Answer | Explanation: The combination reaction involves the joining or combination of two or more elements or compounds to form a new compound. Based on the options provided:
(a) N₂(g) + 3H₂(g) → 2NH₃(g)
This reaction involves the combination of nitrogen gas (N₂) with hydrogen gas (H₂) to form ammonia gas (NH₃). It is a combination reaction.
(b) CaO(s) + CO₂(g) → CaCO₃(s)
This reaction involves the combination of calcium oxide (CaO) with carbon dioxide gas (CO₂) to form calcium carbonate (CaCO₃). It is also a combination reaction.
(c) 250₂(g) + O₂(g) → 250₃(g)
This reaction does not involve a combination of two elements or compounds. It seems to be an incomplete or typo.
(d) NH₃(g) + HCl(g) → NH₄Cl(s)
This reaction involves the combination of ammonia gas (NH₃) with hydrogen chloride gas (HCl) to form ammonium chloride solid (NH₄Cl). It is a combination reaction.
Based on the options provided, both options (a) and (b) involve the combination of two elements or compounds.