The correct answer is combination reaction. Explanation & Key Takeaways: • A combination reaction, also known as a synthesis reaction, is a process where two or more elements or compounds react to form a single new substance. Hence, the correct answer is combination reaction. • The general chemical representation for this type of reaction is A + B → AB, where A and B are the reactants and AB is the sole product. • These reactions are predominantly exothermic in nature because the formation of new chemical bonds typically releases energy in the form of heat or light. • A classic example is the burning of coal, where solid Carbon (C) reacts with Oxygen (O₂) from the air to produce Carbon Dioxide (CO₂) gas. • Another common instance is the reaction between Quicklime (Calcium Oxide, CaO) and water (H₂O) to form Slaked Lime (Calcium Hydroxide, Ca(OH)₂), which is characterized by the evolution of a large amount of heat. Related Concepts: • Decomposition Reaction: • This is the opposite of a combination reaction, where a single reactant breaks down into two or more simpler products. • It usually requires energy in the form of heat, light, or electricity to break the existing bonds. For example, Calcium Carbonate (CaCO₃) decomposes into Calcium Oxide (CaO) and Carbon Dioxide (CO₂) when heated. Hence, statement 1 is incorrect. • Displacement Reaction: • In this reaction, a more reactive element displaces or removes a less reactive element from its compound or salt solution. • A well-known example is Iron (Fe) reacting with Copper Sulfate (CuSO₄) solution, where Iron displaces Copper to form Iron Sulfate (FeSO₄) and solid Copper. Hence, statement 4 is incorrect. • Double Displacement Reaction: • This involves an exchange of ions between two compounds to form two new substances. • Often, one of the products is a precipitate (an insoluble solid). For example, Sodium Sulfate (Na₂SO₄) reacts with Barium Chloride (BaCl₂) to form Barium Sulfate (BaSO₄) precipitate and Sodium Chloride (NaCl). Hence, statement 3 is incorrect. Important Notes: • Industrial Applications: Combination reactions are vital in the Haber Process for the industrial production of Ammonia (NH₃), where Nitrogen and Hydrogen gases combine under high pressure and temperature. • Nature of Reactants: Combination reactions can occur between two elements (like H₂ and O₂), two compounds (like CaO and H₂O), or an element and a compound (like CO and O₂). • Oxidation-Reduction (Redox): Many combination reactions are also Redox reactions, where there is a simultaneous loss and gain of electrons between the reacting species. • Law of Conservation of Mass: In every chemical reaction, including combination reactions, the total mass of the reactants must equal the total mass of the products, necessitating the balancing of chemical equations.