Chemical Reactions and Equations

In a double displacement reaction, the cations and anions of two ionic compounds exchange partners to form new compounds. Its general form is \(AB + CD \rightarrow AD + CB\). Burning a metal is an example of a combination/oxidation reaction, not a double displacement reaction.

Iron is more reactive than copper, so it displaces copper from copper sulfate solution: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). The iron sulfate formed makes the solution light green, and displaced copper deposits as a reddish-brown coating on the nail. Unlike option B, less reactive copper cannot displace iron from its salt solution.

The reaction is \(\mathrm{CuO + H_2 \rightarrow Cu + H_2O}\). Hydrogen combines with oxygen removed from copper oxide and forms water. Addition of oxygen is oxidation, so hydrogen is oxidised. In contrast, copper oxide is reduced because it loses oxygen. Exam tip: Identify oxidation by checking which substance gains oxygen.

In a redox reaction, oxidation and reduction always occur together. One substance loses electrons and is oxidised, while another substance gains those electrons and is reduced. Options B and C are changes of state, which do not necessarily involve transfer of electrons.

Removal of oxygen from a substance is called reduction. For example, in
\(\mathrm{CuO + H_2 \rightarrow Cu + H_2O}\), oxygen is removed from copper(II) oxide, so
\(\mathrm{CuO}\) is reduced. In contrast, oxidation involves the addition of oxygen to a substance.

Rusting is a slow chemical reaction in which iron reacts with oxygen in the presence of moisture, forming hydrated iron(III) oxide, commonly called rust. It is therefore an example of slow oxidation, so option B is correct. Evaporation and freezing are physical changes, while dissolving a precipitate does not describe the formation of rust or the reaction of iron with oxygen and water.

Rusting requires iron to be in contact with both oxygen and water or moisture. A coat of paint forms a protective barrier on the iron surface, so air and moisture cannot easily reach the metal and rusting is reduced. Simply making the surface harder does not prevent rusting.

A displacement reaction occurs when a more reactive element replaces a less reactive element from its compound. In option A, zinc is above copper in the reactivity series, so Zn removes Cu from CuSO₄ and forms ZnSO₄ while copper is released. Option B is a combination reaction, option C is a decomposition reaction, and option D is a double-displacement reaction.

This is a single displacement reaction. Iron is above copper in the reactivity series, so \(Fe\) displaces copper from \(CuSO_4\), forming \(FeSO_4\) and \(Cu\). In a double displacement reaction, ions of two compounds exchange partners; here, one metal replaces another metal from its salt.

Chips contain oil or fat, which can react with oxygen in air and become oxidised. This causes rancidity, producing an unpleasant smell and taste. Nitrogen is relatively unreactive; it reduces the amount of oxygen in the packet and thereby slows oxidation of the oil. Formation of a precipitate refers to separation of an insoluble solid from a solution, so it is not relevant here.

Rancidity is the unpleasant smell and taste that develops when oils and fats undergo oxidation. Antioxidants react preferentially with oxidising substances or interrupt oxidation chains, thereby slowing this process and extending the food’s shelf life. Therefore option C is correct. Option A describes antimicrobial action, option B would increase spoilage, and option D is related to drying rather than antioxidant protection.

Rancidity mainly results from oxidation of unsaturated oils and fats by oxygen in the air. An airtight container limits the entry and contact of oxygen, so the oxidation rate decreases and unpleasant smells and tastes develop more slowly. Thus option B is correct. Heating would generally speed many oxidation processes, while precipitation and conversion into metal have no connection with rancidity.

When milk changes into curd, bacterial action changes components of the milk and curd with new properties is formed. Curd differs from milk in taste, smell, and texture, so this is a chemical change. A change of state alone, such as water freezing into ice, is not a chemical change because no new substance is formed.

In respiration, glucose undergoes complete oxidation in the presence of oxygen and releases energy: \(\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{energy}\). Because energy is released, respiration is an exothermic reaction. Options B and C are true features of respiration, but they do not explain why the reaction is exothermic; the release of energy does.

Photosynthesis is a chemical reaction because reactants are converted into new products. Using light energy and chlorophyll, plants combine carbon dioxide and water to form glucose and release oxygen: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, in simplified form. Option B identifies new-substance formation; leaf shape or evaporation does not establish this reaction.

A subscript tells how many atoms of an element are present in one formula unit or molecule. For example, H₂O and H₂O₂ are different substances because their compositions differ. Changing a subscript during balancing would therefore change the reactant or product itself and violate the original reaction. Only coefficients placed before formulae may be changed to equalise atom counts.

The law of conservation of mass states that atoms are not created or destroyed in a chemical reaction. When an equation is balanced, the coefficient-adjusted number of atoms of every element is identical on the reactant and product sides. Consequently, the total mass is conserved, although atoms may be rearranged into new substances. Therefore option D is correct; physical state or colour is not the basis of balancing.

Balancing a skeletal equation means making the number of atoms of every element equal on both sides while preserving the identities of the substances. This is done by adding suitable whole-number coefficients before formulae, such as changing H₂ + O₂ → H₂O to 2H₂ + O₂ → 2H₂O. Subscripts must not be altered because that would create different substances. Hence A is correct.

If the outside of a reaction container feels cold, the reaction mixture is taking energy from the surroundings faster than it releases energy to them. This indicates an endothermic reaction, in which heat is absorbed. Therefore option B is the best conclusion. A released-heat process is exothermic and would generally warm the container; coldness alone cannot prove combustion or rusting.

A container becoming hot indicates that energy is being transferred from the reacting system to its surroundings. This is evidence that heat has been released and the reaction is likely exothermic, so option C is correct. However, the observation alone does not prove combustion; other exothermic reactions can also warm the container. It also does not represent mere state change or absence of reaction.

The formation of bubbles may indicate that a gas is being produced or released, so the most careful conclusion is that gas formation is possible. Bubbles do not by themselves prove that a precipitate has formed; a precipitate is usually observed as a solid. Bubbles can also arise on heating or when dissolved air escapes, so bubbles alone do not conclusively prove a chemical reaction.

In a chemical equation, an upward arrow (↑) indicates that a gaseous product is evolved during the reaction. For example, an acid reacting with a carbonate may release carbon dioxide gas, which can be shown by this symbol. A downward arrow indicates a precipitate, while heat is written over the reaction arrow rather than represented by an upward arrow. Therefore, option A is correct.

A downward arrow (↓) in a chemical equation shows that an insoluble solid has formed from the reacting solutions. This solid settles out and is called a precipitate; for example, silver chloride may form as a precipitate in a reaction involving soluble silver and chloride compounds. An upward arrow represents gas evolution, whereas combustion and light emission require different descriptions. Thus, option B is correct.

The aqueous state symbol, written as (aq), is used when a substance is dissolved in water and forms an aqueous solution. It does not mean that the substance is merely liquid; water is specifically the solvent. A solid is marked (s), a gas is marked (g), and a pure liquid is marked (l). Therefore, the condition described in option C is the correct one.

Iron is more reactive than copper, so it displaces copper from copper sulphate solution: \(Fe + CuSO_4 \rightarrow FeSO_4 + Cu\). The blue copper sulphate solution turns green due to iron sulphate, and copper deposits on the nail. Therefore, this is a displacement reaction. In contrast, a combination reaction forms one product from two or more reactants.