Chemical Reactions and Equations

The reaction is \(\mathrm{CuO + H_2 \rightarrow Cu + H_2O}\). Oxygen is removed from copper(II) oxide to form copper. Removal of oxygen is reduction, so copper(II) oxide is reduced. In contrast, hydrogen gains oxygen to form water and is oxidised. Copper is the product of reduction, not the reactant that undergoes reduction.

Oxidation traditionally means addition of oxygen, while reduction means removal of oxygen. If these two processes occur simultaneously in the same reaction, the reaction is called a redox reaction, meaning reduction and oxidation together. A precipitation reaction forms an insoluble solid, an exothermic reaction releases heat, and a displacement reaction concerns replacement of one element by another. Hence, option B is correct.

Reduction is defined, in the oxygen-transfer sense, as removal of oxygen from a substance or addition of hydrogen to it. For example, CuO + H₂ → Cu + H₂O involves removal of oxygen from copper(II) oxide and addition of oxygen to hydrogen. Oxidation is generally the reverse process, while combustion and precipitation describe different reaction features. Therefore, option B is correct.

The governing concept is oxidation–reduction. Oxidation is identified by the addition of oxygen or the removal of hydrogen from a substance; loss of electrons is another related definition. Therefore, the reaction described is oxidation, so option A is correct. Reduction is the opposite process, involving removal of oxygen or addition of hydrogen. Displacement and neutralisation describe different reaction patterns.

During rusting, iron is gradually oxidised in the presence of water and oxygen to form hydrated iron(III) oxide. Since the reaction occurs over a long time rather than instantly, it is called slow oxidation. Option B describes dissolution only, whereas rusting is a chemical change that forms a new substance.

Rusting of iron requires both oxygen and water (moisture). A nail in moist open air has a continuous supply of oxygen from air and water from moisture, so it rusts faster. In a dry, airtight bottle, moisture is absent and the oxygen supply is limited; therefore, rusting is very slow or does not occur.

Iron is more reactive than copper. Therefore, iron displaces copper from \(\mathrm{CuSO_4}\), forming \(\mathrm{FeSO_4}\) and free copper. A reaction in which a more reactive element replaces a less reactive element from its compound is a displacement reaction. It is not a double displacement reaction because there is no exchange of ions between two compounds.

Rancidity is the development of unpleasant smell and taste due to oxidation of oils and fats. Open air supplies oxygen, and a higher temperature increases the rate of oxidation; therefore oily food becomes rancid faster. Nitrogen is comparatively unreactive and does not cause this oxidation.

Chips contain oil or fat. On exposure to oxygen in air, the oil can undergo oxidation and become rancid. Nitrogen is relatively unreactive and reduces the amount of oxygen in the packet, so oxidation of the oil slows down. Evaporation of oil is a different physical process, not the main chemical problem prevented by nitrogen.

Oils and fats can react with oxygen in air and become rancid, producing unpleasant taste and smell. Antioxidants slow this oxidation, so rancidity is delayed and the food remains fit for consumption for longer. Option B is the opposite effect because it states that oxidation is increased.

Two observations must be interpreted separately. Gas evolution is evidence that a new gaseous product may have formed, while cooling of the container suggests that the process may have absorbed heat from its surroundings, making it possibly endothermic. These observations do not prove combustion, because combustion specifically requires burning in oxygen. Filtration is a separation process and does not explain gas formation. Hence option A is the most careful conclusion.

In a chemical equation, substances written on the product side are formed during the reaction. Therefore, when heat appears on the product side, heat is produced and released to the surroundings. Such a process is called an exothermic reaction, so option B is correct. If heat were written with the reactants, it would indicate that heat is required or absorbed. A precipitate is an insoluble solid, not a meaning of heat placement.

A reactant is a substance consumed or required in a reaction. Thus, when heat is written on the reactant side, the reaction requires an input of heat and absorbs it from an external source. This indicates an endothermic process, making option A correct. Heat is not a precipitate or a gas, and placing it on the reactant side does not mean that heat is released; release is represented by writing heat among the products.

In combustion, a substance reacts rapidly with oxygen and releases energy as heat and often light. In respiration, glucose undergoes controlled, slow oxidation, and the released energy is mainly captured in ATP. Therefore, both are exothermic oxidation processes, although respiration does not occur with a flame like combustion.

Photosynthesis is a chemical process in which green plants use light energy to convert carbon dioxide and water into glucose and oxygen. In simplified form, carbon dioxide + water + light energy produces glucose + oxygen. Because light energy is taken in and stored as chemical energy in glucose, the process is described as energy-absorbing, so option A is correct. The other options describe unrelated or false effects.

Freezing of water into ice is a change of state from liquid water to solid water; the chemical formula remains \(\mathrm{H_2O}\). Therefore, no new substance is formed, and it is a physical change. In contrast, rusting forms a new substance, iron oxide, so it is a chemical change.

The governing criterion for a chemical change is the formation of one or more substances with different properties. During curd formation, microorganisms convert some milk sugar into lactic acid, changing the taste, texture and acidity and producing curd with new properties. The process is not merely cooling, changing containers or freezing the same substance. Therefore option A is correct, even though the change occurs gradually and may not be easily reversed.

Zinc is above copper in the reactivity series. Therefore, zinc displaces copper from aqueous \(\mathrm{CuSO_4}\), forming \(\mathrm{ZnSO_4}\) and copper. In option B, copper is less reactive than zinc, and in option C, iron is less reactive than magnesium, so neither displacement can occur. Option D is a double-displacement reaction.

In option A, ions from two aqueous ionic compounds exchange partners: \(\mathrm{Ba^{2+}}\) combines with sulfate ions to form insoluble solid \(\mathrm{BaSO_4}\). Therefore, it is a double displacement precipitation reaction. In contrast, option C is a single displacement reaction because zinc displaces copper from its salt.

In a chemical reaction, atoms in the reactants rearrange to form new substances. The products have properties different from those of the reactants, so the formation of new substances is a clear sign of a chemical change. In contrast, a change of state or shape is usually a physical change because no new substance is formed.