Explanation

In the chemical reactivity series, Zinc (Zn) lies above Copper (Cu) and is significantly more reactive. Consequently, zinc readily displaces copper from an aqueous solution of copper sulphate:

Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)

Silver (Ag), Gold (Au), and Mercury (Hg) lie below copper in the activity series and cannot displace it.

Explanation

In this classic thermite reaction, Aluminium (Al) is more electropositive and reactive than Iron (Fe). Aluminium reduces iron(III) oxide by displacing iron from its compound to yield aluminium oxide and molten iron:

Fe₂O₃ + 2Al → Al₂O₃ + 2Fe

Because a more reactive element displaces a less reactive element from its oxide, it is classified as a single displacement reaction (as well as a redox reaction).

Explanation

When an active metal like iron reacts with a dilute mineral acid, it displaces hydrogen to liberate hydrogen gas and forms the corresponding metal chloride:

Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)↑

Hence, iron(II) chloride (ferrous chloride) and hydrogen gas are produced.

Explanation

The microbial biodegradation and breakdown of organic plant matter into compost involves the respiration of aerobic bacteria and fungi. This biochemical process breaks chemical bonds and continuously releases thermal energy into the surroundings, making it a distinctly exothermic reaction.

Explanation

• Chemical Changes: Digestion, cellular respiration, and the burning of candle wax (combustion producing CO₂ and H₂O) all involve the formation of new chemical substances through chemical bonds breaking and forming.
• Physical Change: Melting of candle wax is merely an interconversion of physical states (solid to liquid) due to heat; no new chemical identity or substance is formed.

Explanation

According to the Law of Conservation of Mass, matter can neither be created nor destroyed in a chemical reaction. Therefore, the total mass of elements present in the products must equal the total mass of elements present in the reactants, requiring the total number of atoms of each element to be equal on both sides of a chemical equation.

Explanation

Aluminium has a higher affinity for oxygen and is more electropositive than iron in the activity series. During this reaction, elemental aluminium displaces iron from ferric oxide (Fe₂O₃) to form aluminium oxide (Al₂O₃) and pure iron. Hence, it is a single displacement reaction.

Explanation

When metallic iron combines directly with atmospheric oxygen under standard oxidation conditions, iron(III) oxide (ferric oxide) is formed:

4Fe(s) + 3O₂(g) → 2Fe₂O₃(s)

The product 2Fe₂O₃ (s) balances both atoms (4 Fe atoms and 6 O atoms on both sides) and represents stable iron(III) oxide.

Explanation

The melting of candle wax is a reversible physical change where solid paraffin wax liquefies upon heating without altering its underlying chemical composition. By contrast, burning a candle, biological digestion, and cellular respiration are irreversible processes that break and form chemical bonds, producing completely new substances.

Explanation

When magnesium metal is heated in the presence of atmospheric oxygen, it undergoes a vigorous combination and oxidation reaction, producing intense heat, light, and a fine white powder of magnesium oxide (MgO):

2Mg(s) + O₂(g) → 2MgO(s) + Heat + Light

Note: The ribbon is cleaned with sandpaper before burning to remove the unreactive protective layer of basic magnesium carbonate from its surface.

Explanation

A chemical reaction entails breaking old chemical bonds and creating new bonds to form chemically distinct substances. Any of the following macroscopic observations indicates that a chemical change has occurred:

• Change in state: e.g., burning wax producing water vapor and gaseous carbon dioxide.
• Change in colour: e.g., blue copper sulphate turning light green upon adding iron nails.
• Evolution of a gas: e.g., zinc granules producing effervescent hydrogen gas in dilute acid.
• Change in temperature: e.g., quicklime reacting with water releasing significant heat.

Explanation

A chemical equation in which the total number of atoms of each element on the reactant side is not equal to that on the product side is known as an unbalanced or skeletal chemical equation:

Mg + O₂ → MgO

Here, the left-hand side contains two oxygen atoms while the right-hand side has only one oxygen atom. It violates the Law of Conservation of Mass until balanced by appropriate stoichiometric coefficients.

Explanation

Magnesium has a valency of +2 (Mg²⁺) and oxygen has a valency of -2 (O²⁻), forming the stable binary ionic compound magnesium oxide (MgO). Because oxygen gas exists naturally as diatomic molecules (O₂), balancing the equation to satisfy the Law of Conservation of Mass yields:

2Mg(s) + O₂(g) → 2MgO(s)

Both sides possess exactly 2 magnesium atoms and 2 oxygen atoms.

Explanation

Zinc is more electropositive than hydrogen in the electrochemical activity series. When zinc reacts with dilute sulphuric acid, it displaces hydrogen ions, producing zinc sulphate solution and liberating hydrogen gas:

Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)↑

The presence of hydrogen gas is confirmed experimentally when a burning splinter brought near the mouth of the delivery tube ignites with a characteristic 'pop' sound.

Explanation

• Assertion Analysis: Touching the beaker during the slaking of lime reveals that it becomes scalding hot (True).
• Reasoning Analysis: Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime), releasing substantial thermal energy:
  
  CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat
  
  Because energy is released into the immediate surroundings, it is classified as an exothermic combination reaction (True, and directly explains A).

Explanation

A reaction in which two or more reactants combine to form a single final product is called a combination reaction:

CaO(s) [Quicklime] + H₂O(l) → Ca(OH)₂(aq) [Slaked lime]

Two distinct chemical species (calcium oxide and water) synthesize a single product, calcium hydroxide.

Explanation

Ferrous sulphate crystals (FeSO₄·7H₂O) first lose their seven water molecules of crystallization upon gentle heating, turning from pale green to white anhydrous FeSO₄. On further strong heating, anhydrous ferrous sulphate undergoes thermal decomposition:

2FeSO₄(s) → Fe₂O₃(s) + SO₂(g)↑ + SO₃(g)↑

The solid residue is reddish-brown ferric oxide (Fe₂O₃), accompanied by the choking smell of burning sulphur caused by sulphur dioxide (SO₂) and sulphur trioxide (SO₃) gases.

Explanation

Thermal decomposition of colourless lead nitrate crystals yields a yellow solid residue of lead(II) oxide (PbO), colourless oxygen gas (O₂), and dense suffocating brown fumes of nitrogen dioxide (NO₂):

2Pb(NO₃)₂(s) → 2PbO(s) + 4NO₂(g)↑ [Brown fumes] + O₂(g)↑

Explanation

Silver chloride undergoes photolytic decomposition (photolysis) under solar radiation. Photons of light supply energy to cleave the ionic lattice, liberating greenish-yellow chlorine gas and leaving behind finely divided grey metallic silver (Ag):

2AgCl(s) [White] → 2Ag(s) [Grey] + Cl₂(g)↑

Explanation

• a – 3 (Photolysis): Silver chloride decomposes into silver metal in the presence of sunlight (used in black-and-white photography).
• b – 1 (Electrolysis): Water decomposes into hydrogen and oxygen through the passage of an electric current.
• c – 2 (Thermolysis): Limestone (calcium carbonate) decomposes into quicklime and carbon dioxide upon strong heating.

Explanation

A water molecule consists of two atoms of hydrogen bonded to one atom of oxygen (H₂O). During electrolysis:

2H₂O(l) → 2H₂(g) [Cathode] + O₂(g) [Anode]

According to Avogadro’s law, volumes of gases at identical temperature and pressure are proportional to their moles. Therefore, 2 volumes of hydrogen gas are collected for every 1 volume of oxygen gas, giving a volume ratio of 2 : 1 (by mass, the ratio is 1 : 8).

Explanation

Pure or distilled water is a covalent compound that ionizes poorly into free ions (H⁺ and OH⁻), making it an electrical insulator. Adding a few drops of an acid like dilute H₂SO₄ provides mobile sulphate (SO₄²⁻) and hydronium (H₃O⁺) ions, enabling the electrolyte to conduct an electric current effectively.

Explanation

Iron is more reactive than copper in the activity series. Iron atoms reduce Cu²⁺ ions and enter the solution as Fe²⁺ ions, displacing copper metal:

Fe(s) + CuSO₄(aq) [Blue] → FeSO₄(aq) [Light Green] + Cu(s) [Brown deposit]

The solution’s blue colour (due to Cu²⁺ ions) fades as light green ferrous sulphate (FeSO₄) forms, while displaced elemental copper settles as a reddish-brown coating on the iron nail.

Explanation

In this transformation, a more reactive elemental metal (iron) pushes out and replaces a less reactive metal (copper) from its aqueous compound:

Fe + CuSO₄ → FeSO₄ + Cu

Because one chemical element displaces another from its salt solution, it is classified as a single displacement reaction.

Explanation

Mixing aqueous sodium sulphate with barium chloride triggers a double displacement (precipitation) reaction in which Ba²⁺ and SO₄²⁻ ions pair up to form an insoluble solid:

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ [White ppt] + 2NaCl(aq)

The insoluble white solid that precipitates out of the liquid is barium sulphate (BaSO₄).

Explanation

A reaction where two ionic compounds swap their cationic and anionic components in an aqueous medium is called a double displacement reaction:

AB + CD → AD + CB

These reactions typically result in the formation of an insoluble precipitate, a neutral molecule like water, or an evolved gas.

Explanation

When heated in air, copper metal combines with atmospheric oxygen to undergo surface oxidation, forming black copper(II) oxide or cupric oxide (CuO):

2Cu(s) [Reddish-brown] + O₂(g) → 2CuO(s) [Black coating]

If hot hydrogen gas (H₂) is subsequently passed over this black CuO, it is reduced back to reddish-brown copper: CuO + H₂ → Cu + H₂O.

Explanation

• Oxidation: Involves the gain of oxygen, the loss of hydrogen, or the loss of electrons by a chemical entity.
• Reduction: Involves the loss of oxygen, the gain of hydrogen, or the gain of electrons by a chemical entity.

Explanation

When lipid-containing foodstuffs are exposed to atmospheric oxygen over time, their unsaturated fatty acid chains undergo oxidation, forming volatile aldehydes and carboxylic acids. This oxidative spoilage is called rancidity.

Prevention Methods:

• Flushing packaged food bags with unreactive nitrogen gas (N₂).
• Incorporating synthetic antioxidants such as BHA and BHT.
• Storing food in vacuum-sealed or airtight containers away from direct sunlight.

Explanation

• Assertion Analysis: The combustion of magnesium fulfills the definition of both combination (two reactants yield one product) and oxidation (magnesium gains oxygen) (True).
• Reasoning Analysis: The underlying chemical equation shows that Mg acquires oxygen to form a single product, MgO:
  
  2Mg(s) + O₂(g) → 2MgO(s)
  
  Because magnesium gains oxygen while uniting into one product, it explains why the reaction is simultaneously an oxidation and a combination reaction (True, and explains A).

Explanation

Zinc is more electropositive than hydrogen and displaces it from dilute acids to form zinc chloride and liberate hydrogen gas:

Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)↑

The classic confirmatory test for hydrogen gas is bringing a burning candle or wooden splinter near the mouth of the test tube, causing it to burn with a distinct 'pop' sound.

Explanation

Calcium oxide reacts vigorously with water to form slaked lime (calcium hydroxide), releasing a large amount of thermal energy:

CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat

Reactions that release heat into their surroundings and raise the system's temperature are called exothermic reactions.

Explanation

Anhydrous ferrous sulphate undergoes thermal decomposition upon strong heating:

2FeSO₄(s) → Fe₂O₃(s) + SO₂(g)↑ + SO₃(g)↑

The solid residue left behind is reddish-brown ferric oxide (Fe₂O₃), accompanied by the choking smell of burning sulphur caused by sulphur dioxide (SO₂) and sulphur trioxide (SO₃).

Explanation

Thermal decomposition of lead nitrate is represented by the balanced equation:

2Pb(NO₃)₂(s) → 2PbO(s) [Yellow] + 4NO₂(g)↑ [Brown fumes] + O₂(g)↑ [Colourless]

The third product is colourless, odourless oxygen gas (O₂), which rekindles a glowing wooden splinter.

Explanation

The electrolytic breakdown of water obeys the stoichiometric ratio:

2H₂O(l) → 2H₂(g) [Cathode] + O₂(g) [Anode]

Because water contains two hydrogen atoms for every oxygen atom, hydrogen gas is collected at the cathode in a 2 : 1 volume ratio relative to oxygen at the anode.

Explanation

White silver chloride (AgCl) decomposes in the presence of sunlight into elemental silver and chlorine gas:

2AgCl(s) [White] → 2Ag(s) [Grey] + Cl₂(g)↑

The formation of finely divided metallic silver gives the exposed salt its characteristic grey colour.

Explanation

Iron is more reactive than copper and displaces it from copper sulphate:

Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

The displaced elemental copper (Cu) deposits directly onto the iron nail as a reddish-brown layer, while the blue solution fades to light green ferrous sulphate.

Explanation

A double displacement precipitation reaction takes place between the two soluble salts:

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ [White ppt] + 2NaCl(aq)

The insoluble compound that precipitates out of the aqueous mixture is barium sulphate (BaSO₄).

Explanation

In this redox reaction:

CuO(s) + H₂(g) → Cu(s) + H₂O(l)

Copper(II) oxide loses oxygen to form metallic copper. The loss of oxygen corresponds to reduction, while hydrogen gains oxygen and undergoes oxidation.

Explanation

A reaction where two or more elements or compounds unite chemically to synthesize a single product is called a combination reaction:

A + B → AB

Examples include the formation of water (2H₂ + O₂ → 2H₂O) and the burning of coal (C + O₂ → CO₂).

Explanation

A reaction where a single reactant breaks apart into multiple simpler products is a decomposition reaction:

AB → A + B

Because breaking chemical bonds requires an input of energy, most decomposition reactions are endothermic.

Explanation

The industrial thermal decomposition of limestone produces quicklime and evolves carbon dioxide gas:

CaCO₃(s) → CaO(s) [Quicklime] + CO₂(g)↑

This reaction is an essential step in manufacturing cement and building materials.

Explanation

An exothermic reaction is one in which the total energy of the products is lower than that of the reactants, with the excess energy released as heat or light (e.g., combustion of fuels, respiration, and dilution of strong acids).

Explanation

During respiration, glucose derived from digested food combines with oxygen inside living cells to produce carbon dioxide, water, and metabolic energy (ATP):

C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + Energy

Because energy is liberated to fuel biological activities and maintain body warmth, respiration is an exothermic reaction.

Explanation

In a single displacement reaction, an element higher up in the reactivity series displaces a less reactive element from its dissolved compound:

A + BC → AC + B (where metal A is more reactive than metal B).

Explanation

Any chemical reaction that results in the formation of an insoluble solid phase is called a precipitation reaction:

AgNO₃(aq) + NaCl(aq) → AgCl(s)↓ [White precipitate] + NaNO₃(aq)

Explanation

• Reduction: Characterized by the loss of oxygen, the gain of hydrogen, or the acquisition of electrons.
• Oxidation: Characterized by the gain of oxygen, the loss of hydrogen, or the loss of electrons.

Explanation

When refined metals are gradually deteriorated by atmospheric oxygen, moisture, and acid gases, the process is called corrosion. For iron, it is specifically termed rusting, producing hydrated ferric oxide:

4Fe(s) + 3O₂(g) + 2xH₂O(l) → 2Fe₂O₃·xH₂O(s) [Rust]

Explanation

The slow aerial oxidation of unsaturated fats and oils yields volatile organic compounds like aldehydes and ketones with disagreeable tastes and smells. This spoilage is called rancidity, which manufacturers prevent by flushing bags with inert nitrogen gas (N₂).

Explanation

• Assertion Analysis: Before burning in a laboratory burner, the ribbon must be thoroughly cleaned with abrasive sandpaper (True).
• Reasoning Analysis: Being a reactive metal, magnesium reacts slowly with atmospheric carbon dioxide and oxygen to form a thin surface coating of basic magnesium carbonate [MgCO₃·Mg(OH)₂] or magnesium oxide. This coat shields the underlying metal from oxygen; removing it allows clean, direct contact with air for ignition (True, and directly explains A).

Explanation

• Assertion Analysis: A balanced equation displays identical atom counts for every element across the reaction arrow (True).
• Reasoning Analysis: Because mass is strictly conserved during a chemical transformation, the total mass of the reactants must equal the total mass of the products. This conservation requires atom counts to match across both sides (True, and explains A).

Explanation

• Assertion Analysis: Passing H₂ over heated black CuO restores the elemental reddish-brown colour of copper metal (True).
• Reasoning Analysis: The equation CuO + H₂ → Cu + H₂O demonstrates that hydrogen acts as a reducing agent, abstracting oxygen from CuO to form steam while reducing copper oxide to metallic copper (True, and explains A).

Explanation

• (i) Combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O + Heat (Exothermic).
• (ii) Cellular respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (Exothermic).
• (iii) Decomposition of CaCO₃: Requires continuous external heating (Endothermic).
• (iv) Decomposition of compost: Microbial action releases heat energy (Exothermic).

Hence, statements (i), (ii), and (iv) represent exothermic reactions.

Explanation

• (i), (ii), and (iv): In each case, a single reactant molecule breaks down into multiple simpler products via light or heat (Decomposition reactions).
• (iii): Iron displaces copper from copper sulphate, which is a single displacement reaction, not a decomposition reaction.

Explanation

• A – 4: Two reactants form one product (Combination).
• B – 2: Zinc displaces copper from its salt (Displacement).
• C – 3: Mutual exchange of ions produces a precipitate (Double displacement).
• D – 1: A single substance breaks down upon heating (Decomposition).

Explanation

• A – 4: Corrosion describes the eating away of metals by environmental agents.
• B – 3: Rancidity is the aerial oxidation of dietary lipids leading to foul odors.
• C – 1: Oxidation is fundamentally the gain of oxygen or loss of hydrogen.
• D – 2: Reduction is the removal of oxygen or addition of hydrogen.

Explanation

• Assertion Analysis: The defining criterion of a displacement reaction is that a more electropositive/reactive element displaces a less reactive element from its compound (True).
• Reasoning Analysis: Zinc (Zn) and lead (Pb) lie above copper (Cu) in the reactivity series. When introduced into a CuSO₄ solution, both metals displace copper ions:
  
  Zn + CuSO₄ → ZnSO₄ + Cu
Pb + CuCl₂ → PbCl₂ + Cu
  
  This reactivity difference explains the single displacement process (True, and explains A).

Explanation

• (i): Cu is oxidised (0 to +2) and O₂ is reduced (0 to -2) (Redox).
• (ii): CuO is reduced to Cu, while H₂ is oxidised to H₂O (Redox).
• (iii): Double displacement precipitation without any change in oxidation states (Not a redox reaction).
• (iv): ZnO is reduced to Zn, while Carbon is oxidised to CO (Redox).

Hence, reactions (i), (ii), and (iv) are redox reactions.

Explanation

• Assertion Analysis: Light-sensitive silver bromide (AgBr) emulsions are standard components in black-and-white photographic paper (True).
• Reasoning Analysis: Exposure to light energy cleaves the crystal lattice:
  
  2AgBr(s) → 2Ag(s) [Grey metallic image] + Br₂(g)
  
  The precipitation of metallic silver creates the photographic image, providing the scientific basis for its practical application (True, and explains A).

Explanation

• Assertion Analysis: Including state symbols after chemical formulas is standard practice in chemical notation (True).
• Reasoning Analysis: Adding designations like (s) for solid, (l) for pure liquid, (g) for gas, and (aq) for aqueous solutions conveys state information without needing lengthy descriptive text (True, and explains A).

Explanation

The notation (aq) stands for aqueous. It denotes that the solute species is fully dissolved in water as a solvent, differentiating it from a pure liquid state, indicated by (l).

Explanation

• (i), (ii), and (iii): These reactions require continuous energy input in the form of heat, sunlight, or electricity to drive bond cleavage (Endothermic).
• (iv): The combustion of methane releases heat and light into the surroundings (Exothermic).

Therefore, (i), (ii), and (iii) are endothermic processes.

Explanation

Analyzing the gain and loss of oxygen atoms in the reaction:

3Fe + 4H₂O → Fe₃O₄ + 4H₂

• Iron (Fe): Gains oxygen to form Fe₃O₄, meaning iron is oxidised.
• Water (H₂O): Loses oxygen to form H₂, meaning water is reduced (and acts as the oxidizing agent).

Explanation

Carbon dioxide (CO₂) reacts with aqueous calcium hydroxide (lime water) to form an insoluble white precipitate of calcium carbonate, creating the milky appearance:

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s)↓ [Milky ppt] + H₂O(l)

Passing excess CO₂ dissolves the precipitate by forming soluble calcium hydrogen carbonate, clearing the solution:

CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂(aq) [Colourless]

Explanation

• A – 3: The decomposition of limestone requires strong heat.
• B – 1: The cleavage of silver chloride into silver and chlorine is triggered by light.
• C – 2: The electrolysis of water into hydrogen and oxygen is driven by electricity.

Explanation

• Assertion Analysis: Corrosion degrades iron structures globally, weakening load-bearing frameworks and posing serious safety and financial challenges (True).
• Reasoning Analysis: The continuous expenditure required to repair, replace, and protect rusted components underscores why it is such a serious macroeconomic problem (True, and explains A).

Explanation

By classical chemical definition:

Oxidation is defined as either the gain of oxygen or the loss of hydrogen (statements i and ii).

Conversely, reduction is defined as the gain of hydrogen or the loss of oxygen (statements iii and iv).

Explanation

Examining oxygen and chlorine transfers across the reaction:

MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂

• Manganese dioxide (MnO₂): Loses its oxygen atoms to form MnCl₂. Because it loses oxygen, MnO₂ is reduced (oxidation state of Mn drops from +4 to +2).
• Hydrochloric acid (HCl): Loses hydrogen to form Cl₂ gas, meaning HCl is oxidised.

Explanation

• Assertion Analysis: By definition, double displacement involves mutual partner swapping between positive and negative ions: AB + CD → AD + CB (True).
• Reasoning Analysis: While forming an insoluble precipitate is common in these reactions (e.g., BaSO₄, AgCl), precipitation is simply an outcome of low solubility, not the causal definition of ion exchange itself (True, but R is not the correct explanation of A).