Chemical Reactions and Equations

During rusting, iron is gradually oxidised in the presence of water and oxygen to form hydrated iron(III) oxide. Since the reaction occurs over a long time rather than instantly, it is called slow oxidation. Option B describes dissolution only, whereas rusting is a chemical change that forms a new substance.

Rusting of iron requires both oxygen and water (moisture). A nail in moist open air has a continuous supply of oxygen from air and water from moisture, so it rusts faster. In a dry, airtight bottle, moisture is absent and the oxygen supply is limited; therefore, rusting is very slow or does not occur.

Iron is more reactive than copper. Therefore, iron displaces copper from \(\mathrm{CuSO_4}\), forming \(\mathrm{FeSO_4}\) and free copper. A reaction in which a more reactive element replaces a less reactive element from its compound is a displacement reaction. It is not a double displacement reaction because there is no exchange of ions between two compounds.

Rancidity is the development of unpleasant smell and taste due to oxidation of oils and fats. Open air supplies oxygen, and a higher temperature increases the rate of oxidation; therefore oily food becomes rancid faster. Nitrogen is comparatively unreactive and does not cause this oxidation.

Chips contain oil or fat. On exposure to oxygen in air, the oil can undergo oxidation and become rancid. Nitrogen is relatively unreactive and reduces the amount of oxygen in the packet, so oxidation of the oil slows down. Evaporation of oil is a different physical process, not the main chemical problem prevented by nitrogen.

Oils and fats can react with oxygen in air and become rancid, producing unpleasant taste and smell. Antioxidants slow this oxidation, so rancidity is delayed and the food remains fit for consumption for longer. Option B is the opposite effect because it states that oxidation is increased.

In combustion, a substance reacts rapidly with oxygen and releases energy as heat and often light. In respiration, glucose undergoes controlled, slow oxidation, and the released energy is mainly captured in ATP. Therefore, both are exothermic oxidation processes, although respiration does not occur with a flame like combustion.

Freezing of water into ice is a change of state from liquid water to solid water; the chemical formula remains \(\mathrm{H_2O}\). Therefore, no new substance is formed, and it is a physical change. In contrast, rusting forms a new substance, iron oxide, so it is a chemical change.

Zinc is above copper in the reactivity series. Therefore, zinc displaces copper from aqueous \(\mathrm{CuSO_4}\), forming \(\mathrm{ZnSO_4}\) and copper. In option B, copper is less reactive than zinc, and in option C, iron is less reactive than magnesium, so neither displacement can occur. Option D is a double-displacement reaction.

In option A, ions from two aqueous ionic compounds exchange partners: \(\mathrm{Ba^{2+}}\) combines with sulfate ions to form insoluble solid \(\mathrm{BaSO_4}\). Therefore, it is a double displacement precipitation reaction. In contrast, option C is a single displacement reaction because zinc displaces copper from its salt.

In a chemical reaction, atoms in the reactants rearrange to form new substances. The products have properties different from those of the reactants, so the formation of new substances is a clear sign of a chemical change. In contrast, a change of state or shape is usually a physical change because no new substance is formed.

A colour change may be an observation associated with a chemical reaction, but it is not conclusive evidence by itself. Colour can also change during dissolution, mixing, or another physical change. Confirmation of a chemical reaction requires evidence of formation of new substances and may include observations such as gas evolution, precipitate formation, or a temperature change. Therefore, option B is incorrect because not every colour change is caused by a chemical reaction.

The balanced equation is \(2\mathrm{H}_2 + \mathrm{O}_2 \rightarrow 2\mathrm{H}_2\mathrm{O}\). Two water molecules contain a total of four hydrogen atoms. Each \(\mathrm{H}_2\) molecule contains two hydrogen atoms, so two \(\mathrm{H}_2\) molecules are needed to provide four hydrogen atoms. One \(\mathrm{H}_2\) molecule would provide only two hydrogen atoms.

The balanced equation is

displaystyle \(2\mathrm{Mg}+\mathrm{O_2}\rightarrow2\mathrm{MgO}\). One

displaystyle \(\mathrm{O_2}\) molecule contains 2 oxygen atoms. Since each unit of

displaystyle \(\mathrm{MgO}\) contains 1 oxygen atom, 2 units of

displaystyle \(\mathrm{MgO}\) are formed, requiring 2 magnesium atoms. Therefore, the coefficient of magnesium is 2. A coefficient of 1 would not balance the number of magnesium atoms on both sides.

The reaction is \(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2 + \text{heat}}\). The two reactants, calcium oxide and water, combine to form a single product, calcium hydroxide; therefore, it is a combination reaction. Since heat is released, it is also exothermic. It is not a decomposition reaction because no single compound is breaking down into simpler substances.

Lead nitrate undergoes thermal decomposition: \(2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}\). The brown gas is nitrogen dioxide, \(\mathrm{NO_2}\), and the solid left behind is lead(II) oxide, \(\mathrm{PbO}\). Lead chloride, sulphate, or carbonate would require a source of chloride, sulphate, or carbonate ions, respectively, which is not present here.

Thermal decomposition of lead nitrate produces brown nitrogen dioxide gas as well as colourless oxygen:
\(2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO}+4\mathrm{NO_2}+\mathrm{O_2}\).
Therefore, the visible brown fumes are \(\mathrm{NO_2}\), not oxygen. Oxygen is indeed formed in the reaction, but it is colourless and cannot be identified as brown fumes.

Silver chloride is a photosensitive substance. Energy from sunlight decomposes it: \(2\mathrm{AgCl}\xrightarrow{\text{sunlight}}2\mathrm{Ag}+\mathrm{Cl}_2\). The grey colour is due to the formation of metallic silver. Therefore, this is a photochemical reaction, not a change caused by heat energy.

The balanced equation for electrolysis of water is \(2H_2O \rightarrow 2H_2 + O_2\). It produces 2 moles of hydrogen for every 1 mole of oxygen. At the same temperature and pressure, gas volume is proportional to the number of moles, so hydrogen occupies twice the volume of oxygen. Carbon dioxide, nitrogen, and chlorine are not products of the electrolysis of water.

Iron is more reactive than copper, so it displaces copper from copper sulfate solution: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). The blue solution turns green due to iron sulfate, and displaced copper deposits on the nail. Hydrogen gas evolution is typically associated with a metal reacting with an acid, not with this reaction.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). Some iron from the nail enters the solution, while copper is deposited on the nail. For corresponding equal amounts in the reaction, deposited copper has greater mass than the iron lost, so the nail’s total mass may increase slightly. Option B is incorrect because oxygen is not deposited in this displacement reaction.

The blue colour of copper sulphate solution is due to copper(II) ions. Since iron is more reactive than copper, it displaces copper(II) ions from the solution to form copper metal: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). Thus, the concentration of copper(II) ions decreases and the blue colour fades. Iron(II) ions are formed, but they are not the ions whose decrease causes the fading.

In a displacement reaction, a more reactive metal displaces a less reactive metal from its salt solution. Since zinc displaces copper from copper sulphate, zinc is more reactive than copper. In contrast, copper cannot displace zinc from zinc sulphate because copper is less reactive.

In a displacement reaction, a more reactive metal displaces a less reactive metal from its salt solution. Since A displaces B, A is more reactive than B. Similarly, B is more reactive than C. Thus, the reactivity order is A > B > C, so metal A is the most reactive. Option B is more reactive than C, but it is less reactive than A.

The reaction is \(\mathrm{BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)}\). The \(\mathrm{Ba^{2+}}\) and \(\mathrm{SO_4^{2-}}\) ions form insoluble white \(\mathrm{BaSO_4}\), producing a visible precipitate. In contrast, sodium chloride remains dissolved in water and does not form a precipitate.