A chemical reaction is a process in which one or more substances (reactants) change into new substances (products) with different chemical properties.
A chemical equation represents a chemical reaction using symbols and formulas of reactants and products. Example: 2H2 + O2 → 2H2O.
Because of the law of conservation of mass: matter is neither created nor destroyed. Balancing ensures the same number of each type of atom on both sides.
Balancing steps: (1) Write correct formulas, (2) Count atoms of each element, (3) Use coefficients to equalize atom counts, (4) Simplify coefficients if possible.
A combination reaction is where two or more substances combine to form a single product. Example: 2Mg + O2 → 2MgO.
In decomposition, a compound breaks down into two or more simpler substances. Example: 2KClO3 → 2KCl + 3O2 (on heating).
In a displacement reaction, a more reactive element displaces a less reactive element from its compound. Example: Zn + CuSO4 → ZnSO4 + Cu.
In double displacement, ions of two compounds exchange places to form new compounds. Example: AgNO3 + NaCl → AgCl↓ + NaNO3 (AgCl precipitates).
An exothermic reaction releases heat to the surroundings. Example: combustion of methane: CH4 + 2O2 → CO2 + 2H2O + heat.
An endothermic reaction absorbs heat from the surroundings. Example: photosynthesis overall is endothermic.
Oxidation is loss of electrons (or gain of oxygen/loss of hydrogen). Reduction is gain of electrons (or loss of oxygen/gain of hydrogen). They occur together.
A redox reaction involves simultaneous oxidation and reduction. Example: CuO + H2 → Cu + H2O (CuO reduced, H2 oxidised).
The activity series ranks metals by reactivity. A more reactive metal can displace a less reactive metal from its salt solution (e.g., Zn displaces Cu).
When two soluble salts react to form an insoluble solid (precipitate). Example: BaCl2 + Na2SO4 → BaSO4↓ + 2NaCl.
Rusting of iron: 4Fe + 3O2 + xH2O → 2Fe2O3·xH2O (formation of iron oxide hydrates).
Corrosion is the gradual destruction of metals by chemical reactions with air, water or other chemicals — e.g., iron rusting.
Methods: painting, galvanising (coating with Zn), electroplating, alloying (e.g., stainless steel), and sacrificial protection (using a more reactive metal).
Rancidity is the process where fats and oils become unpleasant in smell/taste due to oxidation. Prevention: storing in airtight containers, refrigeration, adding antioxidants.
Example: CaCl2 + Na2CO3 → CaCO3↓ + 2NaCl (CaCO3 precipitates).
Example: Thermal decomposition of calcium carbonate: CaCO3 → CaO + CO2 (on heating).
Combustion is a rapid exothermic reaction of a substance with oxygen producing heat and light. Example: burning of candle or wood.
In complete combustion, a hydrocarbon reacts with excess oxygen to form CO2 and H2O only. Example: CH4 + 2O2 → CO2 + 2H2O.
In incomplete combustion, limited oxygen causes formation of CO (carbon monoxide), soot (carbon) and less energy released. Example: burning of fuel in a poorly ventilated room.
Bring a burning splint near the gas — it produces a pop sound (test for hydrogen).
Insert a glowing splint into the gas — it relights (test for oxygen).
Bubble the gas through lime water (Ca(OH)2) — it turns milky due to CaCO3 formation; on excess CO2 it clears due to soluble Ca(HCO3)2.
Neutralisation is when an acid reacts with a base to form salt and water. Example: HCl + NaOH → NaCl + H2O.
Zn displaces Cu: Zn + CuSO4 → ZnSO4 + Cu. Copper metal deposits and solution becomes colourless (or light blue to colourless).
Zn will displace H2 from dilute acids because Zn is above H in the activity series. Cu is below H and won't displace it.
An aqueous solution is one where water is the solvent (e.g., NaCl dissolved in water is an aqueous solution).
A catalyst speeds up a reaction without being consumed. It provides an alternate pathway with lower activation energy. Example: MnO2 in decomposition of H2O2.
It predicts which metals can displace others and helps infer whether a reaction will occur between a metal and a salt or acid.
An ionic equation shows dissolved ions separately. A net ionic equation removes spectator ions leaving only species that participate in the reaction. Example: for AgNO3 + NaCl → AgCl↓ + NaNO3, net ionic: Ag+ + Cl- → AgCl↓.
Spectator ions are ions present in solution that do not participate in the chemical change (they remain unchanged on both sides).
A word equation describes a reaction using words. Example: Hydrogen + Oxygen → Water. It is often written before formula equations.
Identify correct chemical formulas of reactants and products and then write them with an arrow. Finally balance using coefficients if needed.
Look at reactants and products: single product (combination), single reactant broken up (decomposition), metal displacing metal (displacement), ions exchanging (double displacement), etc.
Rusting of iron is a redox process. Iron is oxidised (loses electrons) and oxygen is reduced.
Zn + 2HCl → ZnCl2 + H2↑ — hydrogen gas is evolved.
Oxygen is the oxidising agent that reacts with fuel; it accepts electrons (is reduced) and enables combustion to proceed.
A catalytic converter in cars uses catalysts (often Pt, Pd, Rh) to speed up oxidation of CO to CO2 and reduction of NOx, reducing harmful emissions.
A simplified form: 4Fe + 3O2 + xH2O → 2Fe2O3·xH2O (hydrated iron(III) oxide).
In a chemical reaction, the total mass of reactants equals the total mass of products; mass is conserved.
Using antacid tablets (bases) to neutralise excess stomach acid (HCl) is a practical neutralisation.
Fe + CuSO4 → FeSO4 + Cu (iron displaces copper).
Galvanisation coats iron/steel with zinc which is more reactive; zinc corrodes first (sacrificial protection), protecting the iron beneath.
Mixing phenolphthalein in basic solution turns pink; adding acid turns it colourless. Also, copper sulphate solution turns colourless when copper is displaced.
Reaction of metal carbonates with acids: CaCO3 + 2HCl → CaCl2 + CO2↑ + H2O (CO2 gas evolved).
Balanced: 2H2 + O2 → 2H2O.
Chemical equations concisely describe reactions, allow prediction of products, stoichiometric calculations, conservation checks, and communication of chemical changes.
Made for NCERT 10th — Chapter: Chemical Reactions & Equations