Chemical Reactions and Equations

According to the law of conservation of mass, atoms are neither created nor destroyed in a chemical reaction. Therefore, in a balanced chemical equation, the number of atoms of every element is the same on the reactant and product sides, so total mass is conserved. Option B is incorrect because products may be gases, liquids, or solids.

The reaction is \(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}\). Two reactants combine to form a single product, calcium hydroxide, so it is a combination reaction. The container becoming hot indicates that heat is released; therefore, it is also exothermic. In contrast, a decomposition reaction involves one compound breaking down into two or more products.

Heating calcium carbonate produces carbon dioxide:
\[\mathrm{CaCO_3 \xrightarrow{\Delta} CaO + CO_2}\]
The carbon dioxide reacts with calcium hydroxide in lime water to form an insoluble precipitate of calcium carbonate:
\[\mathrm{CO_2 + Ca(OH)_2 \rightarrow CaCO_3\downarrow + H_2O}\]
This insoluble calcium carbonate precipitate makes lime water appear milky. In contrast, passing excess carbon dioxide can form soluble calcium bicarbonate, causing the milkiness to decrease.

Lead nitrate undergoes thermal decomposition on heating:
\[
2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}
\]
\(\mathrm{NO_2}\) is the brown gas, and \(\mathrm{PbO}\) is the solid residue. Although \(\mathrm{O_2}\) is also formed, it is colourless, so it cannot be the brown gas.

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, so the brown fumes confirm the formation of nitrogen dioxide. Although \(\mathrm{O_2}\) is also produced, it is colourless and therefore does not account for the brown colour.

In sunlight, silver chloride decomposes: \(2\mathrm{AgCl}(s) \xrightarrow{\text{sunlight}} 2\mathrm{Ag}(s) + \mathrm{Cl}_2(g)\). This is a photolytic decomposition reaction. The silver metal formed is grey, so white silver chloride gradually appears grey. Chlorine gas is also produced, but it does not cause the grey colour.

In option A, calcium oxide and water combine to form a single product, calcium hydroxide, and heat is released during the process. Therefore, it is an exothermic combination reaction. In contrast, option B shows one compound breaking into two products on heating, so it is a thermal decomposition reaction.

Pure water contains very few ions, so it conducts electricity poorly and electrolysis proceeds very slowly. Adding dilute sulphuric acid supplies \(\mathrm{H^+}\) and other ions, increasing the electrical conductivity of the solution so that current can pass. The acid is added to improve conduction, not to increase the amount of oxygen produced.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution. The reaction is \(\mathrm{Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)}\). The released copper deposits as a reddish-brown coating on the nail, while the iron sulphate formed makes the solution pale green. Rusting in option B is a different process and does not produce a copper coating.

The blue colour of copper sulphate solution is due to hydrated copper(II) ions, \(\mathrm{Cu^{2+}}\). Since iron is more reactive, it displaces copper(II) ions from the solution: \(\mathrm{Fe(s)+CuSO_4(aq)\rightarrow FeSO_4(aq)+Cu(s)}\). Thus, the concentration of \(\mathrm{Cu^{2+}}\) decreases and the blue colour fades. In contrast, \(\mathrm{Fe^{2+}}\) ions are formed during the reaction; they do not decrease.

In a displacement reaction, a more reactive metal displaces a less reactive metal from its salt solution. Zinc displaces copper from copper sulphate, so zinc is more reactive than copper. Copper cannot displace zinc from zinc sulphate because copper is the less reactive metal.

In the double-displacement reaction between barium chloride and sodium sulphate, insoluble white barium sulphate is formed:
\(\mathrm{BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)}\).
Therefore, sodium chloride remains in solution. Option B can also form a white precipitate, but it is calcium carbonate, not barium sulphate.

The reaction is \(\mathrm{BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)}\). The \(\mathrm{Ba^{2+}}\) and \(\mathrm{Na^+}\) ions exchange anionic partners, so it is a double-displacement reaction. The product \(\mathrm{BaSO_4}\) is an insoluble solid that separates as a precipitate, so it is also a precipitation reaction. Unlike option B, no oxidation or reduction occurs in this reaction.

Option B is correct. In the balanced equation, \(2KClO_3\) contains 2 K atoms, 2 Cl atoms, and 6 O atoms. The products \(2KCl\) and \(3O_2\) also contain 2 K atoms, 2 Cl atoms, and 6 O atoms. \(MnO_2\) is not a reactant; it is a catalyst that increases the reaction rate and is written above the arrow. In option A, oxygen atoms are not balanced, and the reaction is a decomposition reaction, not a displacement reaction.

In \(\mathrm{CaCO_3 \rightarrow CaO + CO_2}\), the oxidation numbers of calcium \((+2)\), carbon \((+4)\), and oxygen \((-2)\) remain unchanged on both sides. Therefore, neither oxidation nor reduction occurs, so it is not a redox reaction. In contrast, in option B, Zn changes from \(0\) to \(+2\) and \(\mathrm{Cu^{2+}}\) changes from \(+2\) to \(0\), making it a redox reaction.

Ions exchange between aqueous \(\mathrm{AgNO_3}\) and \(\mathrm{NaCl}\), producing insoluble white \(\mathrm{AgCl(s)}\) as a precipitate; therefore, this is a precipitation and double-displacement reaction. The oxidation numbers remain unchanged: \(\mathrm{Ag}\) is \(+1\), \(\mathrm{Cl}\) is \(-1\), \(\mathrm{Na}\) is \(+1\), and \(\mathrm{N}\) is \(+5\). Hence, no oxidation or reduction occurs, so it is not a redox reaction. The presence of two reactants does not make it a combination reaction, since two products are formed.

Zinc is more reactive than iron. Even if the coating is scratched, zinc oxidises preferentially and supplies electrons that prevent or slow the oxidation of the exposed iron. This is called sacrificial protection. Option C is incorrect because air and moisture can reach a scratch; the protection mainly results from zinc being more reactive than iron.

The balanced equation for electrolysis of water is \(2H_2O \rightarrow 2H_2 + O_2\). At the same temperature and pressure, gas volumes are proportional to their moles, so \(H_2:O_2 = 2:1\). Therefore, 15 mL of \(O_2\) at the anode corresponds to 30 mL of \(H_2\) at the cathode. Option B notes that water contains both elements but does not give the correct volume ratio.

Antioxidants slow oxidation reactions in food, whereas airtight packaging reduces the food's contact with oxygen. Thus, both help prevent or reduce oxidation of fats and oils and hence rancidity. In contrast, forming a precipitate is associated with reactions in solutions, not with preventing oxidation of food.

Heat written on the product side shows that the reaction releases heat to the surroundings. Emission of light is also a sign of energy release. Therefore, the reaction is exothermic. In contrast, an endothermic reaction absorbs heat, so heat is written on the reactant side.

In combustion, oxidation of a fuel releases energy as heat and often light. In respiration, glucose undergoes controlled oxidation and releases energy for cellular activities. Therefore, both are exothermic processes. In contrast, a process that absorbs energy is endothermic.

Iron is more reactive than copper, so it displaces copper from copper sulfate solution: \(Fe + CuSO_4 \rightarrow FeSO_4 + Cu\). Here, \(Fe^0 \rightarrow Fe^{2+} + 2e^-\) is oxidation, while \(Cu^{2+} + 2e^- \rightarrow Cu\) is reduction. The reddish-brown coating is copper metal. Unlike option B, iron loses electrons rather than gaining them.

Chewing breaks food into smaller pieces; its main effect is a change in size and surface area, so it is mainly a physical change. During digestion, enzymes convert complex food substances into new, simpler, absorbable substances, so it is a chemical change. Option B is incorrect because digestion is not merely a reduction in size.

During curd formation, bacterial action produces new substances such as lactic acid and the properties of milk change; therefore, it is a chemical change. In contrast, when water freezes to form ice, its chemical composition remains \(\mathrm{H_2O}\); only its state changes, so it is a physical change. A colour change or precipitate formation is not an essential feature of either process.

When water is added to a copper sulfate solution, its concentration decreases, so the blue colour may become lighter. This dilution is a physical change and does not necessarily form a new substance. In contrast, in option B, the formation of an insoluble precipitate is additional evidence of a new substance being formed.