What defines a substance that is alkaline?

A substance that is considered alkaline is defined by its ability to neutralize acids. Alkaline substances, also known as bases, typically have a pH greater than 7, which allows them to react with acids to form water and salt, effectively reducing the acidity. This characteristic is crucial in various biological processes, including maintaining acid-base balance in living organisms and facilitating biochemical reactions.

The other options, while related to pH and acidity, do not accurately capture the defining trait of alkaline substances. For instance, a pH less than 7 corresponds to acidic substances, and a neutral pH indicates a balance between acids and bases. Additionally, the statement that alkaline substances derive from acidic substances only does not hold true, as many bases can be formed from non-acidic sources or synthetic processes. Thus, the ability to neutralize acids is the most defining feature of alkaline substances.