What distinctive feature is observed with potassium in water?

When potassium is placed in water, it exhibits a distinctive feature of moving rapidly across the surface. This happens because potassium is a very reactive alkali metal. When it comes into contact with water, it reacts vigorously, producing potassium hydroxide and hydrogen gas. The hydrogen gas that is released forms bubbles, causing the potassium to float on the surface and move around as it reacts.

In addition to this movement, the reaction generates enough heat to ignite the released hydrogen gas, which can lead to the characteristic self-ignition of potassium when it is in sufficient quantities. However, the immediate observable feature is the rapid movement across the surface of the water as the reaction unfolds.

The other choices do not accurately describe potassium's behavior with water. Potassium does not sink because it is less dense than water, it does not merely evaporate, and while it can self-ignite, that is a result of the vigorous reaction rather than an observable feature at the moment of initial contact. The key aspect to focus on is the rapid movement, which is prominently associated with the nature of the reaction between potassium and water.