Chemical Reactions and Equations

A displacement reaction occurs only when the free metal is more reactive than the metal present in the salt solution. Copper (Cu) is less reactive than zinc (Zn), so Cu cannot displace Zn from \(\mathrm{ZnSO_4}\). In contrast, in option A, zinc is more reactive than copper and therefore displaces Cu from \(\mathrm{CuSO_4}\).

On mixing the solutions, ions exchange: \(\mathrm{BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)}\). Barium sulphate is insoluble in water, so it appears as a white solid precipitate. Gas evolution or a flame is not characteristic of this double-displacement precipitation reaction.

On heating, calcium carbonate undergoes thermal decomposition:
\(\mathrm{CaCO_3 \xrightarrow{\Delta} CaO + CO_2}\)
Therefore, calcium oxide and carbon dioxide are formed. Option A is incorrect because calcium hydroxide and water are associated with the reaction of calcium oxide with water, not with heating calcium carbonate alone.

On heating, lead nitrate breaks down into simpler substances: \(2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}\). The \(\mathrm{NO_2}\) produced is a brown gas. Since heat causes one compound to decompose, this is a thermal decomposition reaction; in a combination reaction, substances combine to form one product.

The formula of water is \(\mathrm{H_2O}\), so each water molecule contains two hydrogen atoms and one oxygen atom. The balanced electrolysis equation is \(2\mathrm{H_2O} \rightarrow 2\mathrm{H_2} + \mathrm{O_2}\). Therefore, under the same conditions, the volume of hydrogen produced is twice the volume of oxygen. Option A is incorrect because water does not contain equal numbers of hydrogen and oxygen atoms.

Silver chloride is photosensitive. In sunlight, it undergoes photodecomposition: \(2\mathrm{AgCl}(s) \xrightarrow{sunlight} 2\mathrm{Ag}(s)+\mathrm{Cl}_2(g)\). The silver formed is grey, so silver chloride is protected from light. Option B is incorrect because silver chloride does not form silver oxide on exposure to light.

Use the oxygen-transfer rule for redox reactions. CuO loses oxygen and becomes Cu, so copper(II) oxide is reduced. H₂ gains that oxygen and becomes H₂O, so hydrogen is oxidised. Because reduction and oxidation occur simultaneously, the reaction is a redox reaction. Therefore option B is correct; option A reverses both changes, while C and D incorrectly assign the same process to both reactants.

The governing concept is the energy change in a chemical reaction. Calcium carbonate undergoes thermal decomposition only when heated: CaCO3 + heat → CaO + CO2. Therefore, heat is a reactant and must be supplied, making option B correct. Adding water to calcium oxide, burning fuel, and respiration are generally exothermic processes that release heat rather than require continuous heat input.

In a chemical equation, substances and energy shown on the product side are formed or released during the reaction. If heat appears among the products, the reaction gives out heat to its surroundings and is therefore exothermic. Option B is correct. An endothermic reaction absorbs heat, while precipitation and electrolysis describe other reaction features, not the heat direction.

The position of heat in an equation indicates the energy transfer. When heat is written on the reactant side, the reaction must absorb that heat in order to proceed. Such a reaction is called endothermic, so option B is correct. An exothermic reaction places released heat on the product side; combustion is often exothermic, and precipitation refers to solid formation.

Magnesium reacts with oxygen in air to form a thin layer of magnesium oxide on its surface. This layer hinders direct contact between magnesium and oxygen, so the ribbon does not burn readily. Rubbing the ribbon removes this layer, allowing magnesium to burn with a dazzling white flame. This is different from removing magnesium oxide formed after burning.

A combination reaction is identified by its product pattern: two or more reactants join to form one product, represented generally as A + B → AB. Thus option B is correct. Option A describes decomposition, option C describes double displacement, and option D describes single displacement. The number and type of products distinguish these reaction categories.

In a decomposition reaction, one compound breaks down into two or more simpler substances. Its general form is \(AB \rightarrow A + B\). For example, on heating, calcium carbonate decomposes: \(\mathrm{CaCO_3 \rightarrow CaO + CO_2}\). In contrast, option A describes a combination reaction, in which substances join to form a product.

In electrolysis, water, which is a compound, breaks down using electrical energy into hydrogen and oxygen: \(2H_2O(l) \rightarrow 2H_2(g) + O_2(g)\). Therefore, it is a decomposition reaction. In a combination reaction, two or more substances combine to form one product, whereas here one substance forms two products.

In a double displacement reaction, the cations and anions of two ionic compounds exchange partners, forming new compounds. For example, \(\mathrm{AgNO_3 + NaCl \rightarrow AgCl + NaNO_3}\). In contrast, a reaction between two elements does not represent this type of ion exchange.

When aqueous barium chloride and aqueous sodium sulphate are mixed, their ions exchange: \(\mathrm{BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)}\). Insoluble white \(\mathrm{BaSO_4}\) separates as a solid precipitate. Hence, this is both a double-displacement and a precipitation reaction. In contrast, burning magnesium is a combination reaction.

The reaction is: \(\mathrm{BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)}\). Barium sulphate is insoluble in water, so it separates as a white solid precipitate. In contrast, sodium chloride remains soluble, while barium chloride and sodium sulphate are the initial reactants.

At the school-level definition, oxidation means the addition of oxygen to a substance, so option B is correct. For example, magnesium combines with oxygen to form magnesium oxide. Removal of oxygen, addition of hydrogen, and gain of electrons are commonly used indicators of reduction. In the electron-transfer definition, oxidation involves loss of electrons, which also distinguishes it from option D.

Reduction is a process in which oxygen is removed from a substance or hydrogen is added to it. Therefore, removal of oxygen indicates reduction. In contrast, addition of oxygen and removal of hydrogen indicate oxidation. In a physical change alone, the chemical composition of the substance does not change, so it is not reduction.

In a redox reaction, oxidation and reduction occur simultaneously. Oxidation involves loss of electrons or addition of oxygen, whereas reduction involves gain of electrons or removal of oxygen. The formation of a precipitate or a gas alone is not a definite indication of a redox reaction.