ChemistryICSE Class 9 · Study of the First Element: Hydrogen

Oxidation, Reduction and Redox Reactions: Identifying Agents and Counting Electrons (ICSE Class 9)

Updated 17 October 2026 · Fact-checked

Oxidation is addition of oxygen, removal of hydrogen or loss of electrons; reduction is removal of oxygen, addition of hydrogen or gain of electrons. A redox reaction has both together. The oxidising agent is reduced (it accepts electrons); the reducing agent is oxidised (it donates electrons).

Notice that the agent that loses electrons is oxidised, the one that gains them is reduced, and both changes happen together.

Illustration drawn for QuestionBuddy.

Understand Oxidation, Reduction, Oxidising and Reducing Agents and Redox Reactions

Definitions.

	Oxidation	Reduction
Oxygen	gain	loss
Hydrogen	loss	gain
Electrons	loss	gain

A redox reaction is one in which oxidation and reduction occur together; one cannot happen without the other.

Agents. The oxidising agent supplies oxygen, removes hydrogen or accepts electrons; it is itself reduced. Examples: O₂, Cl₂, concentrated HNO₃, CuO. The reducing agent removes oxygen, supplies hydrogen or donates electrons; it is itself oxidised. Examples: H₂, C, CO, metals.

Worked patterns.

$C u O + H_{2} \to C u + H_{2} O$ : CuO loses oxygen (reduced; oxidising agent); H₂ gains oxygen (oxidised; reducing agent).
$H_{2} S + C l_{2} \to 2 H C l + S$ : H₂S loses hydrogen (oxidised); Cl₂ gains hydrogen (reduced).
$Z n + C u S O_{4} \to Z n S O_{4} + C u$ : $Z n \to Z n^{2 +} + 2 e^{-}$ (oxidation); $C u^{2 +} + 2 e^{-} \to C u$ (reduction).
$2 M g + O_{2} \to 2 M g O$ : Mg loses 2 electrons per atom; O gains them.

Electron bookkeeping. In a redox reaction electrons lost equal electrons gained. For $2 A l + 3 C l_{2} \to 2 A l C l_{3}$ : 2 Al atoms lose $2 \times 3 = 6$ electrons; 3 Cl₂ molecules gain $3 \times 2 = 6$ electrons.

Which reactions are redox? Displacement and most combination and decomposition reactions that involve elements are redox. Double decomposition (such as $A g N O_{3} + N a C l \to A g C l + N a N O_{3}$ ) only exchanges ions; no electrons are transferred, so it is not redox.

Mass questions. The mass relations use the balanced equation as usual: $C u O + H_{2} \to C u + H_{2} O$ (Cu = 63.5): 79.5 g of CuO gives 63.5 g of copper.

Oxidation, Reduction, Oxidising and Reducing Agents and Redox Reactions: formulas and key facts

Oxidation: $gain of O ∣ loss of H ∣ loss of e^{-}$; Increase in oxygen or loss of electrons.
Reduction: $loss of O ∣ gain of H ∣ gain of e^{-}$; Opposite of oxidation.
Electrons conserved: $e_{l os t}^{-} = e_{g ain e d}^{-}$; Check the half-changes.

How to solve Oxidation, Reduction, Oxidising and Reducing Agents and Redox Reactions questions

Write what each species gains or loses.

1Compare each reactant with its product for oxygen, hydrogen or electron change.
2Name the species oxidised and the species reduced.
3The reducing agent is the one oxidised; the oxidising agent is the one reduced.
4Write half-changes with electrons for ionic reactions and make the electrons equal.
5Check that a double decomposition is not redox.

Oxidation, Reduction, Oxidising and Reducing Agents and Redox Reactions in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Rusting and burning

Suppose iron rusts and a fuel burns.

Classify both in terms of oxygen.

Try it first, then show the solutionSolution

Iron combines with oxygen: oxidation.
A fuel burning adds oxygen to carbon and hydrogen: oxidation (redox, with oxygen reduced).

Answer: Both are oxidations.

Memory hook: OIL RIG: oxidation is loss, reduction is gain of electrons.

Tricks to remember Oxidation, Reduction, Oxidising and Reducing Agents and Redox Reactions

Short memory aids. Say each one aloud once, then use it on a question the same day.

OIL RIG
Oxidation Is Loss, Reduction Is Gain (of electrons).
Use it when: Electron definitions.
Agent is the opposite
The reducing agent is the one oxidised.
Use it when: Naming agents.
Together always
Oxidation and reduction occur simultaneously: redox.
Use it when: Conceptual questions.

Oxidation, Reduction, Oxidising and Reducing Agents and Redox Reactions: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Reducing agent · 1 mark · MCQBoard level

In $C u O + H_{2} \to C u + H_{2} O$ the reducing agent is:
(a) CuO
(b) H₂
(c) Cu
(d) H₂O

Try it first, then show the solutionSolution

Hydrogen takes oxygen away from CuO and is itself oxidised.

Answer: (b) H₂

Example 2: Define in terms of electrons · 2 marks · Short answerBoard level

Define oxidation and reduction in terms of electrons and show them for the reaction $2 M g + O_{2} \to 2 M g O$ .

Try it first, then show the solutionSolution

Oxidation is loss of electrons; reduction is gain of electrons. (1)
$M g \to M g^{2 +} + 2 e^{-}$ (oxidation); $O + 2 e^{-} \to O^{2 -}$ (reduction). (1)

Answer: Mg oxidised; O reduced

Example 3: Identify the agents · 3 marks · Short answerHarder board level

For each reaction name the substance oxidised, the substance reduced, the oxidising agent and the reducing agent: (a) $F e_{2} O_{3} + 3 C O \to 2 F e + 3 C O_{2}$ (b) $2 N a + C l_{2} \to 2 N a C l$ (c) $Z n + C u S O_{4} \to Z n S O_{4} + C u$ .

Try it first, then show the solutionSolution

(a) CO is oxidised (gains O) and is the reducing agent; Fe₂O₃ is reduced (loses O) and is the oxidising agent. (1)
(b) Na is oxidised (loses e⁻), reducing agent; Cl₂ is reduced (gains e⁻), oxidising agent. (1)
(c) Zn is oxidised, reducing agent; Cu²⁺ is reduced, oxidising agent. (1)

Answer: See working

Example 4: Electrons in aluminium chloride · 3 marks · Short answerHarder board level

For $2 A l + 3 C l_{2} \to 2 A l C l_{3}$ write the half-changes with electrons, show that electrons lost equal electrons gained and name the oxidising and reducing agents.

Try it first, then show the solutionSolution

$A l \to A l^{3 +} + 3 e^{-}$ (oxidation), for 2 Al: 6 electrons lost. (1)
$C l_{2} + 2 e^{-} \to 2 C l^{-}$ (reduction), for 3 Cl₂: 6 electrons gained. (1)
Chlorine is the oxidising agent; aluminium is the reducing agent. (1)

Answer: 6 electrons transferred; Cl₂ oxidises, Al reduces

Example 5: Reducing copper(II) oxide · 4 marks · Case-basedHOTS / competency

7.95 g of copper(II) oxide is heated in a stream of hydrogen (Cu = 63.5, O = 16, H = 1).
(a) Write the equation.
(b) Name the oxidising agent and the reducing agent.
(c) Find the mass of copper formed.
(d) Find the mass of hydrogen used and of water formed, and show that mass is conserved.

Try it first, then show the solutionSolution

(a) $C u O + H_{2} \to C u + H_{2} O$ . (½)
(b) CuO is the oxidising agent; H₂ is the reducing agent. (1)
(c) $7.95 \times \frac{63.5}{79.5} = 6.35$ g. (1)
(d) Hydrogen used $= 7.95 \times \frac{2}{79.5} = 0.2$ g; water formed $= 7.95 \times \frac{18}{79.5} = 1.8$ g. Reactants $7.95 + 0.2 = 8.15$ g; products $6.35 + 1.8 = 8.15$ g: mass is conserved. (1½)

Answer: (c) 6.35 g (d) 0.2 g H₂ used; 1.8 g water; 8.15 g on each side

Where students lose marks in Oxidation, Reduction, Oxidising and Reducing Agents and Redox Reactions

The slips examiners see most often, and what to write instead.

Mistake: Oxidising agent named as the substance oxidised
The oxidising agent is reduced.
Wrong working
H₂ is the oxidising agent.
Right working
CuO is the oxidising agent; H₂ is the reducing agent.
Do this: Agent is the opposite of what happens to it.
Usually costs: 2 marks.
Mistake: Electron loss called reduction
Loss of electrons is oxidation.
Wrong working
Zn → Zn²⁺ + 2e⁻ is reduction.
Right working
It is oxidation.
Do this: OIL RIG.
Usually costs: 1 mark.
Mistake: Half-changes with unequal electrons added
Electrons lost must equal electrons gained.
Wrong working
2Al → 2Al³⁺ + 3e⁻
Right working
2Al → 2Al³⁺ + 6e⁻
Do this: Multiply by the number of atoms.
Usually costs: 1 mark.

Exam tips

Write both definitions (oxygen/hydrogen and electrons) when asked.
Use half-changes with electrons for ionic reactions.
Name the agent along with the species oxidised or reduced.
Note that double decomposition is not redox.

Practice questions on Oxidation, Reduction, Oxidising and Reducing Agents and Redox Reactions

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Oxidation is:
(a) gain of electrons
(b) loss of electrons
(c) loss of oxygen
(d) gain of hydrogen
Hint
OIL RIG.
Answer
(b)
Q2 · 1 mark · Very short answerBoard level
What is a reducing agent?
Hint
It is itself oxidised.
Answer
A substance that donates electrons or removes oxygen (or supplies hydrogen) and is itself oxidised.
Q3 · 2 marks · Short answerHarder board level
In $2 M g + O_{2} \to 2 M g O$ , name the substance oxidised and the substance reduced.
Hint
Mg loses electrons.
Answer
Mg is oxidised (loses electrons, gains oxygen); O₂ is reduced (gains electrons).
Q4 · 3 marks · Short answerHarder board level
In $S O_{2} + 2 H_{2} S \to 3 S + 2 H_{2} O$ , identify the substance oxidised and the substance reduced and explain each in terms of oxygen or hydrogen.
Hint
Look at H and O changes.
Answer
H₂S is oxidised (loses hydrogen to give S). SO₂ is reduced (loses oxygen to give S). Hence H₂S is the reducing agent and SO₂ the oxidising agent.
Q5 · 2 marks · Assertion-reasonHarder board level
Assertion (A): In $Z n + C u S O_{4} \to Z n S O_{4} + C u$ zinc acts as the reducing agent.
Reason (R): Zinc loses electrons and is oxidised.
(a) Both true, R explains A.
(b) Both true, R does not explain A.
(c) A true, R false.
(d) A false, R true.
Hint
Electron loss.
Answer
(a)
Q6 · 3 marks · Short answerHOTS / competency
Decide whether each is a redox reaction and justify with electron transfer: (a) $C l_{2} + 2 K B r \to 2 K C l + B r_{2}$ (b) $A g N O_{3} + N a C l \to A g C l + N a N O_{3}$ .
Hint
Look for a free element on one side.
Answer
(a) Redox: bromide ions lose electrons ( $2 B r^{-} \to B r_{2} + 2 e^{-}$ , oxidised) and chlorine gains them ( $C l_{2} + 2 e^{-} \to 2 C l^{-}$ , reduced). (b) Not redox: the ions simply exchange partners; no electrons are transferred and nothing is oxidised or reduced.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Oxidation in terms of electrons?Show answer
Loss of electrons.
2.Oxidising agent?Show answer
Accepts electrons or supplies oxygen; is itself reduced.
3.Is H₂ an oxidising or reducing agent in CuO + H₂?Show answer
Reducing agent.
4.Redox reaction?Show answer
One in which oxidation and reduction occur together.
5.Name a reducing agent.Show answer
H₂, C, CO or a metal.

Oxidation, Reduction, Oxidising and Reducing Agents and Redox Reactions: frequently asked questions

What is a redox reaction?

A reaction in which oxidation and reduction occur together; one substance loses electrons (or gains oxygen) while another gains them (or loses oxygen).

How do you identify the oxidising agent?

It is the substance that is reduced: it gains electrons or hydrogen or loses oxygen.

Is displacement a redox reaction?

Yes; for example Zn + CuSO₄ involves loss of electrons by zinc and gain by copper ions.