Chemical Reactions and Equations

In an exothermic reaction, heat is released from the reacting system to the surroundings. Therefore, the temperature of the reaction container may rise and it feels hot. In contrast, a container becoming cold can indicate an endothermic reaction, in which heat is absorbed from the surroundings.

An endothermic reaction absorbs heat from its surroundings. Therefore, the temperature of the container and its surroundings may decrease, and the container may feel cold. In contrast, an increase in temperature usually indicates an exothermic reaction. Formation of a precipitate or production of gas may show that a reaction has occurred, but it does not by itself establish that the reaction is endothermic.

The careful scientific conclusion is that gas formation is possible when bubbles appear. Bubbles often result from a gas being released during a chemical reaction, but they may also arise from heating, boiling, or air escaping from a container. Therefore, option A is correct without claiming certainty. A precipitate is a solid formed in a solution, while options C and D incorrectly rule out chemical change.

A colour change can indicate a chemical reaction, but it is not sufficient by itself for a definite conclusion. A more reliable conclusion requires checking for evidence such as formation of a new substance, gas evolution, precipitate formation, or a temperature change. Option D is incorrect because a precipitation reaction normally involves formation of an insoluble solid; a colour change alone does not establish precipitation.

A downward arrow \(\downarrow\) with a product indicates that an insoluble solid forms and settles down as a precipitate. So it is used to show precipitation and settling of the solid product.

Evaporation is a physical change because water changes state from liquid to vapour but no new substance is formed; the chemical identity remains the same.

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.

In a displacement reaction, only a more reactive metal displaces a less reactive metal from its salt solution. Zinc is above copper in the reactivity series, so copper cannot displace zinc from zinc sulphate solution. Hence, no reaction occurs. Option A is incorrect because copper is less reactive than zinc. Exam tip: Compare the metals in the reactivity series to predict a displacement reaction.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution. The reaction is \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). Since a more reactive element replaces a less reactive element from its compound, this is a displacement reaction. In a double displacement reaction, ions from two compounds exchange partners, which does not occur here.