Which characteristic identifies substances that are classified as alkaline?

The classification of substances as alkaline is predominantly based on their pH level. Alkaline substances, also referred to as basic substances, have a pH that is equal to or higher than 7. However, to be more specific, a pH greater than or equal to 12.5 is a common threshold that indicates a strongly alkaline solution. This characteristic is important because it typically aligns with the properties of strong bases, which can react aggressively in chemical processes.

In the context of hazardous waste operations, recognizing the pH level of materials is crucial, as it affects how they may interact with other substances, their corrosivity, and their behavior in various environments, particularly in waste management and treatment scenarios. While a pH of 7 indicates neutrality and anything below that indicates acidity, a pH of 12.5 or higher is specifically indicative of a strong alkaline condition, making it vital for identifying hazardous materials.

The other options refer to different pH ranges, but they do not capture the specific characteristic of what defines a substance as alkaline in the context of hazardous materials.