Chemical Reactions and Equations

In CuO + H₂ → Cu + H₂O, hydrogen combines with oxygen removed from copper(II) oxide and becomes water. Gain of oxygen is oxidation, so hydrogen is oxidised and option A is correct. Copper(II) oxide loses oxygen and is reduced. Displacement describes a reaction pattern, while neutralisation involves an acid and a base, neither of which identifies hydrogen’s process here.

Gain of oxygen is oxidation, while loss of oxygen is reduction. When these two processes occur simultaneously in the same reaction, the reaction is called a redox reaction; therefore, option A is correct. Precipitation produces an insoluble solid, neutralisation involves an acid and a base, and displacement involves replacement of one element by another.

Rusting of iron is an oxidation process that requires both oxygen from air and water or moisture. Moisture provides a thin water layer that supports electrochemical reactions, producing hydrated iron(III) oxide, called rust. Therefore, neither air alone nor moisture alone is considered sufficient for rusting.

Silver chloride undergoes photodecomposition in sunlight, producing grey metallic silver and chlorine gas. The balanced equation is \(2AgCl(s) \xrightarrow{\text{sunlight}} 2Ag(s) + Cl_2(g)\). Option B incorrectly shows chlorine as atomic \(Cl\), whereas chlorine is produced as the diatomic gas \(Cl_2\).

The oils and fats in chips can react with oxygen and become oxidised, causing rancidity and an unpleasant taste or smell. Nitrogen is comparatively unreactive; it displaces oxygen in the packet and reduces oxidation of the oil. In contrast, increasing the oxygen content would promote oxidation.

Rancidity occurs when oils and fats in food undergo oxidation and form substances with an unpleasant smell and taste. Antioxidants slow this oxidation, so food does not spoil as quickly. Unlike option B, antioxidants do not increase the rate of oxidation.

During the conversion of milk into curd, bacterial action changes the components of milk and forms new substances; the change is not easily reversible. Hence, it is a chemical change. In contrast, melting ice is only a change of state, and sugar can be recovered from water, so these are physical changes.

When ice melts, only its state changes from solid to liquid; no new substance is formed, so it is a physical change. Near the wick, wax vapour burns to form new substances such as carbon dioxide and water vapour, so burning is a chemical change. Melting of wax is physical, but burning of wax is chemical.

In a precipitation reaction, mixing two aqueous solutions produces a solid that is insoluble in the solvent. This separated solid is called a precipitate. A constant temperature does not identify the reaction type, whereas the formation of an insoluble solid is a clear indicator of precipitation.

The governing concept is the classification of reactions by the number of reactants and products. In a decomposition reaction, one compound breaks into two or more simpler substances, represented as AB → A + B. Therefore, the described reaction is decomposition, so option A is correct. Combination has several reactants but one product, while displacement reactions involve replacement or ion exchange.

A combination reaction is identified by the pattern A + B → AB: two or more reactants join to form one product. For example, 2Mg + O₂ → 2MgO shows magnesium and oxygen forming magnesium oxide. Thus option A is correct. Decomposition has one reactant and several products, whereas displacement and double displacement involve replacement or exchange rather than simple formation of one product.

In combustion, a fuel reacts with oxygen to form products and releases energy to the surroundings as heat, often with light. Release of heat makes the reaction exothermic. Option B describes an endothermic reaction, in which heat is absorbed from the surroundings.

In respiration, glucose is oxidised and energy is released. Similarly, in combustion, a fuel is oxidised and energy is released as heat and often light. Therefore, both are exothermic oxidation processes. However, respiration is controlled and occurs slowly in cells, whereas combustion is generally much faster.

Balancing follows the law of conservation of mass: the number of atoms of every element must be equal on both sides. A chemical formula represents the fixed composition and identity of a substance, so changing its subscripts would create a different substance. Coefficients change only the number of molecules or formula units. Therefore option A is correct; the other statements misunderstand formulae or coefficients.

According to the law of conservation of mass, atoms are neither created nor destroyed in a chemical reaction. Therefore, in a balanced equation, the number of atoms of every element is equal on the reactant and product sides. A greater number of atoms on either side would indicate an unbalanced equation.

In conventional chemical equations, an upward arrow (↑) indicates that a gas is evolved during the reaction. A downward arrow (↓) indicates the formation of an insoluble solid precipitate from a solution. Thus option A correctly assigns both symbols. They do not specifically represent heat or light; those conditions are usually written above the reaction arrow or shown separately.

The state symbol (aq) means aqueous: the substance is dissolved in water and exists as part of an aqueous solution. For example, NaCl(aq) represents sodium chloride dissolved in water. Therefore option A is correct. The symbols (s), (l), and (g) separately indicate solid, pure liquid, and gaseous states; (aq) is not simply a general symbol for every liquid.

A chemical reaction produces one or more new substances, and observations such as gas evolution, a colour change, or precipitate formation may provide evidence of that change. Merely moving a substance from one place to another changes its position, not its composition or identity. It is therefore a physical change and is not evidence by itself that a chemical reaction occurred. Option A is correct.

An endothermic reaction absorbs heat from its surroundings. If the absorbed energy is taken from the reaction mixture, its temperature generally falls, so option A is correct. An exothermic reaction releases heat and usually raises the mixture or surroundings temperature. Combustion is normally exothermic, while precipitation describes formation of a solid and does not by itself determine the direction of temperature change.

A hot reaction vessel indicates that heat is being released from the reaction to the surroundings. A reaction that releases heat is called an exothermic reaction. In contrast, an endothermic reaction absorbs heat and generally requires energy from outside to keep the reaction going.