Oxidation Reduction Redox
Leave an iron gate out through one Indian monsoon and it grows a flaky brown crust. Leave a packet of chips open overnight and by morning it tastes stale and faintly bitter; a silver chain forgotten in a drawer slowly turns black. Three different objects, one shared chemistry — a slow reaction with the oxygen of air, the everyday face of oxidation and its inseparable partner, reduction.
Definition: Oxidation is the gain of oxygen or the loss of hydrogen by a substance during a chemical reaction. Reduction is the exact opposite — the loss of oxygen or the gain of hydrogen.
Definition: A redox reaction (reduction + oxidation) is a reaction in which one substance is oxidised while another is reduced, both in the same reaction. Oxidation never happens alone — every gain has a giver.
The oxygen story and the hydrogen story
At Class 10 level you decide "oxidised or reduced?" by tracking just two elements.
Track oxygen first. If a substance gains oxygen during a reaction, it is oxidised; if it loses oxygen, it is reduced.
- 2Cu + O₂ --heat--> 2CuO — copper gains oxygen, so copper is oxidised to copper(II) oxide.
- C + O₂ → CO₂ — carbon gains oxygen on burning, so carbon is oxidised.
- CuO + H₂ --heat--> Cu + H₂O — copper oxide loses oxygen, so CuO is reduced to copper.
Track hydrogen when there is no oxygen to follow. If a substance loses hydrogen it is oxidised; if it gains hydrogen it is reduced.
- H₂S + Cl₂ → 2HCl + S — hydrogen sulphide loses hydrogen, so H₂S is oxidised to sulphur; chlorine gains hydrogen, so Cl₂ is reduced to HCl.
Notice that this last equation has no oxygen anywhere, yet it is a perfectly good redox reaction. Board papers use exactly this twist to separate students who memorised one definition from students who know both. (In Class 11 you will meet the most general definition — transfer of electrons — but every question in your Class 10 syllabus can be answered with the oxygen and hydrogen definitions above.)
One reaction, two halves — the redox idea
NCERT introduces redox through a lovely activity. Take about 1 g of copper powder in a china dish and heat it. The shiny brown powder becomes coated with a black substance — copper(II) oxide — because copper is oxidised by the oxygen of the air:
2Cu + O₂ --heat--> 2CuO
Now pass hydrogen gas over this hot black material. The black coating turns brown again, because the reverse change takes place and copper is obtained back:
CuO + H₂ --heat--> Cu + H₂O
Look closely at the second equation. Copper oxide loses oxygen: it is reduced. Hydrogen gains oxygen: it is oxidised. Both changes happen inside one single reaction — one substance's loss is literally the other's gain. That is why oxidation and reduction can never occur separately, and why such reactions are called oxidation-reduction or redox reactions.
Common misconception: students write "CuO is reduced" and stop. A substance cannot be oxidised without something else being reduced at the same time — they are two halves of one process. In any redox question, always name both partners; an answer that has oxidation with no matching reduction is chemically impossible.
Oxidising and reducing agents
Once you can spot who is oxidised and who is reduced, the "agent" vocabulary is one small step away.
- The oxidising agent is the substance that causes oxidation of the other reactant — it gives oxygen (or removes hydrogen). In doing this job, it is itself reduced.
- The reducing agent is the substance that causes reduction of the other reactant — it removes oxygen (or supplies hydrogen). In doing this job, it is itself oxidised.
This cross-over trips up many students, so anchor it to the copper reaction:
| In CuO + H₂ → Cu + H₂O | CuO | H₂ |
|---|---|---|
| What happens to it | loses oxygen → reduced | gains oxygen → oxidised |
| Job done to the other | gives oxygen → oxidising agent | takes away oxygen → reducing agent |
Rule of thumb: an agent is named for the job it does to the other substance, and it suffers the opposite fate itself. The oxidising agent ends up reduced; the reducing agent ends up oxidised. This is not just vocabulary — cheap reducing agents such as carbon (coke) are the workhorses of metallurgy, pulling oxygen off metal oxides like ZnO to release the metal.
Corrosion — oxidation that eats metals
When a metal is attacked by substances around it — moisture, acids, gases in the air — the metal is said to corrode, and the process is called corrosion. It is slow, unwanted oxidation of the metal surface.
- Iron rusts: a new iron article is shiny, but kept in a damp place it acquires a flaky, reddish-brown coating of rust (hydrated iron(III) oxide). Rusting needs both air (oxygen) and moisture (water) together — remove either one and iron stays safe.
- Silver blackens: the black layer on old silver articles is silver sulphide, formed by the action of sulphur compounds present in air.
- Copper turns green: copper exposed to moist air slowly gains a green coating (basic copper carbonate) — the dull green film on old copper vessels and statues.
Corrosion damages car bodies, bridges, iron railings, ships and everything else made of iron; an enormous amount of money is spent every year replacing corroded iron. That is why we apply paint on iron articles — the paint layer cuts off contact between the iron surface and the air and moisture, so the oxidation cannot start. Oiling, greasing and galvanising protect iron in the same way (Chapter 3, Metals and Non-metals, takes this idea further).
Rancidity — oxidation that spoils food
Food containing fats and oils faces the same enemy. When fats and oils are oxidised, the food becomes rancid — its smell and taste change unpleasantly. Stale-smelling chips, butter or fried snacks left exposed to air are everyday examples. Manufacturers fight rancidity by keeping oxygen away from the fat:
- Antioxidants — substances which prevent oxidation — are added to foods containing fats and oils.
- Airtight containers slow down oxidation by limiting the supply of air.
- Nitrogen flushing — bags of chips are flushed with unreactive nitrogen gas so that oxygen never touches the chips.
- Refrigeration slows down the oxidation reactions, keeping food fresh for longer.
Why it matters: the whole game of this topic is controlling oxidation. We deliberately speed it up when it is useful (burning fuels for energy, using carbon to win metals from their oxides) and slow it down when it is harmful (rusting bridges, rancid food). Redox is not a definition to memorise — it is a lever that engineers, metallurgists and even chips manufacturers pull every day.
Work through these examples in order. The first four build the core skill — tracking oxygen and hydrogen — and the later ones are the twists CBSE papers love: redox with no oxygen, both definitions inside one equation, classify-and-apply questions, and activity-based reasoning. Keep both definition pairs in front of you as you solve.
Example 1 — Who is oxidised, who is reduced?
Q: In the reaction CuO(s) + H₂(g) --heat--> Cu(s) + H₂O(l), identify the substance oxidised and the substance reduced.
Given: the equation is balanced (Cu: 1 = 1, O: 1 = 1, H: 2 = 2).
Formula: gain of oxygen = oxidation; loss of oxygen = reduction.
Solve: CuO becomes Cu — it has lost its oxygen, so CuO is reduced. H₂ becomes H₂O — it has gained oxygen, so H₂ is oxidised.
Answer: H₂ is oxidised (to H₂O); CuO is reduced (to Cu). Sanity check: exactly one partner oxidised and one reduced — a complete redox pair. ✓
Example 2 — Agents at work (ZnO + C)
Q: In the reaction ZnO(s) + C(s) --heat--> Zn(s) + CO(g), identify the substance oxidised, the substance reduced, the oxidising agent and the reducing agent.
Given: the equation is balanced (Zn: 1 = 1, O: 1 = 1, C: 1 = 1).
Solve: ZnO becomes Zn — it has lost oxygen, so ZnO is reduced. C becomes CO — it has gained oxygen, so carbon is oxidised. ZnO supplied the oxygen, so it is the oxidising agent; carbon removed the oxygen, so it is the reducing agent.
| Role | Substance |
|---|---|
| Oxidised | C (carbon) |
| Reduced | ZnO |
| Oxidising agent | ZnO |
| Reducing agent | C |
Answer: C is oxidised, ZnO is reduced; ZnO is the oxidising agent and C is the reducing agent. This is exactly how zinc is obtained from its oxide in industry — carbon is the cheap, hard-working reducing agent.
Example 3 — When oxygen itself is a reactant
Q: Identify the substance oxidised and the substance reduced: 4Na(s) + O₂(g) → 2Na₂O(s). (NCERT in-text question)
Formula: the element that gains oxygen is oxidised; the oxygen that is used up is reduced.
Solve: Sodium combines with oxygen to form Na₂O — sodium has gained oxygen, so Na is oxidised. Oxygen is taken up by sodium, so O₂ is reduced.
Answer: Na is oxidised to Na₂O; O₂ is reduced. General rule worth memorising: whenever an element burns or combines with oxygen, the element is oxidised and oxygen itself is reduced (oxygen acts as the oxidising agent).
Example 4 — Everyday reasoning: the nitrogen-flushed packet
Q: Chips manufacturers flush bags of chips with nitrogen gas. Why? Name the process being prevented.
Solve: Chips contain fats and oils. Oxygen from ordinary air slowly oxidises these fats and oils, making the chips rancid — the smell and taste turn unpleasant. Nitrogen is an unreactive gas: filling the packet with nitrogen removes all contact with oxygen, so the oxidation cannot occur.
Answer: Nitrogen prevents oxidation of the fats and oils in the chips, i.e. it prevents rancidity. Related exam favourites: antioxidants, airtight containers and refrigeration all fight the same enemy — oxidation.
Example 5 — Tricky: redox with no oxygen anywhere
Q: In H₂S(g) + Cl₂(g) → 2HCl(g) + S(s), no species contains oxygen. Identify the substance oxidised and the substance reduced, with reasons.
Formula: loss of hydrogen = oxidation; gain of hydrogen = reduction.
Solve: H₂S becomes S — it has lost hydrogen, so H₂S is oxidised. Cl₂ becomes HCl — it has gained hydrogen, so Cl₂ is reduced. For the agents: Cl₂ removes hydrogen from H₂S, so Cl₂ is the oxidising agent; H₂S supplies the hydrogen, so it is the reducing agent.
Answer: H₂S is oxidised to S; Cl₂ is reduced to HCl. Trap: many students freeze when no oxygen appears — switch to the hydrogen definition and the question solves itself.
Example 6 — Tricky: both definitions in one equation
Q: For MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂, identify (i) the substance oxidised, (ii) the substance reduced, (iii) the oxidising agent, (iv) the reducing agent. (NCERT)
Solve: Follow both elements. MnO₂ becomes MnCl₂ — it has lost oxygen, so MnO₂ is reduced. HCl becomes Cl₂ — it has lost hydrogen, so HCl is oxidised. Therefore MnO₂, which causes the oxidation of HCl, is the oxidising agent, and HCl, which removes oxygen from MnO₂, is the reducing agent.
Answer: HCl is oxidised to Cl₂; MnO₂ is reduced to MnCl₂; MnO₂ is the oxidising agent and HCl is the reducing agent. Check: the oxygen leaving MnO₂ appears in the 2H₂O on the product side — every atom is accounted for. ✓
Example 7 — Tricky: classify twice, then apply
Q: Consider Fe₂O₃ + 2Al → Al₂O₃ + 2Fe. (i) Which substance is oxidised and which is reduced? (ii) What type of reaction is this? (iii) State one practical use of this reaction.
Solve: Al becomes Al₂O₃ — aluminium gains oxygen, so Al is oxidised (it is the reducing agent). Fe₂O₃ loses its oxygen to become Fe, so Fe₂O₃ is reduced (it is the oxidising agent). The more reactive aluminium displaces iron from its oxide, so this is a displacement reaction — and since oxidation and reduction occur together, it is a redox reaction too. It is highly exothermic: the iron is produced in the molten state.
Answer: Al is oxidised, Fe₂O₃ is reduced; it is a displacement (and redox) reaction; it is used in thermite welding of railway tracks. One equation, three marks — boards love stacking parts like this.
Example 8 — Tricky: the black-coating puzzle (activity-based)
Q: When copper powder is heated strongly in a china dish, its surface turns black. (i) Why does this happen? (ii) How can the black surface be turned brown again? Write the equations and name the processes involved.
Solve: (i) On heating, copper combines with oxygen from the air and is oxidised to black copper(II) oxide: 2Cu + O₂ --heat--> 2CuO. (ii) Pass hydrogen gas over the hot black material; CuO loses its oxygen and is reduced back to brown copper: CuO + H₂ --heat--> Cu + H₂O.
Answer: The black layer is CuO formed by oxidation of copper; passing H₂ over the hot material reduces CuO back to Cu. Sanity check: the same copper simply cycles Cu → CuO → Cu — oxidation on the way out, reduction on the way back. ✓
- ✓- Oxidation = gain of oxygen or loss of hydrogen; reduction = loss of oxygen or gain of hydrogen.
- ✓- Oxidation and reduction always occur together in the same reaction — that is a redox reaction.
- ✓- The oxidising agent gives oxygen (or removes hydrogen) and is itself reduced.
- ✓- The reducing agent removes oxygen (or supplies hydrogen) and is itself oxidised.
- ✓- Anchor example: CuO + H₂ --heat--> Cu + H₂O — CuO is reduced, H₂ is oxidised.
- ✓- MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂ — HCl is oxidised (loses H), MnO₂ is reduced (loses O).
- ✓- Corrosion is slow oxidation of metals by moisture, acids and gases — iron rusts only when air and moisture are both present.
- ✓- Rancidity is oxidation of fats and oils — prevented by antioxidants, airtight packing, nitrogen flushing and refrigeration.
"Oxidation adds Oxygen — both start with O." For hydrogen, flip it with OIL RIG: Oxidation Is Loss of hydrogen, Reduction Is Gain of hydrogen (in higher classes the same OIL RIG works for electrons). For agents: the agent does the job to the other and suffers the opposite itself — the oxidising agent ends up reduced, the reducing agent ends up oxidised.
- ✓- Follow the oxygen (or, if absent, the hydrogen): gain of O / loss of H means oxidised; loss of O / gain of H means reduced.
- ✓- Oxidation and reduction are two halves of one event — always name both partners in a redox answer.
- ✓- Agents cross over: the oxidising agent is itself reduced, and the reducing agent is itself oxidised.
- ✓- Corrosion of metals and rancidity of food are everyday oxidation — both are controlled by keeping air and moisture away.
Oxidation, Reduction and Redox — Quick Revision
The core definitions and how to identify what is oxidised and reduced.
- Oxidation: gain of oxygen OR loss of hydrogen (loss of electrons).
- Reduction: loss of oxygen OR gain of hydrogen (gain of electrons).
- Redox reaction: oxidation and reduction happen together in the same reaction.
- The oxidising agent is itself reduced; the reducing agent is itself oxidised.
- Example: CuO + H2 → Cu + H2O — CuO loses oxygen (reduced, oxidising agent); H2 gains oxygen (oxidised, reducing agent).
- Example: ZnO + C → Zn + CO — carbon is the reducing agent.
Oxidation, Reduction and Redox — Flashcards
Cover the answer, recall, then check. 7 cards on redox reactions.
Q1. Define oxidation in terms of oxygen and hydrogen.
A1. Gain of oxygen or loss of hydrogen by a substance.
Q2. Define reduction in terms of oxygen and hydrogen.
A2. Loss of oxygen or gain of hydrogen by a substance.
Q3. What is a redox reaction?
A3. A reaction in which oxidation and reduction occur simultaneously.
Q4. In CuO + H2 → Cu + H2O, which substance is oxidised?
A4. H2 is oxidised (it gains oxygen to form water).
Q5. In CuO + H2 → Cu + H2O, which is the oxidising agent?
A5. CuO — it is reduced to Cu and supplies oxygen to hydrogen.
Q6. What happens to the reducing agent in a redox reaction?
A6. It is itself oxidised while it reduces the other substance.
Q7. In terms of electrons, what are oxidation and reduction?
A7. Oxidation is loss of electrons (OIL); reduction is gain of electrons (RIG).