Final answer:
We can tell a substance is a base if it releases hydroxyl ions (OH−) or accepts protons (H+), commonly demonstrated by compounds containing a metal and the hydroxide ion, such as NaOH. Water (H2O) acts as a base when it accepts a proton from NH4+, resulting in NH3(aq) and OH−(aq).
Step-by-step explanation:
To determine if a substance is a base, we look for certain properties. In general, bases are substances that release hydroxyl ions (OH−) in solution, or accept protons (H+) from acids. This is observed in ionic compounds like NaOH and Ca(OH)2 which contain the hydroxide ion, OH−, that accepts a proton from acids to form water (H2O).
Looking at the options provided, (b) starts with a metal/cation/NH4 and ends in -OH is the closest to defining a base, as many bases such as NaOH (sodium hydroxide) are comprised of a metal ion and the hydroxide ion. Ammonia (NH3) is also a common base found in household cleaning products, it becomes a base by reacting with water to increase the OH− ion concentration:
NH3(aq) + H2O(l) → NH4+(aq) + OH−(aq).
In the reaction provided, water (H2O) acts as a base when it accepts a proton:
H2O(l) + NH4+ (aq) → H3O+(aq) + NH3(aq).
Several compounds can act as a Brønsted-Lowry base, as they accept a proton in a reaction, such as H2O, NH3, and anions like OH−.