form a different element and compound.
Double displacement: two compounds form two different compounds.
Combustion: a fuel reacts with oxygen; common carbon-and-hydrogen fuels form carbon dioxide and water.

3. Read the model, then use evidence carefully

A useful reading routine is to find the arrow first. Name everything on its left as a reactant, then name everything on its right as a product. Next, look at the pattern. Does one product form from several reactants? Does one reactant split? Do elements or compounds exchange parts?

For instance, a diagram might show two kinds of particles before a reaction and a new kind of particle afterward. That picture can help you visualize a change. The equation is still the key for naming the reactants and products: particles or formulas before the arrow are starting substances, and those after it are formed substances.

Scientists use observations to decide whether a chemical change may have taken place. Gas production, a new solid, or a lasting colour change can be evidence to investigate. A safe classroom investigation follows teacher instructions and uses approved materials and protective equipment. Do not mix household chemicals to try to make a reaction. Some mixtures can release harmful substances or heat.

Use the arrow before naming the substances.
Use the reaction pattern as a second clue.
Observations can suggest a reaction, but do not identify products unless evidence or a model supports that identification.

Quick pattern guide

Reaction pattern	Reactant arrangement	Product arrangement
Synthesis	Two or more substances	One substance
Decomposition	One substance	Two or more substances
Single displacement	Element and compound	Different element and compound
Double displacement	Two compounds	Two different compounds
Combustion	Fuel and oxygen	Commonly carbon dioxide and water for fuels with carbon and hydrogen

Worked example

Synthesis: magnesium and oxygen

In the equation below, identify the reactants and product. Name the reaction pattern.

Find the arrow
The arrow separates the starting substances from the substances formed.
$\mathrm{2Mg + O_2 \rightarrow 2MgO}$
Name each side
Magnesium and oxygen are on the left, so they are the reactants. Magnesium oxide is on the right, so it is the product.
Recognize the pattern
Two reactants form one product. That arrangement is synthesis.

Answer: The reactants are magnesium and oxygen. The product is magnesium oxide. This is a synthesis reaction.

Check: The number in front of a formula does not change which substance the formula names.

Worked example

Decomposition: hydrogen peroxide

Identify the reactant and products in this decomposition equation.

Locate the starting side
Hydrogen peroxide appears by itself on the left side, so it is the reactant.
$\mathrm{2H_2O_2 \rightarrow 2H_2O + O_2}$
Locate the formed substances
Water and oxygen appear on the right side, so both are products.
Check the pattern
One kind of reactant forms two kinds of products. This is decomposition.

Answer: The reactant is hydrogen peroxide. The products are water and oxygen. This is a decomposition reaction.

Check: Oxygen is a product here because it is written to the right of the arrow.

Worked example

Double displacement: two compounds

Read the equation and identify both reactants and both products. Then name its pattern.

Read the left side
Silver nitrate and sodium chloride are on the left, so they are the reactants.
$\mathrm{AgNO_3 + NaCl \rightarrow AgCl + NaNO_3}$
Read the right side
Silver chloride and sodium nitrate are on the right, so they are the products.
Compare the pattern
Two compounds appear as reactants and two new compounds appear as products. Their parts have exchanged in the equation, so this is double displacement.

Answer: The reactants are silver nitrate and sodium chloride. The products are silver chloride and sodium nitrate. This is a double-displacement reaction.

Check: Do not call silver chloride a reactant: it is on the product side.

Common mistakes and how to avoid them

Calling the substance on the right a reactant.

Correction: Use the arrow: reactants are on the left, and products are on the right.

Thinking a plus sign means the substances are products.

Correction: A plus sign separates substances on the same side. The arrow marks the change from reactants to products.

Using a reaction pattern to ignore the formulas shown.

Correction: First read the substances on each side. Then use the pattern to describe the reaction.

Lesson summary

Reactants are the starting substances written before the arrow.
Products are the substances formed and written after the arrow.
Common patterns include synthesis, decomposition, single displacement, double displacement, and combustion.
Observations may suggest a chemical change, while an equation identifies the reactants and products in the model.

Check your understanding

Question 1

For

\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}

, which substances are the products?

Iron and copper sulfate
Iron sulfate and copper
Iron and iron sulfate
Copper sulfate and copper

Show answer and explanation

Iron sulfate and copper

Iron sulfate and copper are written to the right of the arrow, so they are the products.

Question 2

An equation shows one reactant changing into two products. Which common pattern does this describe?

Synthesis
Decomposition
Single displacement
Combustion

Show answer and explanation

Decomposition

Decomposition has one reactant forming two or more products.

Question 3

In a common combustion equation for a fuel containing carbon and hydrogen, which substance is a reactant?

Carbon dioxide
Water
Oxygen
Neither carbon dioxide, water, nor oxygen

Show answer and explanation

Oxygen

Oxygen is on the reactant side in the common combustion pattern. Carbon dioxide and water are products.

Key terms

Chemical change: A change in which one or more new substances form.
Chemical equation: A symbolic model that shows reactants changing into products.
Reactant: A starting substance in a chemical reaction.
Product: A substance formed in a chemical reaction.
Synthesis: A reaction pattern in which two or more reactants form one product.
Decomposition: A reaction pattern in which one reactant forms two or more products.
Displacement: A reaction pattern in which an element or parts of compounds are replaced or exchanged.
Combustion: A reaction pattern in which a fuel reacts with oxygen.

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 C3.5. 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