ScienceCBSE Class 10 · Chemical Reactions and Equations

Oxidation and Reduction (Redox Reactions)

Updated 14 October 2026 · Fact-checked

Oxidation is the gain of oxygen or loss of hydrogen by a substance; reduction is the loss of oxygen or gain of hydrogen. In a redox reaction both happen together: one reactant is oxidised while the other is reduced. The substance that gets reduced is the oxidising agent; the one that gets oxidised is the reducing agent.

Understand Oxidation and Reduction (Redox Reactions)

Heat a little copper powder in a china dish and its surface turns black. Oxygen from the air has joined the copper to form copper(II) oxide: $2 C u + O_{2} heat 2 C u O$ . Now pass hydrogen over the hot black solid and the brown colour of copper comes back: $C u O + H_{2} heat C u + H_{2} O$ .

Class 10 defines the two processes in terms of oxygen and hydrogen:

A substance is oxidised if it gains oxygen or loses hydrogen.
A substance is reduced if it loses oxygen or gains hydrogen.

In the second reaction, copper(II) oxide loses its oxygen, so it is reduced; hydrogen gains that oxygen to become water, so it is oxidised. The oxygen has to go somewhere, so whenever one substance loses it another gains it. That is why oxidation and reduction always happen together in the same reaction. Such reactions are called oxidation-reduction or redox reactions.

Two more NCERT examples show the method. In $Z n O + C \to Z n + C O$ , zinc oxide loses oxygen (reduced to zinc) and carbon gains oxygen (oxidised to carbon monoxide). In $M n O_{2} + 4 H C l \to M n C l_{2} + 2 H_{2} O + C l_{2}$ , manganese dioxide loses oxygen (reduced) while hydrogen chloride loses hydrogen to become chlorine (oxidised). The second example matters because it uses the hydrogen rule, not the oxygen rule.

Oxidising and reducing agents. The substance that supplies oxygen (or removes hydrogen) makes the other substance oxidised, so it is the oxidising agent; it is itself reduced. The substance that removes oxygen (or supplies hydrogen) is the reducing agent; it is itself oxidised. In $C u O + H_{2} \to C u + H_{2} O$ , $C u O$ is the oxidising agent and $H_{2}$ the reducing agent. Students often swap these, so tie each agent to what happens to it: the oxidising agent gets reduced.

A common trap is to name an element in the product as the thing oxidised or reduced. Always name the reactant: in $2 P b O + C \to 2 P b + C O_{2}$ , lead oxide is reduced and carbon is oxidised; lead and carbon dioxide are products, not substances undergoing change.

Redox reactions turn up across the book: burning, respiration, extracting metals from their oxides using carbon (Chapter 3), and the corrosion of metals. The board typically asks you to identify what is oxidised and reduced, name the agents, judge statements in an MCQ, or explain an activity such as the copper experiment.

Oxidation and Reduction (Redox Reactions): formulas and key facts

Oxidation (Class 10 definition): $gain of oxygen or loss of hydrogen$; Example: $2 C u + O_{2} \to 2 C u O$ (copper oxidised).
Reduction (Class 10 definition): $loss of oxygen or gain of hydrogen$; Example: $C u O + H_{2} \to C u + H_{2} O$ (copper oxide reduced).
Redox with carbon: $Z n O + C \to Z n + C O$; ZnO reduced, C oxidised; carbon is the reducing agent.
Redox by the hydrogen rule: $M n O_{2} + 4 H C l \to M n C l_{2} + 2 H_{2} O + C l_{2}$; $M n O_{2}$ loses oxygen (reduced); $H C l$ loses hydrogen to give $C l_{2}$ (oxidised).
Agents: $oxidising agent is reduced; reducing agent is oxidised$; Oxidising agent gives oxygen or takes hydrogen; reducing agent takes oxygen or gives hydrogen.

How to solve Oxidation and Reduction (Redox Reactions) questions

For any 'identify the substance oxidised, reduced and the agents' question, work through the reactants one at a time.

1Check the equation is balanced and note each reactant.
2For each reactant, follow its oxygen and hydrogen to the product side. Did it gain or lose oxygen? Did it gain or lose hydrogen?
3Gain of oxygen or loss of hydrogen: write 'oxidised'. Loss of oxygen or gain of hydrogen: write 'reduced'.
4Confirm there is one of each; if you found two substances oxidised and none reduced, recheck.
5Name the agents: the substance reduced is the oxidising agent; the substance oxidised is the reducing agent.
6Write the answer naming reactants (for example 'CuO is reduced to Cu'), not just the products.

Oxidation and Reduction (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.

Reduction of a metal oxide by carbon

Suppose you visit a small metal works where zinc oxide powder is mixed with coke (a form of carbon) and heated strongly in a furnace. Shiny zinc metal is collected and a gas burns at the vent.

Write the equation for the reaction, identify the substance oxidised, the substance reduced, the oxidising agent and the reducing agent, and explain why this can be called a redox reaction.

Try it first, then show the solutionSolution

Equation: $Z n O + C \to Z n + C O$ (check: Zn 1 = 1, O 1 = 1, C 1 = 1).
Zinc oxide loses oxygen to become zinc: it is reduced.
Carbon gains oxygen to become carbon monoxide: it is oxidised.
Zinc oxide gives up oxygen to carbon, so it is the oxidising agent; carbon removes oxygen from zinc oxide, so it is the reducing agent.
Oxidation and reduction occur together in one reaction, so it is a redox reaction.

Answer: $Z n O + C \to Z n + C O$ : ZnO reduced (oxidising agent), C oxidised (reducing agent); both changes happen together, so it is redox.

Memory hook: Coke grabs the oxygen, zinc is set free: carbon is the reducing agent.

Tricks to remember Oxidation and Reduction (Redox Reactions)

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

OIL-RIG for oxygen and hydrogen
Oxidation: oxygen In, hydrogen Lost. Reduction: oxygen Gone, hydrogen In. Say it as 'Oxygen In or hydrogen Lost = Oxidation'.
Use it when: Deciding oxidised or reduced for each reactant.
Agents do the opposite
An agent does to others what is not done to itself: the oxidising agent gets reduced, the reducing agent gets oxidised.
Use it when: Naming agents without swapping them.
Name the reactant
Point at the left-hand side when you say 'is oxidised'.
Use it when: Avoids naming a product such as $C O_{2}$ as the thing oxidised.

Oxidation and Reduction (Redox Reactions): solved examples

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

Example 1: Follow the oxygen, then the hydrogen · 2 marks · Very short answerBoard level

Identify the substance oxidised and the substance reduced in: (i) $F e_{3} O_{4} (s) + 4 H_{2} (g) \to 3 F e (s) + 4 H_{2} O (l)$ (ii) $2 H_{2} S (g) + O_{2} (g) \to 2 S (s) + 2 H_{2} O (l)$

Try it first, then show the solutionSolution

(i) $F e_{3} O_{4}$ loses oxygen to become iron, so it is reduced; $H_{2}$ gains that oxygen to form water, so it is oxidised (1 mark).
(ii) $H_{2} S$ loses hydrogen and becomes sulphur, so it is oxidised; $O_{2}$ gains hydrogen to form water, so it is reduced (1 mark).

Answer: (i) $F e_{3} O_{4}$ reduced, $H_{2}$ oxidised. (ii) $H_{2} S$ oxidised, $O_{2}$ reduced.

Example 2: Which statements are wrong? · 3 marks · Short answerHarder board level

For the reaction $2 P b O (s) + C (s) \to 2 P b (s) + C O_{2} (g)$ , which statements are incorrect? (a) Lead is the substance that undergoes reduction. (b) Carbon dioxide undergoes oxidation. (c) Carbon undergoes oxidation. (d) Lead oxide undergoes reduction. Explain each.

Try it first, then show the solutionSolution

(a) Incorrect: lead is a product; the substance reduced is lead oxide (½ mark).
(b) Incorrect: carbon dioxide is a product, not a reactant undergoing change (½ mark).
(c) Correct: carbon gains oxygen to form $C O_{2}$ (1 mark).
(d) Correct: lead oxide loses oxygen to form lead (1 mark).

Answer: (a) and (b) are incorrect; (c) and (d) are correct.

Example 3: Hydrogen rule in action · 3 marks · Short answerHarder board level

In the reaction $M n O_{2} + 4 H C l \to M n C l_{2} + 2 H_{2} O + C l_{2}$ : (a) which substance is oxidised and which is reduced? (b) Name the oxidising and reducing agents. (c) Explain why you cannot decide that HCl is oxidised by looking at oxygen alone.

Try it first, then show the solutionSolution

(a) $M n O_{2}$ loses oxygen (to water), so it is reduced; $H C l$ loses hydrogen to form chlorine, $C l_{2}$ , so it is oxidised (1 mark).
(b) Oxidising agent: $M n O_{2}$ (it is reduced). Reducing agent: $H C l$ (it is oxidised) (1 mark).
(c) HCl contains no oxygen and does not gain any; its oxidation is shown only by the loss of hydrogen as it forms $C l_{2}$ (1 mark).

Answer: (a) $M n O_{2}$ reduced, HCl oxidised. (b) $M n O_{2}$ oxidising agent, HCl reducing agent. (c) HCl is oxidised by losing hydrogen, not by gaining oxygen.

Example 4: Activity: heating copper · 5 marks · Long answerHarder board level

A student heats copper powder in a china dish. (a) What colour change is seen and why? Write the equation. (b) Hydrogen is then passed over the heated product. What change is seen? Write the equation. (c) Identify the substance oxidised and the substance reduced in (b), and name the type of reaction.

Try it first, then show the solutionSolution

(a) The brown surface turns black because oxygen is added to copper, forming copper(II) oxide: $2 C u + O_{2} heat 2 C u O$ (2 marks).
(b) The black coating turns brown again as copper is formed: $C u O + H_{2} heat C u + H_{2} O$ (2 marks).
(c) $C u O$ is reduced (loses oxygen); $H_{2}$ is oxidised (gains oxygen). Both occur together: a redox reaction (1 mark).

Answer: (a) Brown to black, $2 C u + O_{2} \to 2 C u O$ . (b) Black to brown, $C u O + H_{2} \to C u + H_{2} O$ . (c) CuO reduced, $H_{2}$ oxidised; redox reaction.

Example 5: Unfamiliar redox · 3 marks · Short answerHOTS / competency

Hydrogen sulphide reacts with chlorine: $H_{2} S + C l_{2} \to 2 H C l + S$ . (a) Using the Class 10 definitions, identify the substance oxidised and the substance reduced. (b) Name the reducing agent. (c) A student says 'there is no oxygen here, so this cannot be a redox reaction'. Correct the student.

Try it first, then show the solutionSolution

(a) $H_{2} S$ loses hydrogen and becomes sulphur, so it is oxidised; $C l_{2}$ gains hydrogen to become $H C l$ , so it is reduced (1 mark).
(b) The reducing agent is $H_{2} S$ (it is oxidised and supplies hydrogen to chlorine) (1 mark).
(c) Oxidation also includes loss of hydrogen, and reduction includes gain of hydrogen. One substance loses hydrogen and the other gains it, so the reaction is redox even without oxygen (1 mark).

Answer: (a) $H_{2} S$ oxidised, $C l_{2}$ reduced. (b) $H_{2} S$ . (c) Hydrogen transfer alone is enough to make it a redox reaction.

Where students lose marks in Oxidation and Reduction (Redox Reactions)

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

Mistake: Naming a product as the substance oxidised or reduced
Students look at what is formed instead of what changes.
Wrong working
In $2 P b O + C \to 2 P b + C O_{2}$ : lead is reduced, $C O_{2}$ is oxidised.
Right working
$P b O$ is reduced to $P b$ ; $C$ is oxidised to $C O_{2}$ .
Do this: Always name the reactant and, if you like, what it changes into.
Usually costs: Full marks in the NCERT-style 'which statements are incorrect' MCQ.
Mistake: Swapping oxidising and reducing agents
The oxidising agent sounds as if it should be oxidised.
Wrong working
In $C u O + H_{2} \to C u + H_{2} O$ , $H_{2}$ is the oxidising agent because it is oxidised.
Right working
$H_{2}$ is oxidised, so it is the reducing agent; $C u O$ is the oxidising agent.
Do this: The agent is named for what it does to the other substance.
Usually costs: 1 mark per pair.
Mistake: Ignoring the hydrogen rule
Students only track oxygen and conclude a reaction is not redox.
Wrong working
$M n O_{2}$ is reduced but nothing is oxidised.
Right working
$H C l$ loses hydrogen to form $C l_{2}$ , so it is oxidised.
Do this: Check both oxygen and hydrogen for every reactant.
Usually costs: 1 mark in 2-3 mark questions.
Mistake: Writing only one half of the answer
The question says 'identify the substance oxidised and reduced' and only one is given.
Wrong working
Carbon is oxidised.
Right working
Carbon is oxidised and zinc oxide is reduced.
Do this: In a redox reaction always give both, each with its reason (gain or loss of oxygen or hydrogen).
Usually costs: ½ to 1 mark.

Exam tips

State the reason with each label: 'CuO is reduced because it loses oxygen'.
Use only the oxygen and hydrogen definitions in Class 10 answers; they are what the NCERT book and marking schemes use.
In activity questions on copper, describe both colour changes: brown to black on heating in air, black to brown with hydrogen.
If a reaction is both displacement and redox (such as aluminium with iron oxide), name both when asked for the type.
Recheck the balance of any equation you write; an unbalanced equation can lose the equation mark even if the redox labels are right.

Practice questions on Oxidation and Reduction (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
In the reaction $C u O + H_{2} \to C u + H_{2} O$ , the oxidising agent is:
(a) $C u$ (b) $H_{2}$ (c) $C u O$ (d) $H_{2} O$
Hint
The oxidising agent is the reactant that is reduced.
Answer
(c) $C u O$ . It gives its oxygen to hydrogen and is itself reduced.
Q2 · 2 marks · Very short answerBoard level
$F e_{2} O_{3} + 2 A l \to A l_{2} O_{3} + 2 F e$ . (a) Name the type of reaction in two ways. (b) Identify the substance oxidised and the substance reduced.
Hint
Aluminium takes the place of iron; follow the oxygen.
Answer
(a) A displacement reaction, and also a redox reaction. (b) Al gains oxygen (oxidised); $F e_{2} O_{3}$ loses oxygen (reduced).
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): In the reaction $Z n O + C \to Z n + C O$ , carbon acts as a reducing agent.
Reason (R): Carbon gains oxygen in the reaction.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
A substance that gains oxygen is oxidised; what does that make it?
Answer
(a). Carbon gains oxygen, so it is oxidised and acts as the reducing agent by removing oxygen from zinc oxide.
Q4 · 3 marks · Short answerHOTS / competency
Magnesium continues to burn in carbon dioxide gas: $2 M g + C O_{2} \to 2 M g O + C$ . (a) Check that the equation is balanced. (b) Identify the substance oxidised, the substance reduced and the two agents. (c) Why is it wrong to say 'carbon is oxidised' here?
Hint
Follow oxygen from $C O_{2}$ to $M g O$ .
Answer
(a) Mg 2 = 2, C 1 = 1, O 2 = 2. (b) Mg gains oxygen (oxidised, reducing agent); $C O_{2}$ loses oxygen (reduced, oxidising agent). (c) Carbon appears here as a product formed from $C O_{2}$ losing oxygen; it is the result of reduction, not something being oxidised.
Q5 · 3 marks · Short answerHarder board level
Iron ore is reduced in a furnace by carbon monoxide: $F e_{2} O_{3} + 3 C O \to 2 F e + 3 C O_{2}$ . (a) Verify the balance. (b) Name the oxidising agent and the reducing agent. (c) Explain why the reaction is described as reduction of the ore even though something is oxidised too.
Hint
In industry the name is chosen from the useful change.
Answer
(a) Fe 2 = 2, C 3 = 3, O 3 + 3 = 6 = 6. (b) $F e_{2} O_{3}$ is the oxidising agent (it is reduced); CO is the reducing agent (it is oxidised to $C O_{2}$ ). (c) It is a redox reaction; the ore losing oxygen to give the metal is the change we want, so the process is named after the reduction.
Q6 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
Suppose in a school laboratory a teacher shows three reactions:
P: Copper powder heated in air turns black.
Q: Black copper(II) oxide heated in a stream of hydrogen turns brown.
R: Manganese dioxide warmed with concentrated hydrochloric acid gives a greenish-yellow gas.
(a) Write the balanced equation for P and name the substance oxidised. (1 mark)
(b) In Q, name the reducing agent. (1 mark)
(c) In R, identify the gas and explain, using the hydrogen definition, which reactant is oxidised and which is reduced. (2 marks)
Hint
The greenish-yellow gas comes from HCl.
Answer
(a) $2 C u + O_{2} \to 2 C u O$ ; copper is oxidised. (b) Hydrogen ( $H_{2}$ ). (c) The gas is chlorine: $M n O_{2} + 4 H C l \to M n C l_{2} + 2 H_{2} O + C l_{2}$ . HCl loses hydrogen to form $C l_{2}$ , so it is oxidised; $M n O_{2}$ loses oxygen, so it is 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.Define oxidation in terms of hydrogen.Show answer
Loss of hydrogen by a substance.
2.Define reduction in terms of oxygen.Show answer
Loss of oxygen by a substance.
3.Why do oxidation and reduction occur together?Show answer
Oxygen or hydrogen lost by one substance is gained by another in the same reaction.
4.In $Z n O + C \to Z n + C O$ , what is the reducing agent?Show answer
Carbon.
5.What colour does copper turn when heated in air?Show answer
Black, due to copper(II) oxide.
6.The oxidising agent is itself...?Show answer
Reduced.

Oxidation and Reduction (Redox Reactions): frequently asked questions

What is a redox reaction in Class 10?

A reaction in which one reactant is oxidised (gains oxygen or loses hydrogen) and another is reduced (loses oxygen or gains hydrogen) at the same time.

How do I find the oxidising and reducing agent quickly?

First find the substance reduced; that is the oxidising agent. The substance oxidised is the reducing agent.

Do I need the electron-transfer definition of oxidation for the Class 10 board exam?

The Class 10 NCERT chapter defines oxidation and reduction only through gain or loss of oxygen and hydrogen, so answers based on these definitions are what you need.

Is every displacement reaction a redox reaction?

Many Class 10 examples are, such as aluminium with iron oxide, where aluminium gains oxygen and iron oxide loses it. Name the type the question asks for and give both if asked.

Why is burning called oxidation?

The fuel gains oxygen. For example, carbon in coal combines with oxygen to form carbon dioxide.