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Reduction

Reduction is a change in which a substance loses oxygen, gains hydrogen, or gains electrons.

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Student-friendly explanation

Reduction is the opposite side of oxidation. In school chemistry, it is seen when oxygen is removed from a compound or electrons are gained by a substance. Reduction and oxidation usually appear together in the same reaction.

How to write this in exams

  1. 1

    Start with the exact idea

    Reduction is a change in which a substance loses oxygen, gains hydrogen, or gains electrons.

  2. 2

    Then show how to use it

    1. Look for oxygen removal or hydrogen addition. 2. Check electron gain if given. 3. Name the substance as reduced.

  3. 3

    Add one concrete example

    When copper(II) oxide turns into copper in the presence of hydrogen, copper(II) oxide is reduced.

  4. 4

    Avoid this incomplete answer

    Calling it oxidation because the overall reaction has two substances reacting. The correct clue is oxygen loss or electron gain.

Definition

Reduction is a change in which a substance loses oxygen, gains hydrogen, or gains electrons.

Example

When copper(II) oxide turns into copper in the presence of hydrogen, copper(II) oxide is reduced.

Rule to remember

Reduction = removal of oxygen, addition of hydrogen, or gain of electrons.

Memory hook

Reduction: remove oxygen, add hydrogen, gain electrons.

Examples and method

Worked example

CuO + H2 → Cu + H2O. Copper(II) oxide loses oxygen, so it is reduced.

Method to apply

1. Look for oxygen removal or hydrogen addition. 2. Check electron gain if given. 3. Name the substance as reduced.

Diagram support

Show the substance before and after reaction with oxygen removed or electrons gained.

How CBSE asks it

A question may ask which substance is reduced, or whether oxygen is removed or electrons are gained.

Avoid common mistakes

Common confusion

Students sometimes confuse reduction with loss of mass or with decomposition; those are different ideas.

Common wrong answer

Calling it oxidation because the overall reaction has two substances reacting. The correct clue is oxygen loss or electron gain.

Exam tip

If oxygen is removed from a compound, write reduction confidently and then support it with the reason.

Quick check

If a substance gains electrons in a reaction, what is this change called?

This change is called reduction because gaining electrons is one of the main signs of reduction.

Answer writing and exam use

1-mark answer

Reduction is a change in which a substance loses oxygen, gains hydrogen, or gains electrons.

2-mark answer

Reduction is a change in which a substance loses oxygen, gains hydrogen, or gains electrons. Reduction = removal of oxygen, addition of hydrogen, or gain of electrons. When copper(II) oxide turns into copper in the presence of hydrogen, copper(II) oxide is reduced.

3-mark answer

Reduction is the opposite side of oxidation. In school chemistry, it is seen when oxygen is removed from a compound or electrons are gained by a substance. Reduction and oxidation usually appear together in the same reaction. Reduction = removal of oxygen, addition of hydrogen, or gain of electrons. CuO + H2 → Cu + H2O. Copper(II) oxide loses oxygen, so it is reduced. A question may ask which substance is reduced, or whether oxygen is removed or electrons are gained. Calling it oxidation because the overall reaction has two substances reacting. The correct clue is oxygen loss or electron gain.
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