Which of the following is a characteristic of alkaline substances?

Alkaline substances, also known as bases, exhibit a characteristic feel that is often described as slippery or soapy. This slippery sensation is due to the presence of hydroxide ions which interact with the fats and oils on the skin, resulting in a lathering effect that is similar to soap. This property is commonly noted with bases such as sodium hydroxide or potassium hydroxide.

In contrast, other options describe properties typical of acids or do not accurately reflect the nature of alkaline substances. For instance, sour taste is characteristic of acidic substances, while the ability to turn blue litmus paper red is also typical of acids, indicating a decrease in pH. The mention of being highly corrosive does not apply broadly to all alkaline substances as many can be, but not all are considered highly corrosive in the same manner as strong acids. Thus, the slippery feeling is a definitive and notable characteristic of alkaline substances, distinguishing them from acids and other types of compounds.