Chemical Reactions and Equations

In the hydrogen-based description of redox reactions, removal of hydrogen is oxidation, while addition of hydrogen is reduction. Therefore, the two processes in the stated order are oxidation followed by reduction. This is consistent with the broader electron-transfer idea: oxidation involves loss of electrons and reduction involves gain. Option B reverses the sequence; C and D name unrelated reaction types.

Rusting is a slow oxidation reaction in which iron reacts with oxygen in the presence of water or moisture to form hydrated iron(III) oxide, commonly called rust. Both oxygen and moisture are essential conditions. Salt or acids may speed up corrosion by increasing conductivity, but they are not substitutes for oxygen and water. Nitrogen, light, hydrogen, and sound are not the required pair. Thus option A is correct.

In H₂, hydrogen has oxidation number 0; in H₂O it becomes +1, so hydrogen loses electrons overall and H₂ is oxidised. In CuO, copper has oxidation number +2; in elemental Cu it becomes 0, so CuO is reduced. The reaction is therefore a redox reaction, with H₂ acting as the reducing agent and CuO as the oxidising agent. Option A is correct; B reverses the changes.

This is a thermal decomposition reaction: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂. Nitrogen dioxide, NO₂, is a reddish-brown gas, so option B is correct. The oxygen released is colourless, nitrogen is also colourless, and nitric oxide is not the brown gas produced in this reaction. The observation of brown fumes is therefore a useful test for NO₂.

Iron is above copper in the reactivity series, so it can displace copper from copper sulphate. The single-displacement equation is Fe + CuSO₄ → FeSO₄ + Cu. Copper separates and deposits as a reddish-brown coating on the nail, while aqueous iron(II) sulphate gives the solution a pale green colour. Therefore option A is correct. The observations prove that new substances form, so adsorption and the other reaction descriptions are unsuitable.

Option C is a double displacement and precipitation reaction. In aqueous solution, Na⁺ and Ba²⁺ exchange partners with SO₄²⁻ and Cl⁻. Ba²⁺ combines with SO₄²⁻ to produce insoluble barium sulphate, BaSO₄, which appears as a precipitate: Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl. Option A is decomposition, B is single displacement, and D is combination, so C is unambiguous.

Hydrogen combines with oxygen removed from copper(II) oxide and forms water. By the classical definition, gain of oxygen is oxidation, so hydrogen is oxidised and option B is correct. Copper(II) oxide loses oxygen and becomes copper; therefore CuO is reduced. Hydrogen is the reducing agent because it causes CuO to lose oxygen, not the oxidising agent. No precipitate is involved in this reaction.

Rancidity is mainly caused by oxidation of unsaturated oils and fats when they react with oxygen in air. The oxidation products, including unpleasant-smelling compounds, change the original taste and odour. Hence option A is correct. Reduction is the opposite type of redox change, neutralisation requires an acid-base reaction, and displacement with a metal is not the normal cause of rancidity. Antioxidants and airtight storage can slow this oxidation.

The law of conservation of mass states that matter is neither created nor destroyed during a chemical reaction. In a closed container, the hydrogen or other gas produced by the metal–acid reaction remains inside the system, so it is included in the final measurement. Although the substances change into new products, the combined mass of all reactants and products remains constant. Hence option A is correct; an apparent decrease would occur only if gas escaped from an open system.

Conservation of mass applies to a complete, closed system in which every reactant and product is included. In an open container, the gas formed during the reaction escapes into the surroundings, so the balance or measurement records only the material left in the container. The measured mass decreases, but the escaped gas still has mass and has not been destroyed. Therefore option A correctly explains the apparent loss.

When magnesium burns, it reacts with oxygen according to 2Mg + O₂ → 2MgO. It is a combination reaction because two reactants form one main product, magnesium oxide. It is also oxidation because magnesium gains oxygen; in electron terms, magnesium loses electrons and its oxidation state rises from 0 to +2. Thus option A gives both required ideas, whereas the other options describe unrelated changes.

Rusting is an electrochemical corrosion process that needs iron, oxygen and water or moisture. The oil coating acts as a physical barrier, so air and moisture cannot easily reach the iron surface; consequently, the corrosion reaction becomes slower. Oil does not change iron into another metal, dissolve rust completely, or increase oxidation. Therefore, option A gives the correct prevention mechanism.

Antioxidants mainly slow oxidation reactions in foods, especially the oxidation of oils and fats. Oxidation can produce unpleasant tastes and smells called rancidity and may reduce food quality. By reacting preferentially with oxidising species or interrupting chain reactions, antioxidants delay this spoilage. Displacement, electrolysis and galvanisation are different processes and do not describe the usual preservation action here. Thus option A is correct.

A chemical equation is balanced only when the number of atoms of every element is equal on the reactant and product sides. Equality for iron alone is insufficient. If oxygen atoms differ, the equation does not represent conservation of mass correctly and must be balanced by changing suitable coefficients, never subscripts. Precipitation and exothermicity describe other properties, not atom-count balance. Therefore option B is correct.

The formula of magnesium oxide is MgO, so each formula unit contains one magnesium atom and one oxygen atom. Four formula units therefore contain four magnesium atoms and four oxygen atoms. The ratio is 4:4, which simplifies to 1:1. The number of formula units changes the totals but not the simplest ratio. Hence option A is correct; the other ratios do not follow from MgO.

When magnesium burns, it reacts with oxygen according to 2Mg + O₂ → 2MgO. Two reactants combine to form one principal product, magnesium oxide, so the reaction is a combination reaction. At the same time, oxygen is added to magnesium, which satisfies the school-level definition of oxidation. It is not a double displacement, decomposition or dissolution process. Therefore option A is correct.

Calcium oxide reacts with water as CaO + H₂O → Ca(OH)₂. Two reactants form one product, calcium hydroxide, so the reaction is a combination reaction. Considerable heat is released during the reaction, which gives it the additional classification exothermic. It is not decomposition, displacement or photochemical breakdown. Thus option A states both required reasons and is correct.

On heating, lead nitrate decomposes according to 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂. Nitrogen dioxide, NO₂, is a reddish-brown gas, so its brown fumes provide a visible clue during the reaction. Oxygen is colourless, as are hydrogen and carbon dioxide under ordinary conditions. Therefore the gas identified by the brown fumes is nitrogen dioxide, making option B correct.

Many silver salts, such as silver chloride and silver bromide, are photosensitive. Light can supply energy for their decomposition, producing changed substances; for example, silver chloride darkens as silver forms. A dark or amber bottle absorbs or blocks much of the incident light and slows this photochemical reaction. It does not increase solubility, cause combustion, or remove mass. Hence option A is correct.

The electrolysis equation is 2H₂O → 2H₂ + O₂. It shows that hydrogen and oxygen are produced in a volume ratio of 2:1, provided temperature and pressure are the same. Therefore, for 15 mL of oxygen, hydrogen volume = 2 × 15 = 30 mL. Option A is half the oxygen volume, while B ignores the ratio and D uses an incorrect factor of three.

Pure water contains very few ions and is therefore a poor conductor of electricity. Adding a suitable acid, such as dilute sulphuric acid, increases the concentration of mobile ions in the solution. These ions carry charge between the electrodes, allowing electrolysis to proceed more readily. The acid is added to improve conductivity, not to increase water mass or give colour or hardness to the gases.

Decomposition means breaking one compound into simpler substances, but the energy supplied can differ. In electrolytic decomposition, electrical energy drives the reaction, as in the electrolysis of water. In photolytic decomposition, light energy causes the compound to split, as in the decomposition of silver chloride in sunlight. Thus the source of energy is the distinguishing basis; precipitate colour, metal lustre, and gas mass are not defining criteria.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution: Fe + CuSO₄ → FeSO₄ + Cu. The deposited copper appears reddish brown, while aqueous iron(II) sulphate is pale green, causing the solution to become greenish. Sodium chloride, barium sulphate, and magnesium oxide contain elements not involved in this reaction, so they cannot explain the observation.

A precipitate is a solid substance formed when two aqueous solutions react and the product is insoluble in the liquid. It settles or remains suspended separately from the solution, so a downward arrow (↓) is placed after its formula in a chemical equation. A gas is represented by an upward arrow (↑), while heat release and light emission require separate symbols or words and are not the meaning of the downward arrow.

Black copper(II) oxide reacts with hydrogen on heating: CuO + H₂ → Cu + H₂O. The black CuO loses oxygen and forms reddish-brown copper metal. Removal of oxygen from a compound is defined as reduction, while hydrogen is oxidised to water. Thus the colour change and reaction demonstrate reduction of copper oxide, not decomposition or freezing of the other substances.