Chemical Reactions and Equations
How we recognise a chemical change, represent it as a balanced chemical equation obeying the law of conservation of mass, and classify reactions into combination and decomposition (and exothermic) types.
NCERT basic chemistry is a reliable Prelims source, testing identification of reaction types, balancing of equations, and everyday applications such as whitewashing, respiration, electrolysis and photography. For Mains it feeds GS-III Science & Technology and the foundational scientific literacy behind industrial chemistry and the constitutional duty to develop a scientific temper (Article 51A(h)).
Understand the chapter
Recognising a Chemical Reaction
A chemical reaction occurs whenever the nature and identity of a substance change — souring of milk, rusting of iron, fermentation, cooking, digestion and respiration are everyday examples. The chapter fixes four observable clues that tell us a reaction has happened, even without knowing the chemistry.
- Change in state
- Change in colour
- Evolution of a gas
- Change in temperature (heat absorbed or released)
From Words to Symbols: Writing Equations
A reaction described in a sentence is first condensed into a word-equation (reactants on the left, product on the right, joined by an arrow), then into a chemical equation using formulae. The substances that change are reactants (LHS); the new substance formed is the product (RHS). An equation written with correct formulae but unequal atoms is a skeletal (unbalanced) equation.
- Reactants on LHS, products on RHS, separated by + signs
- Arrowhead points towards products, showing reaction direction
- Skeletal equation = formulae written but atoms not yet balanced
- Example: Mg + O2 → MgO is a skeletal equation
Balancing and the Law of Conservation of Mass
Mass can neither be created nor destroyed in a chemical reaction (law of conservation of mass, established by Antoine Lavoisier), so the number of atoms of every element must be equal on both sides. Balancing is done by the hit-and-trial method using the smallest whole-number coefficients placed before formulae. Formulae and subscripts must never be altered; it is convenient to begin with the compound having the maximum number of atoms.
- Atoms of each element must be identical on LHS and RHS
- Adjust coefficients only — write 4H2O, never H2O4 or (H2O)4
- Hit-and-trial uses smallest whole-number coefficients
- Worked example: 3Fe + 4H2O → Fe3O4 + 4H2
Making Equations Informative: States and Conditions
To add information, the physical state of each species is written after its formula. Reaction conditions such as temperature, pressure or catalyst are placed above or below the arrow. Note that H2O(g) indicates water used as steam.
- (s) solid, (l) liquid, (g) gas, (aq) aqueous
- (aq) means present as a solution in water
- Conditions (heat, pressure, catalyst) go above/below the arrow
- States are shown only when it is necessary to specify them
Combination Reactions
When two or more reactants — elements or compounds — combine to form a single product, the reaction is a combination reaction. The classic case is quick lime reacting with water to give slaked lime, releasing a large amount of heat. The slaked lime solution is used for whitewashing, where it slowly absorbs atmospheric CO2 to form a shiny layer of calcium carbonate.
- Two or more reactants → a single product
- CaO(s) + H2O(l) → Ca(OH)2(aq) + heat (quick lime → slaked lime)
- Burning of coal: C + O2 → CO2; and 2H2 + O2 → 2H2O
- Whitewashing: Ca(OH)2 + CO2 → CaCO3 (marble is also CaCO3)
Energy of Reactions: Exothermic Changes
Reactions in which heat is released along with the products are exothermic. Respiration is an exothermic reaction in which glucose combines with oxygen inside cells to release energy; burning of natural gas and the decomposition of vegetable matter into compost are other examples. The vigorous warming felt when CaO meets water is itself an exothermic combination reaction.
- Exothermic = heat released to surroundings
- Respiration: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
- Combustion of natural gas: CH4 + 2O2 → CO2 + 2H2O
- Composting of vegetable matter is exothermic
Decomposition Reactions: Heat, Electricity, Light
A single reactant breaking down into two or more simpler products is a decomposition reaction — essentially the reverse of combination. The energy required is supplied as heat (thermal decomposition), electricity (electrolysis) or light (photolysis). Thermal decomposition of limestone is industrially vital for making quick lime and cement.
- Thermal: CaCO3 → CaO + CO2 (cement); 2FeSO4 → Fe2O3 + SO2 + SO3; 2Pb(NO3)2 → 2PbO + 4NO2 + O2 (brown fumes)
- Electrolytic: electrolysis of water → H2 + O2 in 2:1 volume ratio
- Photolytic: 2AgCl → 2Ag + Cl2 in sunlight (white turns grey)
- Decomposition reactions generally need a continuous energy supply
Key terms
- Chemical reaction
- A change in which the nature and identity of substances alter, producing new substances.
- Reactants
- The starting substances that undergo chemical change, written on the LHS of an equation.
- Product
- A new substance formed during a reaction, written on the RHS of an equation.
- Skeletal equation
- A chemical equation written with formulae but with unequal atoms on the two sides (unbalanced).
- Balanced equation
- An equation having equal numbers of atoms of each element on both sides, satisfying conservation of mass.
- Combination reaction
- A reaction in which two or more reactants combine to form a single product.
- Decomposition reaction
- A reaction in which a single reactant breaks down into two or more simpler products.
- Thermal decomposition
- Decomposition of a substance brought about by heating.
- Exothermic reaction
- A reaction that releases heat along with the formation of products.
- Aqueous (aq)
- A species present dissolved as a solution in water.
Must-know facts exam-ready
- Four signs of a chemical reaction: change in state, change in colour, evolution of a gas, change in temperature.
- Law of conservation of mass (Antoine Lavoisier): mass is neither created nor destroyed — the basis for balancing equations.
- Equations are balanced by the hit-and-trial method using smallest whole-number coefficients; formulae must never be changed.
- State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution in water.
- CaO + H2O → Ca(OH)2 + heat: quick lime to slaked lime, an exothermic combination reaction used for whitewashing.
- Whitewashing: Ca(OH)2 + CO2 → CaCO3 over 2-3 days; marble is also CaCO3.
- CaCO3 → CaO + CO2 is thermal decomposition; the quick lime (CaO) produced is used to manufacture cement.
- Heating ferrous sulphate (green): 2FeSO4 → Fe2O3 (ferric oxide) + SO2 + SO3.
- Heating lead nitrate: 2Pb(NO3)2 → 2PbO + 4NO2 (brown fumes) + O2.
- Photolysis: 2AgCl → 2Ag + Cl2 in sunlight; white silver chloride turns grey (used in photography).
- Electrolysis of water yields hydrogen and oxygen in a 2:1 volume ratio.
- Respiration is an exothermic reaction: glucose + oxygen → carbon dioxide + water + energy.
Memory tricks remember it for good
Traps to avoid
- Balancing changes the coefficients (numbers before a formula), never the subscripts/formula — writing H2O4 instead of 4H2O is wrong.
- A skeletal equation already has correct formulae; having formulae does NOT mean the equation is balanced.
- Quick lime is CaO and slaked lime is Ca(OH)2 — do not swap them; limestone and marble are both CaCO3.
- Decomposition is not always thermal: electrolysis uses electricity and photolysis uses light — only heat-driven decomposition is 'thermal'.
- Watch the gases: brown fumes from lead nitrate are NO2; heated ferrous sulphate gives SO2 and SO3 — don't mix them up.
- H2O(g) denotes steam, not liquid water; (aq) means dissolved in water, which is different from pure liquid (l).
Exam focus
🧠 Prelims angles
- Identify the reaction type (combination / decomposition / thermal / electrolytic / photolytic) from a given equation or example.
- Balance a given skeletal equation or choose the correct set of coefficients.
- Match an everyday process to its reaction type: whitewashing, respiration, photography (AgCl), composting.
- Recall specific observations and products: brown NO2 fumes, AgCl turning grey, green FeSO4 changing colour, 2:1 gases in water electrolysis.
- Formulae and uses of quick lime (CaO), slaked lime (Ca(OH)2) and limestone/marble (CaCO3), including CaO in cement.
- Law of conservation of mass as the principle underlying the balancing of chemical equations.
✍️ Mains angles GS-III
- How basic chemical principles underpin heavy industry and its environmental footprint.Use thermal decomposition CaCO3 → CaO + CO2 for cement; flag the process CO2 emissions as a hard-to-abate decarbonisation challenge (GS-III).
- Promoting scientific temper as a constitutional duty and a developmental tool.Anchor in Article 51A(h); argue that everyday-science literacy — recognising chemical change — builds rational, evidence-based citizens.
- Energy in chemical reactions and its everyday and biological significance.Use respiration and combustion of natural gas as exothermic examples; connect to energy, food and combustion-emission debates.
Last-minute revision tick as you recall
- Four signs of a reaction = CGST: Colour, Gas, State, Temperature.
- Balance by hit-and-trial; change coefficients only, never formulae (Law of Conservation of Mass, Lavoisier).
- State symbols (s),(l),(g),(aq); reaction conditions go above the arrow; H2O(g) = steam.
- Combination = many → one: CaO + H2O → Ca(OH)2 + heat (exothermic, whitewashing).
- Decomposition = one → many, driven by Heat / Electricity / Light (HEL).
- Thermal: CaCO3 → CaO + CO2 (cement); 2Pb(NO3)2 → PbO + NO2(brown) + O2; 2FeSO4 → Fe2O3 + SO2 + SO3.
- Photolysis 2AgCl → 2Ag + Cl2 (grey, photography); electrolysis of water gives H2:O2 = 2:1.
- Respiration and combustion are exothermic; remember CaO (quick lime), Ca(OH)2 (slaked lime), CaCO3 (marble).
- Whitewashing: Ca(OH)2 + CO2 → CaCO3 forms a shiny coat in 2-3 days.
Distilled from NCERT Class 10 · Science (Class 10) for UPSC. Always cross-check facts with the original NCERT.