Chemical Reactions and Equations

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.

Writing heat on the products side means that heat is produced and released to the surroundings during the reaction. Such a reaction is called exothermic. In an endothermic reaction, heat is absorbed, so it is conventionally shown on the reactants side. The labels displacement and decomposition describe possible substance changes, not the heat direction, so they cannot be concluded from this information. Option A is correct.

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 a chemical change. Bacterial action produces lactic acid and changes the constituents of milk into curd, which has properties different from milk. In contrast, freezing water is a physical change because the chemical composition of water, \(\mathrm{H_2O}\), remains unchanged; only its state changes from liquid to solid. Therefore, option A is correct. Option C is incorrect because curd formation is not merely a physical or reversible change.

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.

In chemical equations, the downward arrow (↓) indicates a precipitate, an insoluble solid formed from a solution during the reaction. The upward arrow (↑) indicates a gas that is evolved or released. For example, BaSO₄↓ marks a precipitate, while H₂↑ marks liberated hydrogen gas. Therefore, option A gives the correct order. Option B reverses the meanings, and C and D assign symbols that are not represented by these arrows.

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.

In H₂, hydrogen has oxidation number 0; in H₂O it becomes +1, so hydrogen loses electrons overall and H₂ is oxidised. In CuO, copper has oxidation number +2; in elemental Cu it becomes 0, so CuO is reduced. The reaction is therefore a redox reaction, with H₂ acting as the reducing agent and CuO as the oxidising agent. Option A is correct; B reverses the changes.

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.

The formula of water is \(\mathrm{H_2O}\), so each water molecule contains one oxygen atom. Therefore, two water molecules, \(2\mathrm{H_2O}\), contain two oxygen atoms. In the balanced equation \(2\mathrm{H_2}+\mathrm{O_2}\rightarrow2\mathrm{H_2O}\), one \(\mathrm{O_2}\) molecule supplies two oxygen atoms; hence the answer is two, not one.

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.