Chemical Reactions and Equations

Zinc is more reactive than iron. When a small scratch occurs and moisture is present, zinc acts as the anode and oxidises preferentially. This sacrificial action keeps the exposed iron cathodic and protects it from rusting. Option C reverses these roles: in galvanisation, zinc is the anode, not iron. Iron can rust after the available zinc has been substantially consumed and it is exposed to air and moisture.

Oils and fats, especially unsaturated fats, react with oxygen in air and undergo oxidation. This oxidation produces foul-smelling substances and makes the oils or fats rancid; the process is called rancidity. Reduction is the opposite of oxidation, so it does not cause rancidity.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution:
\(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\).
The \(\mathrm{FeSO_4}\) solution is green, and the reddish-brown coating is copper. This is a displacement reaction because a more reactive metal removes a less reactive metal from its salt solution; it is not a combination reaction, in which reactants form a single product.

Chips contain oils and fats that can oxidise on contact with oxygen in air, causing rancidity and spoiling their taste and smell. Filling the packet with nitrogen greatly reduces the available oxygen, so oxidation of the oils is slowed and the chips remain fresh longer. Nitrogen is generally inert: it neither solidifies the oil nor reacts with it to form a preservative.

Antioxidants slow oxidation reactions in food, especially in foods containing oils and fats. Airtight packing reduces the food’s exposure to oxygen, thereby reducing oxidation and rancidity. Therefore, their common purpose is to slow oxidation. In contrast, electrolysis of water is the chemical decomposition of water using electric current and is unrelated to this purpose.

A cold reaction vessel indicates that the reaction is absorbing heat from the vessel and the surrounding environment. Such a reaction is endothermic, so option A is correct. Option B represents an exothermic reaction, in which heat is released to the surroundings and the vessel or surroundings generally become warmer.

When heat is written as a reactant, the reaction absorbs heat and is therefore endothermic. The breakdown of one compound into two or more products is called a decomposition reaction. Hence, this is an endothermic decomposition reaction. In contrast, a combination reaction forms one product from two or more reactants.

In combustion, oxidation of a fuel releases energy as heat and often as light. In respiration, glucose undergoes controlled oxidation and releases energy, which is used to form ATP. Hence, both are energy-releasing, or exothermic, processes. Option B is incorrect because there is no net absorption of energy, and option D is incorrect because respiration does not release energy as light. Exam tip: A reaction with an overall release of energy is identified as exothermic.

In photosynthesis, plants use light energy to make glucose from carbon dioxide and water. Thus, light energy is stored as chemical energy in glucose. During respiration, glucose is oxidised and energy is released as ATP and heat. Option B is a close distractor because glucose is broken down only in respiration; it is formed during photosynthesis. Exam tip: Remember photosynthesis as energy storage and respiration as energy release.

This is a displacement reaction: \(Fe + CuSO_4 \rightarrow FeSO_4 + Cu\). Iron is more reactive than copper, so it displaces copper from copper sulphate solution. Deposited copper forms the reddish-brown coating on the nail, while the iron(II) sulphate formed makes the solution green. This is not neutralisation, because no acid-base reaction producing water occurs.

Curd formation from milk is due to bacterial activity that produces lactic acid. This acid causes milk proteins to coagulate, so curd formation is a chemical change. Ice formation from water happens when temperature is lowered enough that water freezes; this is a physical change because only the physical state (liquid to solid) changes. Hence the basis differs: \(\text{chemical change (acid + coagulation)}\) for curd and \(\text{physical change (freezing)}\) for ice.

A colour change can indicate a chemical reaction, but it is not conclusive by itself because colour may also change during physical processes such as mixing or dissolving substances. Formation of a new substance should be supported by other evidence, such as gas evolution, precipitate formation, a temperature change, or testing the properties of the product. Option B is incorrect because a colour change does not always prove a chemical change.

Iron \(\mathrm{Fe}\) is above copper \(\mathrm{Cu}\) in the reactivity series. Therefore, iron displaces copper from \(\mathrm{CuSO_4}\) solution to form \(\mathrm{FeSO_4}\) and \(\mathrm{Cu}\). Hence, this is a displacement reaction, not a decomposition or neutralisation reaction.

According to the law of conservation of mass, the total mass remains constant in a chemical reaction if no substance leaves or enters the system. When the mass of the entire closed container and its contents is measured before and after the reaction, all products, including any gas, remain in the system; therefore, the two masses are equal. In an open container, a gas may escape, making the measured mass appear smaller.

In an open container, a gas produced during the reaction may escape, so the measured mass of the material remaining in the container appears lower. This does not mean that mass was destroyed; when the escaped gas is included, the total mass of the system is conserved. Unlike option A, the gas mass was simply excluded from the final measurement.

Subscripts specify the ratio of atoms in one molecule or formula unit; changing them changes the substance itself. For example, \(\mathrm{H_2O}\) and \(\mathrm{H_2O_2}\) are different substances. Coefficients multiply the number of whole molecules or formula units, so they can adjust the total number of atoms of each element without changing a substance’s identity. Option C is incorrect because a coefficient does not change the number of atoms within a single molecule.

A chemical equation is balanced only when the number of atoms of every element is equal on the reactant and product sides. Here, hydrogen is balanced but oxygen is not, so the equation is unbalanced. Option A is incorrect because balancing only one element is not sufficient.

Each molecule of water, $\text{H}_2\text{O}$, contains exactly $1$ oxygen atom. For $2$ water molecules, the number of oxygen atoms required is $2\times 1=2$.

The formula of magnesium oxide is \(\mathrm{MgO}\), so each formula unit contains 1 magnesium atom and 1 oxygen atom. Two formula units therefore contain 2 magnesium atoms and 2 oxygen atoms. Thus, the ratio is \(2:2=1:1\). A ratio of \(2:1\) would incorrectly give fewer oxygen atoms than required.

A thin layer of magnesium oxide
\(\mathrm{MgO}\)
forms on the surface of magnesium ribbon when it is exposed to air. This layer prevents the magnesium underneath from making direct contact with oxygen, so the ribbon may not burn readily or the reaction may not be clear. Sandpaper removes this layer, allowing fresh magnesium to burn with a dazzling white flame and form
\(\mathrm{MgO}\)
; cleaning does not increase the melting point.

When magnesium burns, it reacts with oxygen: \(2\mathrm{Mg} + \mathrm{O}_2 \rightarrow 2\mathrm{MgO}\). Two reactants, magnesium and oxygen, combine to form one type of product, magnesium oxide; therefore, it is a combination reaction. Option B describes a decomposition reaction, in which a compound breaks down.

The reaction is \(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}\). Since two reactants combine to form one product, it is a combination reaction. The container becoming hot shows that heat is released, so the reaction is also exothermic. In a decomposition reaction, one compound breaks into simpler substances, which does not occur here.

Calcium carbonate undergoes thermal decomposition: \(\mathrm{CaCO_3(s) \xrightarrow{\Delta} CaO(s) + CO_2(g)}\). Therefore, calcium oxide is the solid product and carbon dioxide is the gaseous product. Forming calcium hydroxide and hydrogen would require a source of water or hydrogen, which is not present in this reaction.

In this reaction, one compound, \(AgCl\), breaks down to form silver \(Ag\) and chlorine gas \(Cl_2\), so it is a decomposition reaction. Because sunlight supplies the energy for this breakdown, it is specifically a photochemical decomposition reaction. In thermal decomposition, heat causes the substance to break down; here, light energy is used instead.

On heating, lead nitrate undergoes thermal decomposition: \(2\mathrm{Pb(NO_3)_2} \xrightarrow{\Delta} 2\mathrm{PbO} + 4\mathrm{NO_2} + \mathrm{O_2}\). \(\mathrm{NO_2}\) is a brown gas, whereas \(\mathrm{O_2}\) is colourless. Therefore, oxygen is the correct answer; carbon dioxide is not a product of this reaction.