Chemical Reactions and Equations

 ext{(aq)} denotes an aqueous solution, meaning that the substance is dissolved in water; for example,  ext{NaCl(aq)}.  ext{(l)} only indicates that a substance is in the liquid state; for example,  ext{H}_2 ext{O(l)}. Thus, option B incorrectly reverses the meanings of the two symbols.

Formation of a new substance is a key indicator of a chemical reaction. The equality of total mass of reactants and products in a closed container demonstrates the law of conservation of mass. Colour change alone can sometimes occur in a physical change, but the formation of a new substance here confirms a chemical change.

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.

In a balanced equation, the number of atoms of every element is the same on both the reactant and product sides. During a chemical reaction, atoms are neither created nor destroyed; they are only rearranged. Hence, the total mass remains unchanged, demonstrating the law of conservation of mass. The law of constant proportions concerns the fixed mass ratio of elements in a compound, not the balancing of an equation.

Option A is correct. In balancing a chemical equation, coefficients change the number of molecules or formula units without changing the chemical formula or identity of a substance. For example, 2H₂O means two water molecules. In contrast, changing a subscript changes the substance itself; changing H₂O to H₂O₂ changes water into hydrogen peroxide.

In a balanced chemical equation, the number of atoms of each element is the same on the reactant and product sides, satisfying the law of conservation of mass. If the atoms of even one element are unequal, the equation is unbalanced. Reversibility and exothermic nature are different properties of a reaction; they do not describe whether atom counts are balanced.

The molecular formula of water is H₂O. The subscript 2 means that each water molecule contains two hydrogen atoms, and the absence of a subscript after oxygen means one oxygen atom. Thus one molecule contains 2 + 1 = 3 atoms. For two molecules, the total is 2 × 3 = 6 atoms, so option B is correct. The number four would not account for the oxygen atoms properly.

Magnesium oxide has the formula MgO, which shows that each formula unit contains one magnesium atom and one oxygen atom. Three formula units therefore contain three magnesium atoms and three oxygen atoms. The ratio is 3:3, which simplifies by dividing both terms by 3 to 1:1. The number of units changes the quantities but not the fixed composition of the compound, so option A is correct.

A thin layer of magnesium oxide forms on the surface of a magnesium ribbon. This layer hinders direct contact between magnesium and oxygen in air. When the layer is removed, fresh magnesium is exposed and reacts readily with oxygen, burning with a bright flame. The amount of oxygen in the air does not increase; only the exposed magnesium surface becomes available for reaction.

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.

Calcium oxide reacts with water to form calcium hydroxide, also called slaked lime: \(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}\). This is an exothermic reaction. Calcium carbonate is not formed by adding water; its formation requires a calcium compound and carbon dioxide.

The reaction is: \(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2 + heat}\). Two reactants, calcium oxide and water, combine to form the single product calcium hydroxide; therefore, it is a combination reaction. Since heat is released, it is also exothermic. Option B describes a decomposition reaction, in which one compound breaks down.

Copper has oxidation number \(+2\) in \(\mathrm{CuO}\) and \(0\) in \(\mathrm{Cu}\), so copper(II) oxide is reduced. Hydrogen changes from \(0\) in \(\mathrm{H_2}\) to \(+1\) in \(\mathrm{H_2O}\); therefore, hydrogen is oxidised and acts as the reducing agent. Thus, option B is correct, not option A, which reverses the roles.

The thermal decomposition reaction is \(\mathrm{CaCO_3 \xrightarrow{\Delta} CaO + CO_2}\). The evolved \(\mathrm{CO_2}\) reacts with lime water, \(\mathrm{Ca(OH)_2}\), to form an insoluble white precipitate of \(\mathrm{CaCO_3}\), making the lime water milky. Oxygen supports combustion, but it does not turn lime water milky.

Lead nitrate undergoes thermal decomposition:
\[
2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}
\]
\(\mathrm{NO_2}\) is a brown gas, whereas \(\mathrm{O_2}\) is colourless. \(\mathrm{PbO}\) is a solid rather than a gas, so the correct pair is nitrogen dioxide and oxygen.

On heating, lead nitrate decomposes as \(2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}\). The \(\mathrm{PbO}\) formed is the yellow solid residue. Lead chloride, sulphate, or carbonate would require chloride, sulphate, or carbonate ions respectively, none of which are present in this reaction.

Thermal decomposition of lead nitrate produces nitrogen dioxide, lead oxide, and oxygen: \(2\mathrm{Pb(NO_3)_2} \rightarrow 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}\). \(\mathrm{NO_2}\) is a brown gas, so the brown fumes indicate its presence. Oxygen is also formed, but it is colourless.

Zinc is above copper in the reactivity series. Therefore, zinc displaces copper from copper sulphate solution: \(\mathrm{Zn + CuSO_4 \rightarrow ZnSO_4 + Cu}\). The blue \(\mathrm{Cu^{2+}}\) ions are removed from the solution, producing colourless zinc sulphate, while copper is deposited on the zinc strip. Option B reverses the reactivity order.

Silver chloride undergoes photodecomposition in sunlight: \(2\mathrm{AgCl}\xrightarrow{\text{sunlight}}2\mathrm{Ag}+\mathrm{Cl}_2\). The silver metal formed is grey, so it causes the colour change. Chlorine gas is also a product, but it does not cause the grey appearance.