Magnesium ribbon is cleaned with sandpaper to remove the thin layer of magnesium oxide (and other surface impurities) from its surface. This allows fresh magnesium to react properly with oxygen and burn with a bright flame.
Clean Mg = fresh Mg. Remember: “Remove the coating, start the reaction.”
H₂(g) + Cl₂(g) → 2HCl(g).
H and Cl both come in pairs (H₂, Cl₂), so put 2 before HCl: “Pairs need a pair of products.”
3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 3BaSO₄(s) + 2AlCl₃(aq).
Keep the polyatomic groups SO₄ together. Balance SO₄ first, then Ba, then Al/Cl. Think: “3–1–3–2.”
2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g).
Hydrogen is H₂, so make 2 Na and 2 H₂O: “Two sodium, two water, one hydrogen.”
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq).
The white insoluble solid is BaSO₄. Remember: “BaSO₄ = white precipitate.”
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l).
This is a simple 1:1 exchange: Na joins Cl, while H joins OH.
The substance X is calcium oxide (quick lime), CaO. With water: CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat. Calcium hydroxide is slaked lime and its solution is used for whitewashing.
Quick lime + water → slaked lime + heat. Remember: “Quick gets Slaked.”
Water contains hydrogen and oxygen in a 2:1 ratio by volume. During electrolysis, water decomposes into hydrogen and oxygen, so hydrogen is collected in twice the volume of oxygen. The gas collected in double amount is hydrogen (H₂).
Water is H₂O: “2 H for 1 O,” so hydrogen volume is double.
Iron is more reactive than copper, so it displaces copper from copper sulphate: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s). The blue copper sulphate solution changes because iron sulphate is formed, and copper is deposited on the nail.
“More reactive kicks less reactive out.” Iron pushes copper out of CuSO₄.
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq). This is a double displacement reaction because the ions exchange partners, and insoluble AgCl forms as a precipitate.
Double displacement = “partners swap.” Ag swaps with Na; Cl joins Ag.
Sodium (Na) is oxidised because it gains oxygen. Oxygen is reduced because it combines with sodium to form sodium oxide.
Oxidation = gain of oxygen. Think “OIL”: Oxidation Is Loss (of hydrogen) / gain of oxygen in this chapter.
Hydrogen (H₂) is oxidised because it gains oxygen to form water. Copper(II) oxide (CuO) is reduced because it loses oxygen and becomes copper.
“CuO loses O → reduced; H gains O → oxidised.”
The incorrect statements are (b) only: carbon dioxide is a product and is not being oxidised in this reaction. Lead is reduced because PbO loses oxygen to form Pb, carbon is oxidised because it gains oxygen to form CO₂, and lead oxide is reduced. The listed options in the question do not contain “(b) only,” so the options as printed do not provide an exact choice.
Check each substance before choosing an option: “PbO → Pb = reduction; C → CO₂ = oxidation.”
The correct answer is (d) displacement reaction. Aluminium displaces iron from iron(III) oxide, forming aluminium oxide and iron.
One element kicks another element out of its compound = displacement.
The correct answer is (a). Iron reacts with dilute hydrochloric acid to form an iron chloride salt and hydrogen gas: Fe + 2HCl → FeCl₂ + H₂.
Metal + dilute acid → salt + H₂. Remember: “Metal meets acid, hydrogen flies out.”
A balanced chemical equation has the same number of atoms of each element on the reactant side and the product side. Equations must be balanced because mass is conserved in a chemical reaction; atoms are neither created nor destroyed.
“Same atoms, both sides.” Balance atoms, not formulas.
N₂(g) + 3H₂(g) → 2NH₃(g).
Balance N first, then H: 1 N₂ needs 2 NH₃, which needs 6 H atoms = 3H₂.
2H₂S(g) + 3O₂(g) → 2H₂O(l) + 2SO₂(g).
Balance H and S first with 2H₂S; then oxygen becomes 6 on the right, so use 3O₂.
3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 3BaSO₄(s) + 2AlCl₃(aq).
Keep SO₄ as a group. The coefficient pattern is “3–1–3–2.”
2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g).
Like sodium: “2 metal + 2 water → 2 hydroxide + H₂.”
2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O.
There are 2 nitrate groups in Ca(NO₃)₂, so put 2 before HNO₃; then 2 waters balance H.
2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O.
Na₂ needs 2 NaOH; the two OH groups then form 2H₂O.
NaCl + AgNO₃ → AgCl + NaNO₃. It is already balanced.
Count: Na 1, Cl 1, Ag 1, NO₃ 1 on both sides — “1:1:1:1.”
BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl.
Two Cl atoms on the left require 2HCl on the right.
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O.
The equation balances as written: Ca 1, C 1, H 2, O 4 on both sides.
Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag.
Zn(NO₃)₂ needs 2 nitrate groups, so use 2AgNO₃ and get 2Ag.
2Al + 3CuCl₂ → 2AlCl₃ + 3Cu.
LCM of Cl counts 6: 3CuCl₂ and 2AlCl₃.
BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl.
Two Cl on the left means 2KCl; K then automatically becomes 2.
2KBr(aq) + BaI₂(aq) → 2KI(aq) + BaBr₂(s). Type: double displacement reaction.
Both compounds swap ions: K↔Ba partners change. “Swap = double displacement.”
ZnCO₃(s) → ZnO(s) + CO₂(g). Type: decomposition reaction (thermal decomposition when carried out by heating).
One reactant breaks into two products = decomposition. “One breaks, two make.”
H₂(g) + Cl₂(g) → 2HCl(g). Type: combination reaction.
Two reactants form one product = combination. “Many → one.”
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g). Type: displacement reaction.
A reactive metal displaces hydrogen from acid: “Metal kicks H out.”
Exothermic reactions release heat along with the products. Example: CaO + H₂O → Ca(OH)₂ + heat. Endothermic reactions absorb energy. Examples include decomposition of CaCO₃ by heat and decomposition of AgCl by sunlight.
Exo = exit of heat. Endo = energy enters.
During respiration, glucose reacts with oxygen in the cells and produces carbon dioxide, water and energy: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. Because energy is released, respiration is an exothermic process.
Respiration releases energy for life: “Glucose + O₂ → energy.”
In a combination reaction, two or more substances combine to form one product. In a decomposition reaction, one substance breaks down to form two or more products. Examples: Combination: 2H₂ + O₂ → 2H₂O. Decomposition: CaCO₃ → CaO + CO₂ (on heating).
Combination: “many → one.” Decomposition: “one → many.”
Heat: CaCO₃(s) —heat→ CaO(s) + CO₂(g). Light: 2AgCl(s) —sunlight→ 2Ag(s) + Cl₂(g). Electricity: 2H₂O(l) —electricity→ 2H₂(g) + O₂(g).
Three energy forms: “Heat–Light–Electricity” = H-L-E. Match each with a decomposition example.
In a displacement reaction, one element displaces another element from its compound. Example: Fe + CuSO₄ → FeSO₄ + Cu. In a double displacement reaction, two compounds exchange ions/groups. Example: Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl.
Displacement = one element replaces one. Double displacement = two compounds swap partners.
Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s).
Copper is more reactive than silver, so Cu pushes Ag out: “Cu kicks Ag out.”
A precipitation reaction is a reaction in which an insoluble solid, called a precipitate, is formed. Example: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq). Another example is AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq).
Precipitate = “solid appears from solution.” Look for an insoluble (s) product.
Oxidation is the gain of oxygen. Example 1: 2Cu + O₂ → 2CuO; copper gains oxygen. Example 2: 4Na + O₂ → 2Na₂O; sodium gains oxygen.
Oxidation = Oxygen added. Remember “O = Oxidation.”
Reduction is the loss of oxygen. Example 1: CuO + H₂ → Cu + H₂O; CuO loses oxygen and becomes Cu. Example 2: ZnO + C → Zn + CO; ZnO loses oxygen and becomes Zn.
Reduction = oxygen removed. “Remove O = Reduce.”
X is copper (Cu). The black compound is copper(II) oxide (CuO). On heating: 2Cu + O₂ → 2CuO.
Brown copper + oxygen → black CuO. “Copper goes black when oxygen is added.”
Paint forms a protective layer that keeps iron away from oxygen and moisture in the surroundings. This slows down corrosion (rusting) of iron.
Paint = protective barrier. “Block air + water, block rust.”
Nitrogen is used to reduce contact between the food and oxygen. This slows down oxidation of fats and oils and therefore helps prevent rancidity. Nitrogen flushing is used in packets such as chips.
Nitrogen blocks oxygen. “No O₂ → slow oxidation → less rancidity.”
Corrosion is the process in which a metal is attacked by substances in its surroundings such as moisture or acids. Example: iron reacts with oxygen and moisture over time and develops a reddish-brown coating called rust.
Corrosion = metal slowly gets attacked. Iron + moisture → rust.
Rancidity occurs when fats and oils are oxidised, causing the food to develop an unpleasant smell and taste. Example: oil- or fat-containing food left exposed to air for a long time may become rancid.
Rancidity = “old oily food smells/tastes bad” because of oxidation.
Magnesium burns with a dazzling white flame and forms a white powder, magnesium oxide (MgO).
“Mg burns white, makes white MgO.”
A yellow precipitate is formed. It is lead iodide (PbI₂).
“Lead iodide = yellow.”
Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq).
Keep NO₃ together; two iodides are needed for PbI₂.
Yes. The ions exchange partners: Pb²⁺ combines with I⁻ to form PbI₂, while K⁺ combines with NO₃⁻ to form KNO₃.
“Two compounds swap ions = double displacement.”
Yes. Bubbles of gas are produced around the zinc granules because hydrogen gas is evolved.
“Metal + acid → hydrogen bubbles.”
Yes. The container becomes warm because the reaction releases heat.
Warm container = heat released = exothermic.
Yes. The beaker becomes warm because calcium oxide reacts vigorously with water and releases a large amount of heat.
“Quick lime + water = heat.”
Yes. The balanced equation is 2Mg + O₂ → 2MgO.
O₂ needs 2 MgO; then use 2Mg.
It is both a combination reaction and an exothermic reaction. Magnesium and oxygen combine to form one product, MgO, and heat is released.
One product = combination; heat out = exothermic.
No. The volume of hydrogen collected is double the volume of oxygen.
Water is H₂O → 2 parts H, 1 part O.
The hydrogen gas burns with a characteristic ‘pop’ when tested carefully, while oxygen supports combustion and makes a burning candle burn more vigorously.
Hydrogen = pop; Oxygen = supports burning.
Hydrogen is collected at the cathode and oxygen at the anode. Hydrogen is present in double the volume of oxygen.
“H₂ double, O₂ single.”
White silver chloride turns grey because it decomposes into silver and chlorine in sunlight: 2AgCl(s) → 2Ag(s) + Cl₂(g).
White AgCl + sunlight → grey Ag.
The test tube feels cold because the reaction absorbs heat from the surroundings. Therefore, it is an endothermic reaction.
Cold = heat goes in = endothermic.
The precipitate is yellow lead iodide (PbI₂).
Yellow = PbI₂.
Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq).
Keep NO₃ as a group; balance I with 2KI.
Yes. It is a double displacement reaction because the ions exchange between the two reactants: Pb²⁺ and K⁺ exchange their partners with I⁻ and NO₃⁻. The insoluble PbI₂ forms as a precipitate.
“Ion swap + precipitate = double displacement.”
The classification is based on the temperature change: a temperature rise indicates an exothermic reaction, while a temperature fall indicates an endothermic reaction. The activity asks students to determine this experimentally from their recorded measurements.
Rise = release (exo). Fall = absorbed (endo).