Almost every NDA Chemistry paper carries questions on types of chemical reactions. The good news? There are only a handful of patterns to learn. Once you can spot whether a reaction is combination, decomposition, displacement, double displacement or redox, you can answer questions in seconds. This guide from The Cavalier breaks each type down with examples.
Why Reaction Types Matter for NDA
A chemical reaction is a process in which one or more substances (reactants) are converted into one or more new substances (products) with different properties. The atoms simply rearrange — no atom is created or destroyed.
In the NDA General Studies paper, Chemistry questions frequently ask you to classify a given reaction, identify the oxidising or reducing agent, or predict a product. These are scoring questions because the rules are fixed and logical. Unlike numerical physics problems, here you only need to recognise the pattern in front of you.
It also helps to remember the difference between physical and chemical change. In a physical change (melting ice, dissolving sugar) no new substance forms and the change is usually reversible. In a chemical change a new substance with new properties forms, and it is generally not easily reversed. Every reaction type below is a chemical change.
Signs that a chemical reaction has occurred: change in colour, change in temperature (heat released or absorbed), evolution of a gas, formation of a precipitate, or change in state.
Throughout this page, watch for the physical state symbols: (s) solid, (l) liquid, (g) gas, and (aq) aqueous (dissolved in water). Examiners often include or omit these to test whether you understand the products.
Balancing Chemical Equations
Before classifying, you must understand a balanced equation. By the Law of Conservation of Mass, mass is neither created nor destroyed in a chemical reaction. So the number of atoms of each element must be equal on both sides.
Reactants → Products. The total mass of reactants equals the total mass of products. We balance by adjusting coefficients, never by changing the formula (subscripts) of a compound.
Take the formation of water:
Balance: H2 + O2 → H2O
The final equation 2H2 + O2 → 2H2O obeys conservation of mass.
Combination (Synthesis) Reactions
In a combination reaction, two or more reactants combine to form a single product. The general pattern is:
A + B → AB
Common NDA examples:
- Burning of coal: C + O2 → CO2
- Formation of slaked lime: CaO + H2O → Ca(OH)2 (this also releases heat)
- Hydrogen and chlorine: H2 + Cl2 → 2HCl
When the calcium oxide (quick lime) reacts with water to form slaked lime, a large amount of heat is given out. Reactions that release heat are called exothermic; those that absorb heat are endothermic. Slaked lime, Ca(OH)2, is used to whitewash walls; over a few days it slowly reacts with carbon dioxide in air to form a hard, shiny layer of calcium carbonate.
Combination reactions are not limited to elements. A compound can combine with another compound, or an element can combine with a compound. The defining feature is always the same — several reactants but only one product.
If two or more substances become ONE product, it is a combination reaction. Respiration and the burning of fuels are exothermic combination reactions — a favourite NDA fact.
Decomposition Reactions
A decomposition reaction is the opposite of combination: a single compound breaks down into two or more simpler substances. The general pattern is:
AB → A + B
Decomposition needs energy — supplied as heat, light or electricity:
- Thermal decomposition (heat): CaCO3 → CaO + CO2. Limestone on heating gives quick lime, used in cement.
- Electrolytic decomposition (electricity): 2H2O → 2H2 + O2. Passing current through acidified water splits it.
- Photo-decomposition (light): 2AgCl → 2Ag + Cl2. Silver chloride turns grey in sunlight, the basis of photography.
Most decomposition reactions are endothermic because energy must be supplied to break the compound apart.
Displacement Reactions and the Activity Series
In a displacement reaction, a more reactive element pushes out (displaces) a less reactive element from its compound or salt solution:
A + BC → AC + B (where A is more reactive than B)
Classic example: when an iron nail is dipped in copper sulphate solution, the blue colour fades and the nail turns brownish:
Fe + CuSO4 → FeSO4 + Cu
Iron is more reactive than copper, so it displaces copper. To predict displacement, use the reactivity (activity) series, arranged from most to least reactive:
K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Ag > Au. A metal higher in the list can displace one lower down.
The same idea explains why some metals react with dilute acids and others do not. Metals above hydrogen in the series (such as zinc and iron) displace hydrogen from dilute acids and release hydrogen gas: Zn + 2HCl → ZnCl2 + H2. Metals below hydrogen, like copper and silver, do not react with dilute acids in this way.
Gold (Au) and silver (Ag) are at the bottom — that is why they resist corrosion and are used in jewellery and coins. Highly reactive metals at the top (K, Na) are stored under kerosene because they react violently even with moisture in air.
Double Displacement and Precipitation
In a double displacement reaction, two compounds exchange their ions to form two new compounds. The general pattern is:
AB + CD → AD + CB
A very common case forms an insoluble solid called a precipitate:
Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)
The white solid barium sulphate (BaSO4) is the precipitate. Such reactions are also called precipitation reactions.
Neutralisation
Neutralisation is a special double displacement reaction between an acid and a base that gives a salt and water:
HCl + NaOH → NaCl + H2O
Here, hydrochloric acid and sodium hydroxide neutralise each other to form common salt and water. The reaction is exothermic. The H+ ions from the acid combine with the OH− ions from the base to make water, which is why the solution becomes neutral. This is why antacids (mild bases) relieve acidity in the stomach, and why farmers add lime to neutralise acidic soil.
Oxidation, Reduction and Redox
This is the most heavily tested topic, so learn it carefully. Originally:
- Oxidation = gain of oxygen OR loss of hydrogen OR loss of electrons.
- Reduction = loss of oxygen OR gain of hydrogen OR gain of electrons.
Mnemonic — OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).
A reaction where oxidation and reduction happen together is called a redox reaction. Consider copper oxide heated with hydrogen:
CuO + H2 → Cu + H2O
- CuO loses oxygen → it is reduced to Cu.
- H2 gains oxygen → it is oxidised to H2O.
The substance that gets reduced is the oxidising agent; the substance that gets oxidised is the reducing agent. Here CuO is the oxidising agent and H2 is the reducing agent.
Redox reactions power much of daily life. The current from a dry cell or a car battery comes from redox reactions, photosynthesis and respiration are redox processes, and the extraction of metals from their ores almost always involves reduction. So even reactions that look like simple combination or displacement are often redox underneath — if electrons move from one species to another, it is redox.
Corrosion and Rancidity
Two everyday effects of oxidation appear often in NDA papers.
Corrosion
Corrosion is the slow eating away of a metal by reaction with air, moisture or chemicals around it. The rusting of iron is the most common example:
4Fe + 3O2 + xH2O → 2Fe2O3·xH2O (hydrated iron oxide, i.e. rust)
Rusting needs BOTH oxygen and water (moisture). Iron does not rust in dry air or in pure water free of dissolved oxygen. Forgetting one factor loses easy marks.
Corrosion is prevented by painting, oiling, greasing, galvanising (zinc coating), and making alloys like stainless steel.
Rancidity
Rancidity is the spoiling of fats and oils in food when they are oxidised, giving a bad smell and taste. It is slowed by adding antioxidants, flushing packets with nitrogen gas, and refrigeration.
Exothermic and Endothermic Reactions
Reactions are also classified by their heat change, a concept NDA links to physics-style energy questions.
Exothermic: heat is released, surroundings get warmer. Endothermic: heat is absorbed, surroundings get cooler.
- Exothermic examples: respiration, combustion of fuels, mixing quick lime with water, neutralisation.
- Endothermic examples: thermal decomposition of calcium carbonate, photosynthesis, dissolving ammonium chloride in water.
A simple way to test this in everyday life: dissolving washing soda or quick lime in water makes the container feel warm (exothermic), while dissolving certain fertiliser salts makes it feel cold (endothermic). Instant cold packs used for sports injuries work on this endothermic principle.
Respiration is described as a slow exothermic combustion reaction — food (glucose) is broken down with oxygen to release energy. This exact phrasing has appeared in PYQs.
Worked Example: Classify and Balance
Classify and balance: AgNO3 + NaCl → AgCl + NaNO3. Identify any precipitate.
So this is a double displacement / precipitation reaction, and the white precipitate is silver chloride (AgCl). Note it darkens in light by photo-decomposition.
Previous-Year Style Question
Q. In the reaction ZnO + C → Zn + CO, which statement is correct?
Answer: Zinc oxide (ZnO) loses oxygen, so it is reduced to zinc; carbon (C) gains oxygen, so it is oxidised to carbon monoxide (CO). Therefore C is the reducing agent and ZnO is the oxidising agent — this is a redox reaction.
Students often label only one half. Every redox reaction has BOTH oxidation and reduction happening simultaneously — always identify both the oxidising and reducing agent.
Quick Revision
- Combination: A + B → AB (e.g. CaO + H2O → Ca(OH)2).
- Decomposition: AB → A + B; needs heat, light or electricity; usually endothermic.
- Displacement: more reactive metal displaces less reactive one; use the activity series.
- Double displacement: ions swap; can form a precipitate; neutralisation gives salt + water.
- Redox: oxidation (loss of e−) and reduction (gain of e−) together — OIL RIG.
- Rusting needs oxygen + water; rancidity is oxidation of fats, slowed by antioxidants.
Always balance equations using coefficients only, and tag the physical states (s), (l), (g), (aq) where the question asks.
Frequently asked questions
What are the five main types of chemical reactions for NDA?
Combination, decomposition, displacement, double displacement, and oxidation-reduction (redox). Neutralisation and precipitation are special cases of double displacement, and corrosion and rancidity are examples of oxidation.
How do I tell oxidation from reduction quickly?
Use OIL RIG: Oxidation Is Loss of electrons (or gain of oxygen / loss of hydrogen), and Reduction Is Gain of electrons (or loss of oxygen / gain of hydrogen). Both occur together in a redox reaction.
What is the difference between exothermic and endothermic reactions?
Exothermic reactions release heat and warm the surroundings, like respiration and combustion. Endothermic reactions absorb heat and cool the surroundings, like the thermal decomposition of calcium carbonate.
Why does an iron nail change colour in copper sulphate solution?
Iron is more reactive than copper, so it displaces copper from copper sulphate. The blue colour of the solution fades and the nail develops a brown copper coating. This is a displacement reaction.
What two conditions are needed for iron to rust?
Both oxygen (air) and moisture (water) are required. Iron does not rust in dry air or in water free of dissolved oxygen. Rusting is a slow oxidation forming hydrated iron(III) oxide.
Related NDA Chemistry topics
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