In the characteristic reaction method, if the insoluble solid is a base (metal oxide or alkali) reacting with acid, what is formed?

• A. Metal salt and water
• B. Metal salt and hydrogen
• C. Hydrogen gas
• D. Carbon dioxide

Answer: (A)

Neutralization is the key idea here. An insoluble base like a metal oxide or an alkali reacts with an acid by the H+ ions from the acid combining with the OH- ions from the base to form water, while the remaining ions form a salt. So the overall products are a metal salt and water. For example, calcium oxide with hydrochloric acid makes calcium chloride and water, and sodium hydroxide with sulfuric acid makes sodium sulfate and water. Because water is produced from H+ and OH-, no gas is released in this type of reaction, unlike metal–acid reactions that produce hydrogen gas or reactions with carbonates that release CO2.