Chemical Reactions and Equations

Correct answer: A. Heating calcium carbonate

When calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide: \(\mathrm{CaCO_3 \xrightarrow{\Delta} CaO + CO_2}\). This is thermal decomposition because heat causes the breakdown, not light. Silver chloride and silver bromide undergo photolytic decomposition in sunlight. In contrast, iron reacting with copper sulfate is a displacement reaction.

Correct answer: A. The product is \(Fe_3O_4\), not \(FeO\); the correct equation is \(3Fe + 4H_2O \to Fe_3O_4 + 4H_2\).

Red-hot iron reacts with steam to form magnetite, \(Fe_3O_4\), and hydrogen. The balanced equation is \(3Fe + 4H_2O \to Fe_3O_4 + 4H_2\); each side contains 3 Fe, 4 O, and 8 H atoms. Option D balances the atom count but still incorrectly identifies the iron oxide as \(FeO\).

Correct answer: A. A displacement reaction in which iron displaces copper from its salt

Iron is more reactive than copper, so it displaces copper from copper sulphate solution: \(Fe + CuSO_4 \rightarrow FeSO_4 + Cu\). Reddish-brown copper deposits on the nail, and green iron sulphate solution forms. This is not a combination reaction because it produces two products rather than a single product.

Correct answer: D. Formation of new substances with properties different from those of the reactants

In a chemical reaction, reactants are converted into new substances with properties different from those of the reactants. Therefore, the formation of new substances with new properties is the most reliable indication of a chemical reaction. In contrast, changing position, shape, or container usually forms no new substance; these are physical changes.

Correct answer: A. Law of conservation of mass

In a closed container, no matter enters or leaves. During a chemical reaction, atoms are only rearranged, so the total mass of the reactants equals the total mass of the products. This illustrates the law of conservation of mass. The law of definite proportions concerns the fixed mass ratio of elements in a compound, not the total mass before and after a reaction.

Correct answer: B. Precipitation reaction

An insoluble solid formed when two aqueous solutions are mixed is called a precipitate. Its formation indicates a precipitation reaction. In contrast, combustion involves burning in oxygen, whereas photolytic and electrolytic decomposition involve breakdown of a compound by light and electricity, respectively.

Correct answer: C. There is a strong possibility that a chemical reaction has occurred

A change in colour, formation of a gas, and release of heat are common indicators of a chemical reaction. When these observations occur together, there is a strong possibility that a chemical reaction has taken place. In contrast, a change only in position is a physical change and does not indicate formation of a new substance.

Correct answer: A. Gas formed and the reaction may have absorbed heat

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.

Correct answer: A. It uses light energy to form glucose

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.

Correct answer: A. Because curd is a substance with new properties

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.

Correct answer: A. A precipitation reaction is possible

A white insoluble solid formed from solution is called a precipitate, so its appearance is strong evidence that a precipitation reaction may have occurred. Temperature change is not required for every chemical reaction; some reactions produce too little heat to cause an observable change. Therefore option A is the most justified conclusion. The observation does not prove combustion or photolytic decomposition, and absence of temperature change does not prove that no reaction occurred.

Correct answer: A. A chemical reaction occurred, and the law of conservation of mass was demonstrated.

Formation of a new substance and a change in chemical properties identify a chemical reaction rather than a merely physical change. Because the container is closed, no reactant or product can escape, so the total mass before and after the reaction remains equal. This demonstrates conservation of mass. Option A combines both conclusions; option B ignores the new substance, while C and D contradict the evidence.

Correct answer: A. Ten grams

On heating, calcium carbonate decomposes according to the reaction CaCO₃ → CaO + CO₂. Although carbon dioxide is produced, the container is closed, so this gas remains inside and is included in the measured products. The law of conservation of mass therefore gives: total mass of products = total mass of reactant = 10 g. Hence option A is correct; the other values incorrectly assume mass is lost, doubled, or absent.

Correct answer: C. Carbon dioxide

Heating calcium carbonate causes thermal decomposition: CaCO₃ → CaO + CO₂. The released carbon dioxide reacts with calcium hydroxide in limewater to form insoluble calcium carbonate, producing a milky appearance: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. Thus option C is correct. Oxygen, hydrogen, and nitrogen do not give this characteristic limewater test under these conditions.

Correct answer: A. It decomposes in light

Silver bromide is photosensitive, meaning that it undergoes a chemical decomposition when exposed to light. The reaction can be represented as 2AgBr → 2Ag + Br₂ in the presence of light. The deposited silver forms the dark image on a photographic material, so option A explains its use. The remaining choices describe unrelated properties and do not account for image formation.

Correct answer: B. Hydrogen

In CuO + H₂ → Cu + H₂O, hydrogen combines with oxygen from copper(II) oxide and becomes part of water. By the school-level definition, addition of oxygen is oxidation, so hydrogen is oxidised. At the same time, CuO loses oxygen and changes to copper, so CuO is reduced. Water is a product, not the substance being oxidised, making option B correct.

Correct answer: A. CuO + H₂ → Cu + H₂O

In CuO + H₂ → Cu + H₂O, copper in CuO changes from oxidation state +2 to 0, so it is reduced. Hydrogen changes from oxidation state 0 to +1 in water, so it is oxidised. Since oxidation and reduction occur together, this is a redox reaction and option A is correct. The other equations are combination, double displacement, or decomposition reactions without simultaneous oxidation and reduction.

Correct answer: A. Contact with oxygen decreases

The governing concept is oxidation and rancidity. Oils and fats in chips react slowly with oxygen, producing unpleasant smell and taste. Nitrogen is comparatively unreactive and displaces much of the oxygen from the packet, so the oil has less contact with oxygen and oxidation becomes slower. Therefore option A is correct. Nitrogen does not make oxygen precipitate or convert it into a metal.

Correct answer: A. They decrease the rate of oxidation

The governing concept is prevention of oxidation. Oils and fats react with oxygen during storage, and this process can produce rancid odours, unpleasant flavours, and reduced food quality. Antioxidants react preferentially with reactive species or interrupt the oxidation chain, thereby slowing spoilage. Hence option A is correct. They do not decompose water, rust iron, or necessarily form a precipitate.

Correct answer: A. The container feels cold

An endothermic reaction absorbs heat from its surroundings, so the surroundings lose thermal energy and the reaction vessel may feel cooler. Thus option A is the appropriate observation. A hot vessel generally indicates an exothermic process that releases heat. A bright flame and burning food are also commonly associated with combustion, which is exothermic, not endothermic.

Correct answer: A. A reddish-brown copper coating forms on the iron nail and the blue solution gradually turns green

The reactivity series places iron above copper, so iron displaces copper from copper sulphate: Fe + CuSO₄ → FeSO₄ + Cu. Reddish-brown copper deposits on the nail, while pale green FeSO₄ forms in solution and reduces the blue colour. Thus A is correct. Silver deposition, no reaction, or rapid gas bubbles do not describe this displacement reaction.

Correct answer: A. Comparing total mass after carrying out reaction in a closed container

The law of conservation of mass states that total mass remains constant in a chemical reaction when matter is not allowed to enter or leave the system. A closed container retains reactants and any gaseous products, so measuring and comparing the total mass before and after the reaction provides the best test. Therefore A is correct; colour and smell are qualitative observations, not mass measurements.

Correct answer: B. Heat is the energy that drives the reaction

Thermal decomposition requires an input of energy to break the bonds in calcium carbonate. On heating, calcium carbonate decomposes as CaCO₃ → CaO + CO₂, so heat supplies the activation energy and is written over the reaction arrow as a condition, not as a product. Hence option B is correct. The products are calcium oxide and carbon dioxide, not heat or a precipitate formed by heat.

Correct answer: A. To prevent its decomposition by light energy

Silver bromide is photosensitive. Light supplies energy that can cause it to decompose, producing silver and bromine; this property is related to photochemical reactions and was used in photographic materials. A dark or amber bottle absorbs or blocks much of the incident light, reducing decomposition during storage. Therefore A is correct; the bottle is not intended to promote burning, dissolving, or mass gain from air.

Correct answer: B. Ten millilitres

The balanced equation for electrolysis of water is 2H₂O → 2H₂ + O₂. It shows that hydrogen and oxygen are produced in a volume ratio of 2:1 under the same conditions. Therefore, for 5 mL of oxygen, hydrogen volume = 2 × 5 = 10 mL. Thus option B is correct; the other options do not follow the stoichiometric gas ratio.