Chemical Reactions and Equations

Magnesium reacts with oxygen in air to form a thin layer of magnesium oxide on its surface. This layer hinders direct contact between magnesium and oxygen, so the ribbon does not burn readily. Rubbing the ribbon removes this layer, allowing magnesium to burn with a dazzling white flame. This is different from removing magnesium oxide formed after burning.

A combination reaction is identified by its product pattern: two or more reactants join to form one product, represented generally as A + B → AB. Thus option B is correct. Option A describes decomposition, option C describes double displacement, and option D describes single displacement. The number and type of products distinguish these reaction categories.

In a decomposition reaction, one compound breaks down into two or more simpler substances. Its general form is \(AB \rightarrow A + B\). For example, on heating, calcium carbonate decomposes: \(\mathrm{CaCO_3 \rightarrow CaO + CO_2}\). In contrast, option A describes a combination reaction, in which substances join to form a product.

In electrolysis, water, which is a compound, breaks down using electrical energy into hydrogen and oxygen: \(2H_2O(l) \rightarrow 2H_2(g) + O_2(g)\). Therefore, it is a decomposition reaction. In a combination reaction, two or more substances combine to form one product, whereas here one substance forms two products.

In a double displacement reaction, the cations and anions of two ionic compounds exchange partners, forming new compounds. For example, \(\mathrm{AgNO_3 + NaCl \rightarrow AgCl + NaNO_3}\). In contrast, a reaction between two elements does not represent this type of ion exchange.

When aqueous barium chloride and aqueous sodium sulphate are mixed, their ions exchange: \(\mathrm{BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)}\). Insoluble white \(\mathrm{BaSO_4}\) separates as a solid precipitate. Hence, this is both a double-displacement and a precipitation reaction. In contrast, burning magnesium is a combination reaction.

The reaction is: \(\mathrm{BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)}\). Barium sulphate is insoluble in water, so it separates as a white solid precipitate. In contrast, sodium chloride remains soluble, while barium chloride and sodium sulphate are the initial reactants.

At the school-level definition, oxidation means the addition of oxygen to a substance, so option B is correct. For example, magnesium combines with oxygen to form magnesium oxide. Removal of oxygen, addition of hydrogen, and gain of electrons are commonly used indicators of reduction. In the electron-transfer definition, oxidation involves loss of electrons, which also distinguishes it from option D.

Reduction is a process in which oxygen is removed from a substance or hydrogen is added to it. Therefore, removal of oxygen indicates reduction. In contrast, addition of oxygen and removal of hydrogen indicate oxidation. In a physical change alone, the chemical composition of the substance does not change, so it is not reduction.

In a redox reaction, oxidation and reduction occur simultaneously. Oxidation involves loss of electrons or addition of oxygen, whereas reduction involves gain of electrons or removal of oxygen. The formation of a precipitate or a gas alone is not a definite indication of a redox reaction.

Rusting is an oxidation process because iron gradually combines with oxygen in the presence of moisture, producing hydrated iron(III) oxide, commonly called rust. The reaction occurs over a long period and does not show the rapid energy release of burning. Therefore, option B correctly explains why rusting is called slow oxidation; the other statements contradict the role of oxygen, moisture, or reaction rate.

Rusting requires contact between iron, oxygen, and water or moisture. Painting iron creates a continuous protective coating that separates the metal surface from air and moisture, so option B is correct. Leaving the metal exposed or placing it in moisture increases contact with the necessary reactants. Scratching can remove protection and expose fresh iron, making corrosion more likely rather than preventing it.

In galvanisation, iron or steel is coated with zinc. The zinc layer prevents air and moisture from reaching the iron; moreover, because zinc is more reactive than iron, it can oxidise first even if the coating is scratched. Copper is not the metal used for galvanisation.

Oils and fats can react slowly with oxygen from the air, especially when exposed to heat, light, or air for a long time. This oxidation produces rancid substances responsible for the unpleasant taste and smell, so option B is correct. Freezing is a physical state change, precipitation forms an insoluble solid from a solution, and filtration only separates materials; none causes rancidity.

Rancidity mainly results from oxidation of oils and fats by oxygen. Replacing much of the air in a food packet with nitrogen reduces the amount of oxygen available to react with the food. Nitrogen is comparatively unreactive under these conditions, so option A is correct. It does not burn food, cause rusting, or convert food into a precipitate; those choices confuse preservation with unrelated processes.

Zinc is more reactive than iron. The zinc coating acts as a barrier to air and moisture, and if the coating is scratched, zinc oxidises in preference to iron. This sacrificial protection prevents iron from rusting. Options B, C, and D do not describe a mechanism of corrosion prevention.

Rancidity is the oxidation of oils and fats. In warm, open air, the food has ample access to oxygen and the higher temperature increases the rate of oxidation, so rancidity develops fastest. In contrast, a nitrogen-filled packet slows rancidity because it contains very little oxygen.

In a chemical change, the chemical composition of substances changes and one or more new substances are formed. In contrast, a physical change usually changes only the shape, state, or appearance while the chemical identity remains the same. For example, melting ice is a physical change, whereas rusting of iron is a chemical change.

A chemical change is recognized when one or more new substances with different properties are formed. During curd formation, bacteria convert some lactose into lactic acid, changing the taste, texture, and other properties of milk and producing curd. Thus option B is correct. Cooling, changing colour alone, or transferring milk to another container does not by itself establish formation of a new substance.

Iron is more reactive than copper, so it displaces copper from copper sulphate: \(Fe + CuSO_4 \rightarrow FeSO_4 + Cu\). The iron sulphate solution is light green, while copper forms a reddish-brown deposit on the nail. This is not a combination reaction because the reactants do not combine to form a single product.

During respiration, glucose is oxidised and new substances, carbon dioxide and water, are formed; energy is also released. It can be represented as \(\text{glucose} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water} + \text{energy}\). Formation of new substances is the defining feature of a chemical reaction. In contrast, the entry of air in option B is only a physical process and is not itself the chemical reaction of respiration.

In photosynthesis, chlorophyll absorbs light energy. This energy drives the process in which carbon dioxide and water form glucose, with oxygen being released. Therefore, option B is correct. In contrast, rusting of iron is a separate oxidation reaction and has no role in photosynthesis.

According to the law of conservation of mass, atoms are neither created nor destroyed in a chemical reaction. In a balanced equation, the number of atoms of every element is the same on the reactant and product sides; therefore, total mass is conserved. Reactants and products need not have the same colour or physical state, so options A, C, and D are incorrect.

Subscripts give the fixed number of atoms of each element in a compound, so they are part of the formula and identity of the substance. For example, changing H₂O to H₂O₂ changes water into hydrogen peroxide. Only coefficients are changed when balancing an equation because coefficients change the number of molecules without changing the identity of a substance.

While balancing, the chemical formulae and identities of substances are not changed. Suitable coefficients are placed before formulae so that the number of atoms of every element is equal on the reactant and product sides. For example, \(\mathrm{H_2 + O_2 \rightarrow H_2O}\) becomes \(\mathrm{2H_2 + O_2 \rightarrow 2H_2O}\), giving equal numbers of both H and O atoms.