What defines an alkali in chemistry?

An alkali in chemistry is specifically defined as a water-soluble base that releases hydroxide ions (OH⁻) when dissolved in water. This characteristic is crucial because it distinguishes alkalis from other types of bases that may not dissolve well in water. The release of hydroxide ions is significant as it contributes to the basicity or alkalinity of the solution, allowing it to neutralize acids and influence pH levels.

When considering the other options, they do not align with the accepted definition of alkalis. A water-insoluble base does not interact with water to produce hydroxide ions, and thus cannot be classified as an alkali. Similarly, while it is true that some bases can absorb carbon dioxide, this is a property of certain compounds rather than a defining feature of alkalis. Lastly, a substance that increases acidity directly contradicts the basic nature of alkalis, as acids provide protons (H⁺) which decrease pH, thus further emphasizing why the understanding of alkalis revolves around their ability to release hydroxide ions in aqueous solutions.