Final answer:
An air gap should exist between the faucet and the sink rim to prevent backflow of contaminated water, ensuring that the potable water system remains safe from contaminants.
Step-by-step explanation:
To prevent the backflow of contaminated water into a clean water supply at a sink, an air gap should be provided between the faucet and the floor rim of the sink. An air gap is simply an open space that separates a water outlet from any potential source of contamination. In case of backpressure or a siphoning event, the air gap prevents dirty water from being drawn back into the potable water system. Plumbing systems often include air gaps as a simple, yet effective, backflow prevention measure. They are crucial to maintaining water quality and protecting health.