Chemical Reactions and Equations

In a displacement reaction, a more reactive metal removes a less reactive metal from its salt solution. Since A displaces B, A is more reactive than B, written A > B. Since B displaces C, B > C. Combining both relationships gives A > B > C, so option A is correct. Equal reactivity would not produce these displacements.

Removal of oxygen from substance A is reduction, while addition of oxygen to substance B is oxidation. Because these two complementary processes occur simultaneously in one reaction, the overall reaction is classified as a redox reaction. Option B describes only oxidation and option C only reduction; neither represents the complete reaction. Neutralisation requires acid-base interaction.

In option A, the ions of two aqueous compounds exchange partners: Ba²⁺ combines with SO₄²⁻ to form insoluble BaSO₄(s), while Na⁺ and Cl⁻ remain in solution. This is both a double displacement reaction and a precipitation reaction. Option B is single displacement, C is combination, and D is neutralisation without a solid precipitate.

Ghee contains fats that can react with oxygen and become rancid. Keeping it open increases contact with atmospheric oxygen, while warmth generally speeds up the oxidation reaction. Consequently, rancidity develops more rapidly. Option A gives both relevant factors; nitrogen formation, conversion into a metal, and precipitate dissolution do not explain oxidation of ghee.

If the container and its surroundings feel colder during a reaction, the reacting system is absorbing thermal energy from the surroundings. Such a heat-absorbing reaction is called endothermic. Therefore option A is the best explanation. A reaction that releases heat would warm the surroundings, while colour change alone does not describe the energy transfer.

The governing concept is decomposition: one compound splits into two or more simpler substances. Because heat supplies the energy required for this breakdown, the reaction is specifically called thermal decomposition. For example, calcium carbonate decomposes on heating: CaCO₃ → CaO + CO₂. A combination reaction forms one product from several reactants, while double-displacement and precipitation involve exchange of ions, so option A is correct.

A careful scientific conclusion must rely on evidence without claiming more than the observation proves. Evolution of a gas may indicate that a new substance has formed, so it can be evidence of a chemical reaction. However, absence of colour change does not rule out a reaction, because many reactions show no visible colour change. The observation alone cannot prove that the change is physical or specifically rusting; therefore option A is best.

The law of conservation of mass states that matter is neither created nor destroyed in a chemical reaction, provided the complete system is considered. A closed container prevents gases, vapours, or solids from leaving, so measuring the total mass before and after gives a reliable comparison. In an open vessel, escaping gas makes the remaining material appear lighter. Colour and smell are observations, not measurements of total mass; therefore option A is correct.

The coefficient 2 before H₂O means that two water molecules are present on the product side. The subscript 2 in each H₂O molecule shows that each molecule contains two hydrogen atoms. Therefore, the total number of hydrogen atoms is coefficient × subscript = 2 × 2 = 4. The value 2 counts hydrogen atoms in only one water molecule, whereas 6 incorrectly mixes coefficients and subscripts. Thus option C is correct.

On heating, lead nitrate undergoes thermal decomposition: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂. Nitrogen dioxide, NO₂, is a reddish-brown gas, so its characteristic colour identifies it as the gas observed. Oxygen is colourless, hydrogen is also colourless and is not produced in this decomposition, and carbon dioxide is not a product of lead nitrate decomposition. Therefore option C is correct.

Silver chloride is photosensitive and decomposes when exposed to sunlight according to 2AgCl → 2Ag + Cl₂. Light energy initiates this decomposition, and the metallic silver produced appears grey, causing the visible colour change. The reaction is not driven by sound or electrical energy in this observation, and water or hydrogen is not formed. Thus the correct combination is light energy and silver, given in option A.

Pure water contains very few ions and therefore conducts electricity poorly. During electrolysis, adding a small amount of a suitable acid increases the concentration of mobile ions, allowing electric current to pass through the water more effectively. The acid is not added to colour oxygen, solidify hydrogen, or produce a precipitate. It supports conduction; the water is then decomposed at the electrodes into hydrogen and oxygen. Hence option A is correct.

Barium chloride and sodium sulphate undergo a double-displacement reaction: BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq). Barium sulphate is a white insoluble precipitate, while sodium chloride remains dissolved. The other choices do not provide the required pair of products or do not represent two suitable aqueous salt solutions.

The reaction is CuO + H₂ → Cu + H₂O on heating. Oxygen is removed from copper(II) oxide and combines with hydrogen to form water. Removal of oxygen is the defining sign of reduction, so CuO is reduced to copper. Oxidation would involve gaining oxygen or losing electrons; precipitation and neutralisation do not describe this change.

In CuO, copper has oxidation number +2 and becomes elemental copper with oxidation number 0, so CuO is reduced. Hydrogen in H₂ has oxidation number 0 and becomes +1 in H₂O, so hydrogen is oxidised. Thus option A correctly identifies both changes. A decrease in oxidation number means reduction, while an increase means oxidation.

In Zn + CuSO₄ → ZnSO₄ + Cu, zinc displaces copper from its salt, so it is a displacement reaction. Zinc changes from oxidation state 0 to +2 and loses electrons, while Cu²⁺ changes to Cu with oxidation state 0 and gains electrons. Thus it is also redox. Option D is redox but not displacement; B is decomposition and C is neutralisation.

Electrolysis requires the solution to conduct electric current. Pure or distilled water contains very few ions, so its electrical conductivity is too low for efficient electrolysis. Adding a small amount of a suitable acid increases the concentration of mobile ions, allowing current to pass and the water to decompose into hydrogen and oxygen. Therefore option B is correct; the other choices are not problems of electrical conduction.

A double displacement reaction generally occurs between two ionic compounds. The positive ions, or cations, exchange partners with the negative ions, or anions, of the other compound. For example, in AB + CD, the new combinations are AD and CB, provided the resulting substances are chemically possible, such as a precipitate, gas, or water. Therefore option B gives the governing principle; the other choices describe unrelated physical or chemical processes.

In the classical description of redox processes, removal of hydrogen from a substance is oxidation, while addition of hydrogen is reduction. The question asks for these processes in that exact order: first hydrogen is removed, so oxidation occurs; then hydrogen is added, so reduction occurs. Consequently, option D is correct. Option A contains the same two terms but reverses their order, and the other options name unrelated reaction types.

Rusting is an electrochemical corrosion process that requires both water or moisture and oxygen. In moist open air, iron receives a continuous supply of moisture and oxygen, so rust formation is greatest. A dry airtight bottle lacks sufficient moisture and fresh oxygen; an oil coating blocks contact with both, and a nitrogen-filled container lacks oxygen. Therefore option A is the only condition that strongly supports rusting.

Calcium oxide reacts with water as CaO + H₂O → Ca(OH)₂. Two reactants combine to produce a single product, calcium hydroxide, so the reaction is a combination reaction. Because heat is released to the surroundings, it is also exothermic. Therefore, option A correctly identifies both features; the other options describe different reaction types or energy changes.

On heating, lead nitrate undergoes thermal decomposition: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂. Nitrogen dioxide, NO₂, is a reddish-brown gas, so its brown fumes provide the identifying observation. Oxygen is colourless, hydrogen is colourless and combustible, and carbon dioxide is colourless and does not produce brown fumes. Thus option B is correct.

When lead nitrate is heated, it decomposes according to 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂. Lead oxide, PbO, remains as the solid residue, while nitrogen dioxide and oxygen escape as gases. Lead sulphate, lead chloride, and lead carbonate are not products of this decomposition because their anions are absent from the reactant. Therefore, option A is correct.

Mixing BaCl₂(aq) and Na₂SO₄(aq) causes exchange of ions: BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl. Barium sulphate is insoluble in water, so it separates as a white precipitate. Sodium chloride remains dissolved, while barium oxide and sodium oxide are not formed because no oxide ions are present. Hence option A is correct.

In the oxygen-based definition of redox, removal of oxygen from a substance is reduction, whereas addition of oxygen is oxidation. Since A loses oxygen, A is reduced. Since B gains that oxygen, B is oxidised. These processes occur together as a redox change. Option B reverses the definitions, and options C and D incorrectly assign the same process to both substances.