Chemical Reactions and Equations

A white insoluble solid formed from solution is called a precipitate, so its appearance is strong evidence that a precipitation reaction may have occurred. Temperature change is not required for every chemical reaction; some reactions produce too little heat to cause an observable change. Therefore option A is the most justified conclusion. The observation does not prove combustion or photolytic decomposition, and absence of temperature change does not prove that no reaction occurred.

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.

In conventional chemical-equation notation, an upward arrow (↑) placed beside a product indicates that a gas is evolved, while a downward arrow (↓) indicates that an insoluble solid precipitate has formed. Thus option A gives both meanings correctly. These arrows do not represent heat, light, liquid, or metal. State symbols such as (g), (l), (s), and (aq) provide different information about physical state.

The state symbol (aq) means aqueous. It is written when a substance is dissolved in water and exists as part of an aqueous solution, for example NaCl(aq). Therefore option A is correct. A solid substance is shown by (s), a liquid by (l), and a gas by (g). Light emission is not represented by a physical-state symbol, so options B, C and D do not give the meaning of (aq).

A chemical equation represents a reaction from starting substances to formed substances. The starting substances are called reactants, while the new substances produced are called products. By convention, reactants are written on the left side of the arrow and products on the right side; conditions may be written above the arrow. Therefore, option A is correct, whereas the other options reverse or misstate the standard arrangement.

Formation of a new substance and a change in chemical properties identify a chemical reaction rather than a merely physical change. Because the container is closed, no reactant or product can escape, so the total mass before and after the reaction remains equal. This demonstrates conservation of mass. Option A combines both conclusions; option B ignores the new substance, while C and D contradict the evidence.

On heating, calcium carbonate decomposes according to the reaction CaCO₃ → CaO + CO₂. Although carbon dioxide is produced, the container is closed, so this gas remains inside and is included in the measured products. The law of conservation of mass therefore gives: total mass of products = total mass of reactant = 10 g. Hence option A is correct; the other values incorrectly assume mass is lost, doubled, or absent.

Balancing a chemical equation means making the number of atoms of each element equal on both sides while keeping the identities of the substances unchanged. This is done by placing suitable whole-number coefficients before chemical formulae. Changing a subscript changes the substance itself, and removing a reactant or reversing the arrow changes the reaction rather than balancing it. Therefore, option B is the correct method.

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.

Heating calcium carbonate causes thermal decomposition: CaCO₃ → CaO + CO₂. The released carbon dioxide reacts with calcium hydroxide in limewater to form insoluble calcium carbonate, producing a milky appearance: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. Thus option C is correct. Oxygen, hydrogen, and nitrogen do not give this characteristic limewater test under these conditions.

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.

Silver bromide is photosensitive, meaning that it undergoes a chemical decomposition when exposed to light. The reaction can be represented as 2AgBr → 2Ag + Br₂ in the presence of light. The deposited silver forms the dark image on a photographic material, so option A explains its use. The remaining choices describe unrelated properties and do not account for image formation.

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.