Chemical Reactions and Equations

Zinc is more reactive than copper. Therefore, zinc displaces copper from aqueous \(\mathrm{CuSO_4}\), forming \(\mathrm{ZnSO_4}\) and \(\mathrm{Cu}\). Zinc changes from oxidation state 0 to \(+2\), while copper changes from \(+2\) to 0. Option B is a decomposition reaction, whereas option C is a double-displacement reaction.

Iron is more reactive than copper, so it displaces copper from copper sulphate. The reaction is \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). The iron sulphate solution is green, while displaced copper forms the reddish-brown coating on the nail. This is not a double displacement reaction because iron replaces copper from its salt.

Zinc is more reactive than copper, so it displaces copper from copper sulphate solution: \(\mathrm{Zn + CuSO_4 \rightarrow ZnSO_4 + Cu}\). As blue \(\mathrm{Cu^{2+}}\) ions are removed, the solution fades and reddish-brown copper is deposited on the zinc strip. The hydrogen bubbles in option B are typically produced when a metal reacts with an acid, not with copper sulphate solution.

Antioxidants mainly slow oxidation reactions in foods, especially the oxidation of oils and fats. Oxidation can produce unpleasant tastes and smells called rancidity and may reduce food quality. By reacting preferentially with oxidising species or interrupting chain reactions, antioxidants delay this spoilage. Displacement, electrolysis and galvanisation are different processes and do not describe the usual preservation action here. Thus option A is correct.

In \(\mathrm{FeCl_3}\), Fe has oxidation number \(+3\), which becomes \(+2\) in \(\mathrm{FeCl_2}\). A decrease in oxidation number is reduction, so Fe is reduced. The oxidation number of Sn increases from \(+2\) in \(\mathrm{SnCl_2}\) to \(+4\) in \(\mathrm{SnCl_4}\); therefore, Sn is oxidised. Hence, option A is correct; both elements are not undergoing the same process.

This is a photodecomposition reaction. A single compound, \(AgCl\), decomposes in sunlight to form silver \(Ag\) and chlorine gas \(Cl_2\). The equation is balanced, with 2 Ag atoms and 2 Cl atoms on each side. In a displacement reaction, a free element replaces another element in a compound; no such free reactant is present here.

Heat on the reactant side indicates that the reaction absorbs heat, so it is endothermic. The breakdown of one compound into two or more products is a decomposition reaction. Therefore, the reaction is an endothermic decomposition reaction. Option D is also a decomposition reaction, but an exothermic reaction releases heat on the product side.

In option A, ions from two aqueous solutions exchange, producing insoluble \(\mathrm{AgCl}\) as a solid. Formation of an insoluble solid from aqueous solutions identifies a precipitation reaction. In option B, zinc displaces copper from its salt solution, so it is a displacement reaction rather than a precipitation reaction.

This is a photochemical decomposition reaction because the single compound \(AgCl\) breaks down in sunlight to form silver and chlorine gas. In a displacement reaction, a second, more reactive element removes another element from its compound; no such reactant is present here.

q] means an aqueous solution: the substance is dissolved in water, for example NaCl(aq). In contrast, ] means that the substance itself is in the liquid state, for example H₂O(l). Thus, q] indicates water as the solvent, whereas ] indicates only the physical state.

In \(\mathrm{CuO + H_2 \rightarrow Cu + H_2O}\), copper in \(\mathrm{CuO}\) changes from oxidation state \(+2\) to \(0\), so copper is reduced. Hydrogen in \(\mathrm{H_2}\) changes from \(0\) to \(+1\), so hydrogen is oxidised. Therefore, this is a redox reaction. In contrast, oxidation numbers do not change in option B.

According to the law of conservation of mass, total mass is neither created nor destroyed in a chemical reaction. The gas formed remains inside the closed container, so the combined mass of the container and all its contents is the same before and after the reaction. A rise in temperature indicates an energy change, not an increase in mass.

The law of conservation of mass applies when the complete system is considered. In an open container, a gaseous product can leave before weighing, so the measured system contains less material and its apparent mass decreases. The gas has not been destroyed; it is simply outside the chosen system boundary. The reaction may still be chemical and has produced a product. Hence option B is correct.

A chemical reaction rearranges existing atoms into new substances; it does not create or destroy atoms. Therefore, each element must have the same number of atoms among the reactants and products. This atom balance expresses the law of conservation of mass because the total mass contributed by each type of atom is retained. Energy may change, but balancing coefficients are not primarily used to show energy, colour or temperature conservation. Option B is correct.

Subscripts in a chemical formula show the fixed ratio of atoms in a compound. For example, \(\mathrm{H_2O}\) contains two hydrogen atoms and one oxygen atom. Changing the subscript to make \(\mathrm{H_2O_2}\) gives hydrogen peroxide, not water. Therefore, only coefficients are changed to balance an equation; subscripts must remain unchanged.

A chemical equation is balanced only when the number of atoms of every element is equal on the reactant and product sides. Equality for iron alone is insufficient. If oxygen atoms differ, the equation does not represent conservation of mass correctly and must be balanced by changing suitable coefficients, never subscripts. Precipitation and exothermicity describe other properties, not atom-count balance. Therefore option B is correct.

The chemical formula of water is \(H_2O\). One water molecule has 2 hydrogen atoms and 1 oxygen atom, so it contains 3 atoms in total. Therefore, two water molecules contain \(2 \times (2+1)=6\) atoms. Option 5 is incorrect because it does not represent the total number of hydrogen and oxygen atoms in two complete water molecules.

The formula of magnesium oxide is MgO, so each formula unit contains one magnesium atom and one oxygen atom. Four formula units therefore contain four magnesium atoms and four oxygen atoms. The ratio is 4:4, which simplifies to 1:1. The number of formula units changes the totals but not the simplest ratio. Hence option A is correct; the other ratios do not follow from MgO.

Magnesium reacts slowly with oxygen in air to form a thin magnesium oxide layer on its surface. This coating reduces direct contact between magnesium and oxygen. Rubbing with sandpaper removes the coating, so magnesium burns more readily with a dazzling white flame. The rubbing does not change magnesium’s melting point or remove oxygen from the air.

When magnesium burns, it reacts with oxygen according to 2Mg + O₂ → 2MgO. Two reactants combine to form one principal product, magnesium oxide, so the reaction is a combination reaction. At the same time, oxygen is added to magnesium, which satisfies the school-level definition of oxidation. It is not a double displacement, decomposition or dissolution process. Therefore option A is correct.