What defines an alkali in terms of hydroxide ions?

An alkali is defined as a substance that produces hydroxide ions (OH⁻) when dissolved in water. This characteristic is fundamental to the behavior of alkali substances in chemical reactions, particularly in their ability to neutralize acids. When alkalis dissolve in water, they dissociate to release hydroxide ions, which can combine with hydrogen ions (H⁺) from acids to form water.

This definition highlights the role of hydroxide ions in defining not just the chemical properties of alkalis, but also their applications in various chemical processes, such as in titrations with acids. The presence of hydroxide ions is what gives alkalis their basicity, allowing them to counteract acidic solutions effectively.

In contrast to the other options, the correct answer emphasizes the importance of hydroxide ions in the identity of alkalis. For instance, while carbonates can react with acids, they are not necessarily classified as alkalis. Additionally, the production of hydrogen ions pertains to acids rather than alkalis, and not all reactive metals are classified as alkalis since they may not dissolve in water to produce hydroxide ions.