A Level Chemistry OCR Practice Exam

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What distinguishes an alkali from other bases?

It only reacts with acids
It is insoluble in water
It forms hydroxide ions in solution
It is a strong acid

The correct answer is: It forms hydroxide ions in solution

An alkali is distinguished from other bases primarily by its ability to dissolve in water and produce hydroxide ions (OH⁻) in solution. When an alkali such as sodium hydroxide (NaOH) is dissolved in water, it dissociates to yield hydroxide ions, which are responsible for the basic properties of the solution. This characteristic is what sets alkalis apart from other bases, which may not be soluble in water or may not produce hydroxide ions when they do dissolve. The definition of an alkali typically includes the stipulation that it must be a soluble base. Therefore, while all alkalis are bases, not all bases are alkalis. This is an essential distinction in the study of acid-base chemistry, particularly when considering reactions involving acids and bases and the role of ionic species in those reactions. Understanding this concept is critical for topics in pH, neutralization reactions, and the behavior of different substances in aqueous solutions, which are fundamental to higher-level chemistry courses.