Chemical Reactions and Equations: Types, Oxidation-Reduction and Redox Notes with MCQs
Milk turning into curd, iron getting rusted, food being digested, a matchstick bursting into flame – all these are chemical reactions. In a chemical reaction, one or more substances (reactants) change into new substances (products) with different properties. Chemists describe these changes in a short, exact way using chemical equations, and classify them into types such as combination, decomposition, displacement, double displacement and redox reactions.
Chemical reactions and equations is one of the most important Chemistry chapters for SSC CGL, CHSL, MTS, GD, RRB NTPC, Group D, WBCS, WBPSC, Police and State PSC exams. Questions come on balancing equations, types of reactions with examples, exothermic and endothermic reactions, oxidation and reduction, oxidising and reducing agents, corrosion and rancidity. This guide explains everything in simple language with diagrams, solved examples, memory tricks and 20 MCQs with answers.
➤ Chemical equation: Reactants → Products · Must be balanced (law of conservation of mass)
➤ Five main types: Combination · Decomposition · Displacement · Double displacement · Redox
➤ Oxidation: Gain of oxygen / loss of hydrogen / loss of electrons
➤ Reduction: Loss of oxygen / gain of hydrogen / gain of electrons
➤ Memory key: "OIL RIG" – Oxidation Is Loss, Reduction Is Gain (of electrons)
➤ Exothermic: Heat released (burning, respiration) · Endothermic: Heat absorbed (photosynthesis)
➤ Corrosion: Rusting of iron (Fe2O3·xH2O) needs both air and water
Physical Change vs Chemical Change
| Physical change | Chemical change |
|---|---|
| No new substance is formed | A new substance with new properties is formed |
| Usually reversible | Usually irreversible |
| Only shape, size or state changes | Composition changes |
| Melting of ice, dissolving sugar, breaking glass, magnetising iron | Burning of paper, rusting, cooking food, curdling of milk, digestion |
How Do We Know a Chemical Reaction Has Taken Place?
| Sign | Example |
|---|---|
| Change in colour | Iron nail turns brown in copper sulphate solution; blue solution turns light green |
| Evolution of a gas | Zinc with dilute sulphuric acid gives hydrogen bubbles |
| Change in temperature | Quicklime with water becomes very hot |
| Formation of a precipitate | White precipitate of barium sulphate when sodium sulphate is mixed with barium chloride |
| Change in state | Burning of a candle (wax melts, burns to gases) |
Chemical Equations and Balancing
A chemical equation represents a reaction using symbols and formulae. Reactants are written on the left and products on the right, separated by an arrow. For example, when magnesium ribbon burns in air:
2Mg + O2 → 2MgO
According to the law of conservation of mass (Antoine Lavoisier), mass can neither be created nor destroyed in a chemical reaction. So the number of atoms of each element must be the same on both sides – the equation must be balanced. State symbols are added to make it more informative: (s) solid, (l) liquid, (g) gas and (aq) aqueous solution.
Example – balancing step by step: Iron reacts with steam to form iron oxide and hydrogen.
Skeletal equation: Fe + H2O → Fe3O4 + H2
Step 1 – Balance oxygen: 4 O atoms on the right, so put 4 before H2O → Fe + 4H2O → Fe3O4 + H2
Step 2 – Balance hydrogen: 8 H atoms on the left, so put 4 before H2 → Fe + 4H2O → Fe3O4 + 4H2
Step 3 – Balance iron: 3 Fe on the right → 3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)
Types of Chemical Reactions
Figure 1 shows the pattern of the four basic types of reactions using simple letters.
Figure 1: The four basic types of chemical reactions shown with letters – combination, decomposition, displacement and double displacement
1. Combination Reaction
Two or more substances combine to form a single new product.
| Reaction | Equation | Note |
|---|---|---|
| Quicklime + water | CaO(s) + H2O(l) → Ca(OH)2(aq) + heat | Forms slaked lime; strongly exothermic; slaked lime is used for whitewashing |
| Burning of coal | C(s) + O2(g) → CO2(g) | Exothermic |
| Formation of water | 2H2(g) + O2(g) → 2H2O(l) | – |
| Burning of magnesium | 2Mg + O2 → 2MgO | Dazzling white light |
Whitewashing: Slaked lime [Ca(OH)2] applied on walls slowly reacts with carbon dioxide in air to form a thin, shiny layer of calcium carbonate (CaCO3) in two to three days.
2. Decomposition Reaction
A single compound breaks down into two or more simpler substances. It needs energy in the form of heat, electricity or light, so most decomposition reactions are endothermic.
| Type (energy used) | Example |
|---|---|
| Thermal decomposition (heat) | CaCO3 → CaO + CO2 (limestone to quicklime – used in cement industry); 2FeSO4 → Fe2O3 + SO2 + SO3; 2Pb(NO3)2 → 2PbO + 4NO2 (brown fumes) + O2 |
| Electrolytic decomposition (electricity) | 2H2O → 2H2 + O2 – hydrogen and oxygen are released in the ratio 2 : 1 by volume |
| Photolytic decomposition (light) | 2AgCl → 2Ag + Cl2 (white silver chloride turns grey in sunlight); 2AgBr → 2Ag + Br2 – used in black-and-white photography |
Digestion of food in our body is also a set of decomposition reactions – starch breaks into glucose and proteins into amino acids.
3. Displacement Reaction
A more reactive element pushes out (displaces) a less reactive element from its compound. The reactivity series of metals tells us which metal can displace which.
| Reaction | Observation |
|---|---|
| Fe + CuSO4 → FeSO4 + Cu | Iron nail gets a brown coating of copper; blue solution turns light green |
| Zn + CuSO4 → ZnSO4 + Cu | Blue colour fades |
| Zn + H2SO4 → ZnSO4 + H2 | Hydrogen gas is released (used in laboratories) |
| Cu + AgNO3 solution | Copper displaces silver, solution turns blue |
| Cu + FeSO4 | No reaction – copper is less reactive than iron |
4. Double Displacement Reaction
Two compounds exchange their ions to form two new compounds. If one product is an insoluble solid, it is called a precipitation reaction.
| Reaction | Note |
|---|---|
| Na2SO4 + BaCl2 → BaSO4↓ + 2NaCl | White precipitate of barium sulphate |
| Pb(NO3)2 + 2KI → PbI2↓ + 2KNO3 | Yellow precipitate of lead iodide |
| AgNO3 + NaCl → AgCl↓ + NaNO3 | White precipitate of silver chloride |
| NaOH + HCl → NaCl + H2O | Neutralisation reaction – acid + base → salt + water |
Exothermic and Endothermic Reactions
| Exothermic reactions (heat released) | Endothermic reactions (heat absorbed) |
|---|---|
| Burning of natural gas: CH4 + 2O2 → CO2 + 2H2O + energy | Photosynthesis: 6CO2 + 6H2O + sunlight → C6H12O6 + 6O2 |
| Respiration: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy | Thermal decomposition of limestone |
| Quicklime + water | Electrolysis of water |
| Decomposition of vegetable matter into compost | Dissolving ammonium chloride in water (solution becomes cold) |
Exam point: Respiration is an exothermic process – it releases the energy we need. Photosynthesis is endothermic – plants absorb energy from sunlight.
Oxidation and Reduction (Redox Reactions)
In many reactions, one substance is oxidised while another is reduced at the same time. Such reactions are called redox reactions (reduction + oxidation).
| Basis | Oxidation | Reduction |
|---|---|---|
| Oxygen | Gain of oxygen | Loss of oxygen |
| Hydrogen | Loss of hydrogen | Gain of hydrogen |
| Electrons (modern definition) | Loss of electrons | Gain of electrons |
| Oxidation number | Increases | Decreases |
Example: When hydrogen is passed over heated copper oxide:
CuO + H2 → Cu + H2O
Copper oxide loses oxygen – it is reduced to copper. Hydrogen gains oxygen – it is oxidised to water (Figure 2).
Figure 2: A redox reaction – copper oxide is reduced and hydrogen is oxidised at the same time
Oxidising and Reducing Agents
An oxidising agent gives oxygen (or removes hydrogen, or accepts electrons) and itself gets reduced. A reducing agent takes oxygen (or gives hydrogen, or donates electrons) and itself gets oxidised. In the example above, CuO is the oxidising agent and H2 is the reducing agent.
| Reaction | Oxidised | Reduced |
|---|---|---|
| ZnO + C → Zn + CO | Carbon (C → CO) | Zinc oxide (ZnO → Zn) |
| MnO2 + 4HCl → MnCl2 + 2H2O + Cl2 | HCl (loses hydrogen → Cl2) | MnO2 (loses oxygen) |
| Fe2O3 + 2Al → 2Fe + Al2O3 (thermite reaction) | Aluminium | Iron oxide |
| 2Na + Cl2 → 2NaCl | Sodium (loses an electron) | Chlorine (gains an electron) |
Common oxidising agents: Oxygen, ozone, hydrogen peroxide, potassium permanganate (KMnO4), potassium dichromate (K2Cr2O7), chlorine. Common reducing agents: Hydrogen, carbon, carbon monoxide, aluminium.
Effects of Oxidation in Daily Life
Corrosion
Corrosion is the slow eating away of metals by air, moisture or chemicals. Iron forms reddish-brown rust (hydrated iron(III) oxide, Fe2O3·xH2O); copper gets a green coating (basic copper carbonate); silver turns black (silver sulphide). Rusting needs both oxygen (air) and water, and is faster in salty or humid air – which is why coastal areas see more rusting.
| Prevention method | How it works |
|---|---|
| Painting, oiling, greasing | Keeps air and moisture away from the iron surface |
| Galvanisation | Coating iron with a thin layer of zinc; zinc corrodes first and protects iron |
| Electroplating / tinning | Coating with chromium, nickel or tin |
| Alloying | Making stainless steel (iron + chromium + nickel) which does not rust easily |
Rancidity
When fats and oils are oxidised, they become rancid – their smell and taste change. It is prevented by adding antioxidants, keeping food in airtight containers or in the refrigerator, and by flushing packets with nitrogen, as is done in packets of potato chips.
Combustion
Combustion is a fast oxidation reaction in which a substance burns in oxygen and gives heat and light. A fire needs fuel, heat (ignition temperature) and oxygen. Rusting is slow oxidation; burning is fast oxidation.
Factors Affecting the Rate of a Reaction
| Factor | Effect | Example |
|---|---|---|
| Temperature | Higher temperature → faster reaction | Food cooks faster in a pressure cooker; food spoils slowly in a refrigerator |
| Concentration | More concentrated reactants → faster reaction | Concentrated acid reacts faster than dilute acid |
| Surface area | Powder reacts faster than lumps | Powdered chalk reacts faster with acid than a piece of chalk |
| Catalyst | Speeds up a reaction without being used up | Iron in the Haber process for ammonia; nickel in hydrogenation of oils; enzymes in our body |
Important Reactions at a Glance
| Reaction | Type |
|---|---|
| CaO + H2O → Ca(OH)2 | Combination, exothermic |
| CaCO3 → CaO + CO2 | Thermal decomposition |
| 2H2O → 2H2 + O2 | Electrolytic decomposition |
| 2AgCl → 2Ag + Cl2 | Photolytic decomposition |
| Fe + CuSO4 → FeSO4 + Cu | Displacement |
| Na2SO4 + BaCl2 → BaSO4 + 2NaCl | Double displacement (precipitation) |
| NaOH + HCl → NaCl + H2O | Neutralisation |
| CuO + H2 → Cu + H2O | Redox |
| Fe2O3 + 2Al → 2Fe + Al2O3 | Displacement and redox (thermite – used to weld railway tracks) |
West Bengal Connection – Acharya Prafulla Chandra Ray
Kolkata is closely linked with the growth of chemistry in India. Acharya Prafulla Chandra Ray, known as the "Father of Indian Chemistry", taught at Presidency College, Calcutta, and discovered the stable compound mercurous nitrite in 1896. In 1901 he founded Bengal Chemicals and Pharmaceutical Works in Kolkata, one of India's first chemical and pharmaceutical companies. He also wrote A History of Hindu Chemistry. In 2011, the Royal Society of Chemistry honoured his work with a Chemical Landmark Plaque in Kolkata – the first such plaque outside Europe. Kolkata's iron bridges, such as the Howrah Bridge, also need regular painting to protect their steel from rusting in the humid, salty air near the Hooghly.
For related topics, read Carbon and Organic Chemistry Complete Study Notes (combustion and reactions of carbon compounds), Biotechnology and Life Sciences (enzymes as biological catalysts) and Climatology (how burning fuels adds CO2 to the air). Practise with the CSIR NAL MTS Mock Test.
Quick Revision Box – Easy Tricks to Memorise
Trick 1 – "OIL RIG"
Oxidation Is Loss (of electrons), Reduction Is Gain.
Trick 2 – "LEO the lion says GER"
Lose Electrons = Oxidation; Gain Electrons = Reduction.
Trick 3 – Oxygen–Hydrogen rule
Oxidation = gain O or lose H; Reduction = lose O or gain H.
Trick 4 – Agents work "opposite"
Oxidising agent gets reduced; reducing agent gets oxidised.
Trick 5 – Types "C-D-D-DD"
Combination (A + B → AB), Decomposition (AB → A + B), Displacement (C + AB → CB + A), Double displacement (AB + CD → AD + CB).
Trick 6 – Decomposition needs "Heat, Light, Current"
Thermal (CaCO3), Photolytic (AgCl), Electrolytic (H2O).
Trick 7 – Water electrolysis "H is double"
H2 : O2 = 2 : 1 by volume.
Trick 8 – "Exo goes Out, Endo goes In"
Exothermic releases heat; endothermic absorbs heat.
Trick 9 – "Respiration Releases, Photosynthesis takes"
Respiration is exothermic; photosynthesis is endothermic.
Trick 10 – Precipitate colours
BaSO4 white, AgCl white, PbI2 yellow.
Trick 11 – Rust needs "Air + Water"
Galvanisation = zinc coating; rust = Fe2O3·xH2O.
Trick 12 – Chips packets have "Nitrogen"
Nitrogen flushing prevents rancidity (oxidation of fats).
20 MCQs on Chemical Reactions and Equations
Q1. Which of the following is a chemical change?
(a) Melting of ice
(b) Dissolving sugar in water
(c) Rusting of iron
(d) Breaking of glass
Answer: (c) – Rusting forms a new substance, hydrated iron oxide.
Q2. A chemical equation is balanced to satisfy the law of:
(a) Conservation of mass
(b) Constant proportions
(c) Multiple proportions
(d) Conservation of energy
Answer: (a) – Given by Antoine Lavoisier; atoms are neither created nor destroyed.
Q3. The balanced equation for the reaction of iron with steam is:
(a) Fe + H2O → Fe3O4 + H2
(b) 3Fe + 4H2O → Fe3O4 + 4H2
(c) 2Fe + 3H2O → Fe2O3 + 3H2
(d) 3Fe + 2H2O → Fe3O4 + 2H2
Answer: (b) – 3 Fe, 8 H and 4 O atoms on each side.
Q4. CaO + H2O → Ca(OH)2 is an example of:
(a) Decomposition reaction
(b) Displacement reaction
(c) Double displacement reaction
(d) Combination reaction
Answer: (d) – Two reactants form one product; the reaction is also exothermic.
Q5. The chemical name of slaked lime is:
(a) Calcium oxide
(b) Calcium carbonate
(c) Calcium hydroxide
(d) Calcium sulphate
Answer: (c) – Ca(OH)2; quicklime is CaO and limestone is CaCO3.
Q6. In the electrolysis of water, hydrogen and oxygen are released in the volume ratio of:
(a) 1 : 1
(b) 2 : 1
(c) 1 : 2
(d) 8 : 1
Answer: (b) – Because water (H2O) has two hydrogen atoms for each oxygen atom.
Q7. White silver chloride turns grey in sunlight. This is an example of:
(a) Photolytic decomposition
(b) Thermal decomposition
(c) Combination
(d) Neutralisation
Answer: (a) – 2AgCl → 2Ag + Cl2; used in black-and-white photography.
Q8. When an iron nail is dipped in copper sulphate solution, the solution changes colour from:
(a) Green to blue
(b) Colourless to blue
(c) Blue to colourless
(d) Blue to light green
Answer: (d) – Iron displaces copper and forms green iron sulphate.
Q9. Which of the following reactions will NOT take place?
(a) Zn + CuSO4
(b) Fe + CuSO4
(c) Cu + FeSO4
(d) Mg + CuSO4
Answer: (c) – Copper is less reactive than iron and cannot displace it.
Q10. The yellow precipitate formed when lead nitrate reacts with potassium iodide is:
(a) Potassium nitrate
(b) Lead iodide
(c) Lead oxide
(d) Potassium iodate
Answer: (b) – Pb(NO3)2 + 2KI → PbI2↓ + 2KNO3, a double displacement reaction.
Q11. The reaction between an acid and a base to form salt and water is called:
(a) Oxidation
(b) Decomposition
(c) Combination
(d) Neutralisation
Answer: (d) – NaOH + HCl → NaCl + H2O.
Q12. Which of the following is an endothermic process?
(a) Photosynthesis
(b) Respiration
(c) Burning of coal
(d) Reaction of quicklime with water
Answer: (a) – Plants absorb energy from sunlight; the other three release heat.
Q13. Oxidation is defined as:
(a) Gain of electrons
(b) Loss of oxygen
(c) Loss of electrons
(d) Gain of hydrogen
Answer: (c) – "OIL RIG": Oxidation Is Loss of electrons.
Q14. In the reaction CuO + H2 → Cu + H2O, the substance that is oxidised is:
(a) CuO
(b) Cu
(c) H2O
(d) H2
Answer: (d) – Hydrogen gains oxygen to form water; CuO is reduced.
Q15. In a redox reaction, the oxidising agent itself gets:
(a) Reduced
(b) Oxidised
(c) Unchanged
(d) Decomposed
Answer: (a) – It gives oxygen or takes electrons, so it is reduced.
Q16. The chemical formula of rust is:
(a) FeO
(b) Fe2O3·xH2O
(c) Fe3O4
(d) FeCO3
Answer: (b) – Hydrated iron(III) oxide, reddish-brown in colour.
Q17. Galvanisation is the process of coating iron with:
(a) Tin
(b) Copper
(c) Zinc
(d) Chromium
Answer: (c) – Zinc protects iron from rusting even if the coating is scratched.
Q18. Chips packets are flushed with which gas to prevent rancidity?
(a) Oxygen
(b) Carbon dioxide
(c) Hydrogen
(d) Nitrogen
Answer: (d) – Nitrogen prevents oxidation of fats and oils.
Q19. The thermite reaction (Fe2O3 + 2Al → 2Fe + Al2O3) is used for:
(a) Welding railway tracks
(b) Making cement
(c) Purifying water
(d) Making soap
Answer: (a) – The reaction releases so much heat that molten iron is produced.
Q20. Bengal Chemicals and Pharmaceutical Works was founded in Kolkata in 1901 by:
(a) J.C. Bose
(b) Meghnad Saha
(c) S.N. Bose
(d) Prafulla Chandra Ray
Answer: (d) – Acharya P.C. Ray, the "Father of Indian Chemistry", also discovered mercurous nitrite in 1896.
Want more practice? Try our Daily GK Quiz and Mock Tests.
Conclusion
Chemical reactions are happening all around us and inside us. A balanced chemical equation shows exactly what changes, and reactions can be grouped as combination, decomposition, displacement, double displacement and redox. Oxidation and reduction always go together, and their effects – corrosion, rancidity and combustion – are part of daily life. For exams, remember the examples of each type, the colours of precipitates, OIL RIG, oxidising and reducing agents, exothermic and endothermic reactions, and how rusting and rancidity are prevented. Revise the tricks above and practise the MCQs regularly.
Reference: NCERT Science Class X ("Chemical Reactions and Equations"); NCERT Chemistry Class XI ("Redox Reactions").
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