Chemical Reactions and Equations

In a chemical equation, an upward arrow is commonly used to show that a gaseous product is evolved and escapes from the reaction mixture. Therefore option A is correct. A precipitate is an insoluble solid and is conventionally indicated by a downward arrow, not an upward arrow. Freezing and filtration describe physical processes and do not determine this gas-evolution symbol.

In a chemical equation, a downward arrow denotes a precipitate. A precipitate is an insoluble solid formed during a reaction in solution. Therefore, a downward arrow is used when an insoluble solid forms. In contrast, the evolution of a gas is generally shown by an upward arrow.

Freezing of water is a change of state. Water changes from a liquid to a solid, but its chemical composition remains H₂O and no new substance is formed. Therefore, it is a physical change. In contrast, curd formation, rusting of iron, and burning magnesium produce new substances, so they are chemical changes.

A thin magnesium oxide layer, \(\mathrm{MgO}\), forms on the ribbon on exposure to air. This layer reduces direct contact between magnesium and oxygen. Rubbing with sandpaper removes the coating, exposing fresh magnesium that reacts readily with oxygen to form \(\mathrm{MgO}\). Cleaning does not increase magnesium's melting point.

Zinc is more reactive than copper. Therefore, zinc displaces copper from copper sulfate solution: \(Zn + CuSO_4 \rightarrow ZnSO_4 + Cu\). The zinc sulfate solution formed is colourless, so the blue colour fades, while displaced copper deposits as a reddish-brown coating on zinc. Option B is incorrect because copper is less reactive than zinc.

When iron rusts, it reacts with oxygen and moisture in air to form rust, a new substance with properties different from those of iron. Therefore, rusting is a chemical change. In contrast, breaking glass, folding paper, and melting ice are physical changes because no new substance is formed.

In a closed container, no matter enters or leaves. During a chemical reaction, atoms are only rearranged; they are neither created nor destroyed. Therefore, the total mass before and after the reaction remains the same. This is the law of conservation of mass, whereas the law of reflection concerns the reflection of light.

In a chemical reaction, new substances with properties different from those of the reactants are formed. A colour change, evolution of gas, temperature change, or formation of a precipitate may be observed as evidence. In contrast, changing only the position, container, or shape of a substance is generally a physical change and does not produce a new substance.

The formation of an insoluble solid usually indicates the formation of a precipitate, so a precipitation reaction is the most likely conclusion. No noticeable temperature change does not prove that no reaction occurred; in some reactions, the thermal change may be too small to observe easily. Therefore, option B is incorrect, and a white precipitate alone cannot establish that the reaction is definitely exothermic or a decomposition reaction.

Magnesium burns in oxygen present in air to form white magnesium oxide: \(2\mathrm{Mg}+\mathrm{O}_2\rightarrow2\mathrm{MgO}\). Since two reactants combine to form one product, this is a combination reaction. In contrast, a decomposition reaction involves one compound breaking down into two or more substances.

When two or more reactants combine to form a single product, the reaction is called a combination reaction. The release of heat shows that it is exothermic. Therefore, the reaction is classified as combination and exothermic. In contrast, a decomposition reaction involves one compound breaking down into two or more products.

A hot container shows that heat is released to the surroundings during the reaction, so it is an exothermic reaction. Calcium oxide reacts with water to form calcium hydroxide: \(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2 + heat}\). It is not merely a physical change because a new substance, calcium hydroxide, is formed.

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 the number of moles. Hence, the volume ratio of hydrogen to oxygen is \(2:1\). The ratio \(1:2\) would be correct only if the order were oxygen : hydrogen. Exam tip: In a balanced equation, the coefficients of gaseous products give their volume ratio under the same conditions.

During electrolysis of acidified water, reduction occurs at the cathode and hydrogen gas is released there. The balanced equation is 2H2O(l) → 2H2(g) + O2(g). It shows a 2:1 ratio of hydrogen to oxygen, so hydrogen has twice the volume under the same conditions. Therefore option A is correct; oxygen forms at the anode, while nitrogen and carbon dioxide are not the expected products.

On strong heating, lead nitrate undergoes thermal decomposition: 2Pb(NO3)2 → 2PbO + 4NO2 + O2. Nitrogen dioxide, NO2, is a reddish-brown gas, so option B is correct. Oxygen is also produced in the reaction but is colourless, which is why it does not account for the brown fumes. Hydrogen and sulphur dioxide are not products of lead nitrate decomposition.

On heating, lead nitrate undergoes thermal decomposition: \(2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}\). Lead oxide is the yellow solid formed, while nitrogen dioxide and oxygen are released as gases. Therefore, lead sulphate, lead chloride, and lead carbonate are not products of this reaction.

Silver chloride is photosensitive. In sunlight, it undergoes photodecomposition: \(2\mathrm{AgCl}(s) \xrightarrow{\text{sunlight}} 2\mathrm{Ag}(s)+\mathrm{Cl}_2(g)\). The silver metal formed is grey, so the silver chloride turns grey. Although chlorine gas is also produced, the grey colour is due to silver metal.

Silver bromide is a photosensitive substance. In the presence of light, it undergoes photochemical decomposition: \(2\mathrm{AgBr}\xrightarrow{\text{light}}2\mathrm{Ag}+\mathrm{Br}_2\). A dark-coloured bottle reduces the amount of light reaching it and thus prevents its decomposition. It is not stored this way to prevent dissolution in water or precipitation.

Iron is more reactive than copper. Therefore, iron displaces copper from copper sulphate solution: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). A reddish-brown coating of copper forms on the nail, and the blue solution turns pale green. In contrast, copper cannot displace iron from its salt because copper is less reactive.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). The displaced copper deposits on the iron nail as a reddish-brown solid. Iron sulphate is formed in the solution; it is not the reddish-brown coating on the nail.