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C2.1 · Use reactant, product, compound, and related terminology

Learn to use reactant, product, compound, and related terminology through clear examples and targeted practice.

Ontario Grade 10 Science

Chemistry: Chemical Reactions

Using reaction vocabulary to describe substances

Think about a simple word-based model: hydrogen and oxygen form water. The names before “form” are the starting substances. Water is the substance formed. In science, the starting substances are called reactants, and the substances formed are called products. Water is also a compound, because it contains more than one kind of element. These words answer different questions: reactant and product describe a role in a reaction; element and compound describe what a substance is made of. This lesson practises those terms using words and simple models, not chemical formulas.

What you will learn

Identify reactants and products in a word-based reaction model.
Explain what the terms element and compound describe.
Tell the difference between a substance’s composition and its role in a reaction.
Use reaction terminology to describe a hypothetical observation safely.

1. Grade 9 bridge: substances, atoms, and elements

A substance is a kind of matter, such as water or oxygen. To describe what a substance is made of, it helps to recall two Grade 9 ideas. An atom is a tiny building block used to describe matter. An element is a substance that contains only one kind of atom. For example, oxygen is an element because it contains oxygen atoms.

A compound is a substance made of two or more different elements joined together. Water is a compound because it contains hydrogen and oxygen. The word compound describes the substance’s composition: what kinds of elements it contains. It does not tell us what the substance is doing in a particular reaction.

A chemical reaction is a change in which starting substances form different substances. The starting substances are called reactants. The substances formed are called products. These names describe the substances’ roles in that reaction. A substance might be a product in one reaction and a reactant in another.

An element contains only one kind of atom.
A compound contains different elements joined together.
Reactant and product name roles in a reaction, not kinds of composition.

2. A word-based model: identify what starts and what forms

Consider this model: hydrogen and oxygen form water. Hydrogen and oxygen are the reactants because they are the starting substances. Water is the product because it is the substance formed. The model describes the reaction using substance names rather than chemical formulas.

A reaction model helps organize the information. First, find the starting substances. Then find the substance or substances formed. In a word-based model, words such as “form” or “produce” can help show that change. Do not mistake the order of the names for a description of their composition.

Now ask a separate question: what is each substance made of? Hydrogen and oxygen are elements. Water is a compound because it contains hydrogen and oxygen. So water is both a product in this model and a compound. Those descriptions do not conflict: one names its role, and the other names its composition.

A plus sign is not needed for this word-based model. When people list more than one starting substance in words, the word “and” simply separates their names. It does not mean they have become one substance.

Reactants are the starting substances in the reaction model.
Products are the substances formed in the reaction model.
A substance can have both a reaction-role description and a composition description.

3. Use two questions to keep the terms clear

When describing a substance, ask two questions. First: what is it made of? Use element or compound when the information lets you decide. Second: what is its role in this reaction? Use reactant or product. Keeping the questions separate prevents common mix-ups.

For example, imagine a word-based model in which carbon and oxygen form carbon dioxide. Carbon and oxygen are the reactants. Carbon dioxide is the product. Carbon dioxide is also a compound because it contains the different elements carbon and oxygen. Carbon and oxygen are elements.

A substance does not stop being a compound when it is used as a reactant. Imagine a model in which water forms hydrogen and oxygen. Water is the reactant in that model, but it remains a compound because its composition has not changed as a description of the substance. The model’s role words depend on the particular reaction.

The word “element” refers to one kind of atom, not to the number of atoms in a sample. A substance made of many atoms of the same element is still an element. A compound requires different elements joined together.

Element or compound answers, “What is it made of?”
Reactant or product answers, “What is its role in this reaction?”
The role can change from one reaction model to another; composition terms describe the substance.

4. Observe and describe safely

In an approved classroom activity, an observable change may provide evidence that a reaction has taken place. Evidence could include a new substance being indicated by the teacher or by the activity’s observations. An observation is what is noticed or recorded; it is not the same as guessing which substances are involved. Use only the evidence supplied in the activity to name reactants and products.

For practice, suppose a teacher provides this hypothetical information: a model says that substance A and substance B form substance C. This is invented practice information, not a measured result. The names A and B stand for the starting substances, and C stands for the substance formed. You can identify the roles from the model, but you cannot decide whether any of them is an element or compound unless information about their composition is also provided.

Models help us discuss reactions, but they are not instructions for carrying them out. Do not try reactions at home. In class, follow the teacher’s directions and the safety rules for the activity. If the evidence does not say what a substance is made of, do not guess its composition.

Use evidence and the reaction model to identify roles.
Do not infer composition when the model gives no composition information.
Practise with hypothetical information; follow classroom safety directions.

Two questions for describing a substance

Question	Useful terms	Example
What is it made of?	Element or compound	Water is a compound.
What is its role in this reaction?	Reactant or product	Water is a product in one model and a reactant in another.

Worked example

Hydrogen and oxygen form water

A word-based model says that hydrogen and oxygen form water. Name the reactants and product. Which substances are elements, and which is a compound?

Name the reaction roles
Hydrogen and oxygen are the starting substances in the model, so they are the reactants. Water is the substance formed, so it is the product.
Describe composition
Hydrogen and oxygen are each elements because each name refers to one kind of atom. Water is a compound because it contains hydrogen and oxygen.

Answer: Hydrogen and oxygen are the reactants and are elements. Water is the product and is a compound.

Check: Reactant and product identify roles in this model. Element and compound describe composition.

Worked example

Carbon dioxide forms from carbon and oxygen

A word-based model says that carbon and oxygen form carbon dioxide. Identify the reactants and product. Is carbon dioxide an element or a compound?

Find the starting substances and substance formed
Carbon and oxygen are the starting substances, so they are the reactants. Carbon dioxide is named as the substance formed, so it is the product.
Use the composition information
Carbon dioxide contains carbon and oxygen, which are different elements. A substance made from different elements joined together is a compound.

Answer: Carbon and oxygen are the reactants. Carbon dioxide is the product and is a compound.

Check: Carbon dioxide can be described as a product and as a compound because the terms answer different questions.

Worked example

Water is a reactant in another model

A word-based model says that water forms hydrogen and oxygen. Identify the reactant and products. Is water an element or a compound?

Identify the roles in this model
Water is the starting substance, so it is the reactant. Hydrogen and oxygen are the substances formed, so they are the products.
Describe water’s composition
Water contains hydrogen and oxygen. Since these are different elements joined together, water is a compound, even though it is the reactant in this model.

Answer: Water is the reactant and is a compound. Hydrogen and oxygen are the products and are elements.

Check: Water’s role is reactant in this model. Its composition description remains compound.

Common mistakes and how to avoid them

Calling every compound a product.

Correction: A compound can be a reactant or a product. Check its role in the particular reaction model.

Calling a substance a compound because it contains many atoms.

Correction: A compound contains different elements joined together. Many atoms of one element do not make a compound.

Treating reactant and product as types of substance.

Correction: They describe a substance’s role in a reaction. Element and compound describe what it is made of.

Deciding whether a substance is an element or compound when the model gives no composition information.

Correction: Use only the information provided. A substance’s reaction role does not reveal its composition.

Lesson summary

Reactants are the starting substances in a reaction; products are the substances formed.
An element contains one kind of atom. A compound contains different elements joined together.
Composition and reaction role are separate descriptions.
A substance can be a product in one reaction and a reactant in another.

Check your understanding

Question 1

A word-based model says that magnesium and oxygen form magnesium oxide. Which substance is the product?

Magnesium
Oxygen
Magnesium oxide
Magnesium and oxygen

Show answer and explanation

Magnesium oxide

Magnesium oxide is named as the substance formed, so it is the product.

Question 2

A model says that carbon and oxygen form carbon dioxide. Which description fits carbon dioxide?

A reactant and an element
A product and a compound
A reactant and a compound
A product and an element

Show answer and explanation

A product and a compound

Carbon dioxide is the substance formed, so it is a product. It contains different elements, so it is a compound.

Question 3

A model says that water forms hydrogen and oxygen. What is water’s role, and what kind of substance is it?

A reactant and an element
A product and an element
A reactant and a compound
A product and a compound

Show answer and explanation

A reactant and a compound

Water is the starting substance, so it is a reactant. It contains hydrogen and oxygen, so it is a compound.

Key terms

Substance: A kind of matter, such as water or oxygen.
Atom: A tiny building block used to describe matter.
Element: A substance that contains only one kind of atom.
Compound: A substance made of different elements joined together.
Reactant: A starting substance in a chemical reaction.
Product: A substance formed in a chemical reaction.
Observation: Something noticed or recorded during an activity.
Model: A simplified way to represent or describe something.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 10 Science (SNC2D), expectation C2.1. 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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